Preview Inorganic Chemistry for the JEE Mains and Advanced by K. Rama Rao (2014)

Preview Inorganic Chemistry for the JEE Mains and Advanced by K. Rama Rao (2014) Preview Inorganic Chemistry for the JEE Mains and Advanced by K. Rama Rao (2014) Preview Inorganic Chemistry for the JEE Mains and Advanced by K. Rama Rao (2014) Preview Inorganic Chemistry for the JEE Mains and Advanced by K. Rama Rao (2014) Preview Inorganic Chemistry for the JEE Mains and Advanced by K. Rama Rao (2014)

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Inorganic Chemistry is an outcome of several years of teaching chemistry to the students preparing for different competitive

examinations When the thought of bringing out this book came to my mind, three pertinent questions required consideration

• First, is there really a need to bring out yet another book on the subject ‘inorganic chemistry’ when there are already so many standard books available in the market?

• Second, how this book would tackle the limitations in the presentation of the subject in case of preparation of different competitive examinations after the completion of Class XII?

• Third, what must be the right order or sequence of topics so that the preparation will be easier for the students?

I believe that the answer to the first question lies in the rich content offered by this book While dealing with various topics

of inorganic chemistry, one needs to refer to different books for clarifications on various topics Naturally, an absence of a stop solution to all the problems is felt by the students This justifies the need to bring out a book which can serve as a single

one-source of reference on inorganic chemistry and make learning simpler and facilitate problem-solving

An analysis of the questions asked in various competitive examinations conducted over the last few years was done It was seen that the knowledge required to answer the questions is much more than what is offered at the Class XII level In fact,

a deeper study of the concepts is needed to attempt these questions This book goes a little deeper into the concepts/topics included at the Class XII level This answers the second question mentioned above Moreover, this analysis also fulfills the need to study the difficulty level of questions, types of questions and important topics etc., as the sole purpose of this book is to equip the student with sufficient knowledge to solve multiple-choice questions pertaining to inorganic chemistry As far as the third question is concerned, my own personal experience in teaching the subject in a student-friendly manner has been helpful

in planning the sequence in which the topics should be dealt with

Earlier, the study of inorganic chemistry was thought to be a very complex and elongated process; one requiring the need

to study and remember all the properties and uses of various elements and their compounds thoroughly But with the passage of time, the requirements of teachers and students necessitated the need to deal with each topic in a logical manner For instance, there has been a steady infiltration of physical chemistry into inorganic chemistry and it has resulted in the subject being made rigorous and more comprehensive for studying as compared to the past scenario As such, it is a futile attempt if one writes a

book on inorganic chemistry without laying stress on structural and energy considerations which are the two kingpins upon

which a satisfactory development of the subject rests The Class XII syllabus tends to reflect this trend For this purpose, concepts such as enthalpy, entropy, free energy changes, equilibrium and equilibrium constant, acid base etc., are explicitly touched upon wherever necessary In explaining the stability and solubility of inorganic compounds such as carbonates, sul-phates, halides, oxides, hydroxides, etc., the concept of thermodynamics is aptly used Similarly, an attempt has been made

to explain the trend in solubility of inorganic compounds based on the acid base theory and thermodynamic data Mainly, the focus is on explaining the different aspects with a logical approach so that students may retain a steady interest in the subject

The book goes beyond the immediate needs of the existing Class XII syllabus and fulfills all the requirements of a tory tool required to attempt questions asked in various competitive examinations I hope that not only students would benefit from this book but teachers would also find it as a valuable resource for referring to the explanations of various concepts in a simple and detailed manner

prepara-I am grateful to all those who directly or indirectly encouraged me to author this book prepara-I am also very grateful to the staff of Pearson Education, especially Rajesh Shetty, Bhupesh Sharma and Vamanan Namboodiri, for their continuous encouragement and hard work in bringing out this book in this fascinating manner

Good Luck!

K Rama Rao

1.1 iNTRODUCTiON

The introduction of atomic theory by John Dalton early in the

nineteenth century marks the beginning of modern era in

the research initiatives in the field of Chemistry The virtue

of Dalton’s theory was not that it was new or original, for

theories of atoms are older than the science of chemistry,

but that it represented the first attempt to place the

cor-puscular concept of matter upon a quantitative basis

The theory of the atomic constitution of matter dates back at

least 2,500 years to the scholars of ancient Greece and early

Indian philosophers who were of the view that atoms are

fundamental building blocks of matter According to them,

the continued subdivision of matter would ultimately yield

atoms which would not be further divisible The word

‘atom’ has been derived from the Greek word ‘a-tomio’

which means ‘uncuttable’ or non-divisible Thus, we

might say that as far as atomic theory is concerned, Dalton

added nothing new He simply displayed a unique ability to

crystallize and correlate the nebulous notions of the atomic

constitution prevalent during the early nineteenth century

into a few simple quantitative concepts

1.2 ATOmiC TheORy

The essentials of Dalton’s atomic theory may be

summa-rized in the following postulates:

1 All matter is composed of very small particles called atoms.

2 Atoms are indestructible They cannot be subdivided,

created or destroyed

3 Atoms of the same element are similar to one another

and equal in weight

4 Atoms of different elements have different properties

and different weights

5 Chemical combination results from the union of atoms

in simple numerical proportions

Thus, to Dalton, the atoms were solid, hard, trable particles as well as separate, unalterable individuals

impene-Dalton’s ideas of the structure of matter were born out of considerable amount of subsequent experimental evidence

as to the relative masses of substances entering into cal combination Among the experimental results and rela-tionship supporting this atomic theory were Gay-Lussac’s law of combination of gases by volume, Dalton’s law of multiple proportions, Avagadro’s hypothesis that equal vol-umes of gases under the same conditions contain the same number of molecules, Faraday’s laws relating to electrolysis and Berzelius painstaking determination of atomic weights

chemi-modern Atomic Theory

Dalton’s atomic theory assumed that the atoms of elements were indivisible and that no particles smaller than atoms

John Dalton was born in England in 1766 His

family was poor, and his formal education stopped when he was eleven years old He became a school teacher He was colour blind His appearance and manners were awkward, he spoke with difficulty in public As an experiment he was clumsy and slow

He had few, if any outward marks of genius

In 1808, Dalton published his celebrated New Systems of Chemical Philosophy in a series of publica-

tions, in which he developed his conception of atoms as the fundamental building blocks of all matter It ranks among two greatest of all monuments to human intel-ligence No scientific discovery in history has had a more profund affect on the development of knowledge

Dalton died in 1844 His stature as one of the greatest scientists of all time continues to grow

exist As a result of brilliant era in experimental physics

which began towards the end of the nineteenth century

extended into the 1930s paved the way for the present

modern atomic theory These refinements established that

atoms can be divisible into sub-atomic particles, i.e.,

elec-trons, protons and neutrons—a concept very different from

that of Dalton The major problems before the scientists at

that time were

(i) How the sub-atomic particles are arranged within the

atom and why the atoms are stable?

(ii) Why the atoms of one element differ from the atoms

of another elements in their physical and chemical

properties?

(iii) How and why the different atoms combine to form

molecules?

(iv) What is the origin and nature of the characteristics of

electromagnetic radiation absorbed or emitted by atoms?

1.3 SUb-ATOmiC PARTiCleS

We know that the atom is composed of three basic sub-atomic

particles namely the electron, the proton and the neutron The

characteristics of these particles are given in Table 1.1

It is now known that many more sub-atomic particles

exist, e.g., the positron, the neutrino, the meson, the hyperon

etc, but in chemistry only those listed in Table 1.1 generally

need to be considered The discovery of these particles and

the way in which the structure of atom was worked out are

discussed in this chapter

1.3.1 Discovery of electron

The term ‘electron’ was given to the smallest particle

that could carry a negative charge equal in magnitude to

the charge necessary to deposit one atom of a 1-valent

element by Stoney in 1891 In 1879, Crookes

discov-ered that when a high voltage is applied to a gas at low pressure streams of particles, which could communicate momentum, moved from the cathode to the anode It did

not seem to matter what gas was used and there was strong evidence to suppose that the particles were com-mon to all elements in a very high vacuum they could not

be detected The cathode ray discharge tube is shown

in Fig 1.1

The properties of the cathode rays are given below:

(i) When a solid metal object is placed in a discharge tube in their path, a sharp shadow is cast on the end

of the discharge tube, showing that they travel in straight lines

(ii) They can be deflected by magnetic and electric fields, the direction of deflection showing that they are neg-atively charged

(iii) A freely moving paddle wheel, placed in their path, is set in motion showing that they possess momentum, i.e., particle nature

(iv) They cause many substances to fluoresce, e.g., the familiar zinc sulphide coated television tube

(v) They can penetrate thin sheets of metal

J.J Thomson (1897) extended these experiments and

determined the velocity of these particles and their charge/

mass ratio as follows

The particles from the cathode were made to pass through a slit in the anode and then through a second slit

They then passed between two aluminium plates spaced about 5 cm apart and eventually fell onto the end of the tube, producing a well-defined spot The position of the spot was noted and the magnetic field was then switched

on, causing the electron beam to move in a circular arc while under the influence of this field (Fig 1.2)

Table 1.1 The three main sub-atomic particles

No charge

To vacuumpumpGas at low pressure

e e

e m

By applying a potential of several thousand volts across the parallel metal plates, the oil droplet can either be speeded

up or made to rise, depending upon the direction of the tric field Since, the speed of the droplet can be related to its weight, the magnitude of the electric field, and the charge it picks up, the value of the charge can be determined

elec-Thomson proposed that the amount of deviation of the

particles from their path in the presence of electrical and

magnetic field depends on

(i) Greater the magnitude of the charge on the particle,

greater is the interaction with the electric and

mag-netic fields, and thus greater is the deflection

(ii) Lighter the particle, greater the deflection

(iii) The deflection of electrons from its original path

increase in the voltage across the electrodes, or the

strength of the magnetic field

By careful and quantitative determination of the

mag-netic and electric fields on the motion of the cathode rays,

Thomson was able to determine the value of charge to mass

ratio as

e

e m

me is the mass of the electron in kg and e is the

magnitude of the charge on the electron in Coulomb

1.3.2 Charge on the electron

Thomson’s experiments show electrons to be negatively

charged particles Evidence that electrons were discrete

particles was obtained by Millikan by his well known oil

drop experiment during the years 1910−14 By a series of

very careful experiments Millikan was able to determine

the value − electronic charge, and the mass Millikan

found the charge on the electron to be −1.6 × 10−19 C

The present-day accepted value for the charge on the

elec-tron is 1.602 × 10−19 C When this value for ‘e’ is

com-pared with the most modern value of e/m, the mass of the

electron can be calculated

Fig 1.2 Thomson’s apparatus for determining e/m for the electron

Anode (+)

Cathode²

Spot of light

Spot of light

Fluorescent screen

Deflecting plates

Gas at low pressure

when top plate is positive

when plate isnot charged

²+

Millikan Oil Drop Method

To vacuum pumpM

V+

−B

V

Oil sprayerA

Oilglobules

E1

x – RaysLight

v

Fig 1.3 Millikan’s apparatus for determining the value of

the electronic charge

9 2 12 1

Where the superscript refers to the atomic mass and the subscript refers to the atomic number (the number of protons in the nucleus) Notice that a new element, carbon, emerges from this reaction

1.4 ATOmiC mODelS

The discovery that atoms contained electrons caused some consternation Left to themselves, atoms were known to be electrically neutral So, the negative charge of the electrons had to be balanced by an equal amount of positive charge

The puzzle was to work out how the two types of charges were arranged To explain this, different atomic models were proposed Two models proposed by J.J Thomson and Earnest Rutherford are discussed here though they cannot explain about the stability of atoms

1.4.1 Thomson model of Atom

“A theory is a tool not a creed.” J.J Thomson

Sir Joseph John Thomson 1856–1940

Thomson’s researches on discharge of electricity through gases led to the discovery of the electron and isotopes

In 1898, Sir J.J Thomson proposed that the electrons are embedded in a ball of positive charge (Fig 1.5) This

model of the atom was given the name plum pudding or

raisin pudding or watermelon According to this model

we can assume that just like the seeds of a watermelon are embedded within the reddish juicy material, the electrons are embedded in a ball of positive charge It is important

to note that in Thomson’s model, the mass of the atom is

1.3.3 Discovery of Proton

If the conduction of electricity, through gases is due to

particles, which are similar to those involved during

electrolysis, it was to be expected that positive as well as

negative ones should be involved, and that they would be

drawn to the cathode By using a discharge tube

contain-ing a perforated cathode, Goldstein (1886) had observed

the formation of rays (shown to the right of the cathode in

Fig 1.4)

J.J Thomson (1910) measured their charge/mass ratio

from which he was able to deduce that the particles were

positive ions, formed by the loss of electrons from the

residual gas in the discharge tube The proton is the

small-est positively charged particle equal in magnitude to that

on the electron and is formed from the hydrogen atom by

the loss of an electron

H → H+ + e−

Unlike cathode rays, the characteristics of

posi-tively charged particles depend upon the nature of the

gas present in the cathode ray tube These are positively

charged ions The charge to mass ratio of these

parti-cles is found to be dependent upon the gas from which

these originate Some of the positively charged particles

carry a multiple of the fundamental unit of electrical

charge The behaviour of these particles in the magnetic

or electric field is opposite to that observed for electron

or cathode rays

1.3.4 Discovery of Neutron

The neutron proved to be a very elusive particles to track

down and its existence, predicted by Rutherford in 1920,

was first noticed by Chadwick in 1932 Chadwick was

bombarding the element beryllium with α-particles and

noticed a particle of great penetrating power which was

unaffected by magnetic and electric fields It was found to

have approximately the same mass as the proton (hydrogen

ion) The reaction is represented as

Fig 1.5 The Thomson model of atom The positive

charge was imagined as being spread over the entire atom and the electrons were put in this background

Anode

Perforated cathodeH

H+e

_e_e_

Fig 1.4

g-rays are high energy radiations like X-rays, are neutral in nature and do not consist of particles As regards penetrat-ing power, α-particles have the least followed by b-rays (100 times that of α-particles) and g-rays (1000 times that

he became director of the celebrated Cavendish oratory at Cambridge In 1908, he received Nobel prize for chemistry Rutherford made many of the basic discoveries in the field of radioactivity With Bohr and others, he elaborated a theory of atomic structure In 1919, he produced the first artificial transmutation of an element (that of nitrogen into oxygen) For many years he was a vigorous leader

lab-in laylab-ing the foundation of the greatest ments in atomic science, which he did not live to see He died in 1937

develop-In 1911, Ernest (later Lord) Rutherford demonstrated

a classic experiment for testing the Thomson’s model

Rutherford, Geiger and Harsden studied in detail the effect

of bombarding a thin gold foil by high speed α-particle (positively charged helium particles) A thin parallel beam

of α-particles was directed onto a thin strip of gold and the subsequent path of the particles was determined Since the α-particles were very energetic, it was thought they would

go right through the metal foils To their surprise they observed some unexpected results which are summarized

Rutherford’s Explanation Rutherford pointed out

that his results are not in agreement with Thomson’s model

uniformly distributed over the atom Though this model

could explain the overall neutrality of the atom, it could not

explain the results of later experiments

The Discovery of X-rays and Radioactivity

In 1895, Rontgen noticed when electrons strike a

mate-rial in the cathode ray tubes, produces a penetrating

radia-tion emitted from the discharge tubes, and it appeared to

originate from the anode The radiation had the following

properties:

(i) It blackened the wrapped photographic film

(ii) It ionized gases, so allowing them to conduct

electricity

(iii) It made certain substances fluoresce, e.g., zinc

sulphide

Furthermore, the radiation was shown to carry no

charge since it could not be deflected by magnetic or

elec-tric fields Since Rontgen did not know the nature of the

radiation, he named them as X-rays and the name is still

carried on The true nature of X-rays was not discovered

until 1912, when it became apparent that its properties

could be explained by assuming to be wave like in

char-acter, i.e., similar to light but is much smaller wavelength

It is now known that X-rays are produced whenever

fast-moving electrons are stopped in their tracks by impinging

on a target, the excess energy appearing mainly in the form

of X-radiation

The year after Rontgen discovered X-rays, Henry

Becquerel observed that uranium salts emitted a

radia-tion with properties similar to those possessed by X-rays

The Curies followed up this work and discovered that

the ore pitchblend was more radioactive than purified

uranium oxide; this suggested that something more

intensely radioactive than uranium was responsible for

this increased activity and eventually the Curies

suc-ceeded in isolating two new elements called polonium

and radium, which were responsible for this increased

activity

In 1889, Becquerel reported that the radiation from

the element radium could be deflected by a magnetic field

and in the same year, Rutherford noticed that the

radia-tion from uranium was composed of at least two distinct

types Subsequently, it was shown that the radiation from

both sources contained three distinct components and are

named as α – b and g-rays Rutherford found that α-rays

consist of high energy particles carrying two units of

posi-tive charge and four units of atomic mass He concluded

that α-particles are helium nuclei as when α-particles

combined with two electrons yielded helium gas b-rays

are negatively charged particles similar to electrons The

(i) An atom has a centre of the nucleus, in which tive charge and mass are concentrated and he called

posi-this centre as the nucleus.

The quantitative results of scattering experiments such as Rutherford’s indicate that the nucleus of an atom has a diameter of approximately one to six Fer-mis (1 Fermi = 10−10 m) and atoms have diameters about

100000 times as great as the size of the nucleus, i.e., of the order 10−5 m

(ii) The atom as a whole is largely an empty space and the nucleus is located at the centre of the atom

(iii) The nucleus is surrounded by the electrons which are revolving round the nucleus in closed paths like the planets.

(iv) Electrons and the nucleus are held together by trostatic forces of attraction

elec-Rutherford, therefore, contemplated the dynamical stability and imagined the electrons to be whirling about

the nucleus, similar to the planetary motion.

Drawbacks of the Rutherford model

Following objections were raised against the Rutherford’s model:

(i) It did not explain how the protons could be packed to give a stable nucleus

(ii) When an electron revolves round the nucleus, it will radiate out energy, resulting in the loss of energy This loss of energy will make the electron

to move slowly and consequently, it will be ing in a spiral path and ultimately falling onto the nucleus So, the atom should be unstable, but the atom is stable

(iii) If an electron starts losing energy continuously, the observed spectrum would be continuous and have broad bands merging into one another But most of the atoms give line spectra Thus, Rutherford’s model failed to explain the origin of line spectra

He explained the above results by drawing the following

conclusions:

(i) As most of the alpha particles passed very nearly

straight through the foil, it means that the atom is

extra ordinarily hollow with a lot of empty space

inside

(ii) The only way to account for the large deflection is

to say that the positive charge and mass in the gold

foil are concentrated in very small regions Although

most of the alpha particles can go through without

any deflection, occasionally some of them which

come closer to the region of positive charge, they

repel each other, and the repulsion may be big enough

to cause the α-particles to undergo large deviations

from its original path

(iii) Due to the rigid nature of the nucleus, some

α-particles on colliding with the positive charge, turn

back on their original paths

On the basis of above observations and

conclu-sions, Rutherford proposed the nuclear model of atom as

follows:

Fig 1.7 An electron that is accelerating, radiates energy

As it loses energy, it spirals onto the nucleus

Electron–+

Nucleus

+ + + +

Fig 1.6 Results of Rutherford’s experiment (a) One

layer of metal atoms each with nucleus (b) One atom of

the metal with nucleus

Mass number (A) = Number of protons (Z) + Number

of neutrons (n)

1.5.1 isobars, isotopes and isotones

In modern methods, the symbols of elements are written

as A[ ]n,

Z X x where the left hand superscript A is the mass number, left hand subscript Z is the atomic number, right hand superscript n is the number of charges and right hand

subscript is the number of atoms

Different atoms having same mass number but with

different atomic numbers are called isobars, e.g., 14

6C and 7

14N The atoms having same atomic number but with

dif-ferent mass numbers are called isotopes The difference

between isotopes is due to the difference in the number

of neutrons present in the nucleus, e.g., 1 2 3

1H, D and T1 1are isotopes of hydrogen namely protium, deuterium and tritium respectively

Isotopes exhibit similar chemical properties because they depend on the number of protons in the nucleus Neu-trons present in the nucleus show very little effect on the chemical properties of an element So, all the isotopes of a given element show same chemical behavior

Sometimes atoms of different elements contain same

number of neutrons Such atoms are called isotones For

example, 13

6C and 14

7 N Isotones differ in both atomic number and mass numbers but the difference in atomic number and mass number is the same

1.6 DevelOPmeNTS leADiNg TO The bOhR mODel OF ATOm

During the period of development of new models to improve Rutherford’s model of atom, two new concepts played a major role They are

(i) Dual behaviour of electromagnetic radiation: This

means that light has both particle like and wave like

properties

(ii) Atomic spectra: The experimental results

regard-ing atomic spectra of atoms can only be explained by assuming quantized (fixed) electronic energy levels

in atoms

Let us briefly discuss about these concepts before

studying a new model proposed by Niels Bohr known as

Bohr Model of Atom.

1.6.1 Nature of light and electromagnetic Radiation

A radiation is a mode of transference of energy of ent forms Light, X-rays and g-radiations are examples of

differ-1.5 ATOmiC NUmbeR

In 1913, H.G.J Moseley, a young English Physicist and

one of Rutherford’s brilliant students, examined the X-ray

spectra of 38 elements When Moseley bombarded

differ-ent elemdiffer-ents with cathode rays, the X-rays were generated

which had different frequencies He suggested that the

frequency of X-rays produced in this manner was related

to the charge presented on the nucleus of an atom of the

element used as anode When he took the frequencies of a

particular line in all elements such as Kα or Kb lines, then

the frequencies were related to each other by the equation

Where, v is the frequency of any particular Kα or Kb etc,

line Z is the atomic number of the element and ‘a’ and ‘b’

are constants for any particular type of line A plot of v

vs Z gives a straight line showing the validity of the above

equation This is shown in Fig 1.8

However, no such relationship was obtained when the

frequency was plotted against the atomic mass Moseley

further found that the nuclear charge increases by one unit

in passing from one element to the next element arranged

by Mendeleef in the order of increasing atomic weight The

number of unit positive charges carried by the nucleus of

an atom is termed as the atomic number of the element

Since, the atom as a whole is neutral, the atomic number

is equal to the number of positive charges present in the

nucleus.

Atomic number = Number of protons present in the

nucleus

= Number of electrons present outside the nucleus of

the same atom.

While the positive charge of the nucleus is due to

pro-tons, the mass of the nucleus is due to protons and

neu-trons The total number of protons and neutrons is called

the mass number The particles present in the nucleus are

(mm), nanometer (nm), picometer (pm) etc These units are related to SI unit (m) as

1 Å = 10−10 m; 1 m = 10−6 m, 1 mm = 10−9 m,

1 nm = 10−9 m; 1 pm = 10−12 m

2 Frequency (n): The number of waves passing through

a given point in a unit time is known as its frequency

It is denoted by the Greek letter n (nu) The frequency

is inversely proportional to the wavelength Its unit in cycles per second (cps) of Hertz (Hz); 1 cps = 1 Hz

A cycle is said to be completed when a wave consisting crest and trough passes through a point

3 Velocity (c): The distance travelled by the wave in one

second is called the velocity of wave It is equal to uct of wavelength and frequency of the wave Thus,

prod-c = vl

or v = c

4 Amplitude: It is the height of crest or depth of wave’s

trough and is generally expressed by the letter ‘a’

The amplitude of the wave determines the intensity or brightness of radiation

5 Wave number ( )v : It is equal to the reciprocal of

wavelength In other words, it is defined as the number

of wavelengths per centimeter It is denoted by v and

λ

electromagnetic Radiation Maxwell in 1873 proposed that light and other forms of

radiant energy propagate through space in the form of waves These waves have electric and magnetic fields

associated with them and are, therefore, called

electro-magnetic radiations and electroelectro-magnetic waves.

The important characteristics of electromagnetic radiations are

(i) These consist of electric and magnetic fields that oscillate in the directions perpendicular to the direc-tion in which the wave is travelling as shown in Fig 1.11 The two field components have the same wavelength and frequency

(ii) All electromagnetic waves travel with the same speed In vacuum, the speed of all types of electro-magnetic radiation is 3 00 × 108 ms−1 This speed is called the velocity of light

the radiant energy The earliest view of light, due to

New-ton, regarded light as made up of particles (commonly

termed as corpuscles of light) The particle nature of light

explained some of the experimental facts such as

reflec-tion and refracreflec-tion of light However, it failed to explain the

phenomenon of interference and diffraction The

corpuscu-lar theory was therefore, discarded and Huygens proposed

a wave-like character of light With the help of wave theory

of light, Huygens explained the phenomena of interference

and diffraction

We know that when a stone is thrown into water

of a quiet pond, on the surface of water waves are

produced The waves originate from the centre of the

disturbance and propagate in the form of up and down

movements The point of maximum upward

displace-ment is called the crest and the point of maximum

downward displacement is called trough Thus, waves

may be considered as disturbance which originate from

some vibrating source and travel outwards as a

continu-ous sequence of alternating crests and troughs as shown

in Fig 1.10

Characteristics of the Wave motion

1 Wavelength (l): It is the distance between two

near-est crnear-ests or troughs It is denoted by the Greek letter

Lambda l and is expressed in Angstrom units (Å) It is

also expressed as micron meter (m), milli micron meter

Fig 1.9 Diffraction of the waves while passing through

electromagnetic radiations, such as X-rays, ultraviolet rays, infra-red rays, microwaves and radiowaves.

The arrangement of different types of electromagnetic radiations in the order of increasing wavelengths (or decreas-

ing frequencies) is known as electromagnetic spectrum.

Different regions of electromagnetic spectrum are identified by different names The complete electromag-netic spectrum is shown in Fig 1.12

The various types of electromagnetic radiations have different energies and are being used for different purposes

as listed in Table 1.2

(iii) These electromagnetic radiations do not require any

medium for propagation For example, light reaches

us from the sun through empty space

electromagnetic Spectrum

The different electromagnetic radiations have different

wavelengths The visible light in the presence of which

our eyes can see, contains radiations having wavelength

between 3800 Å to 7600 Å The different colours in the

visible light corresponds to radiations of different

wave-lengths In addition to visible light there are many other

Fig 1.12 Complete electromagnetic spectrum

Increasing wavelengthDecreasing freqency

INDIGO

Direction ofpropagation of

em wave

X

YZ

Fig 1.11 Electric and magnetic fields associated with an

frequency

10 5 – 10 8 10 2 Signal

transmission

1.6.2 Quantum Theory of Radiation

When an object is heated, its colour gradually changes

For example, a black coal becomes red, orange, blue and finally ‘white hot’ with increasing temperature This shows that red radiation is most intense at a particular tempera-ture, blue radiation is most intense at another temperature and so on It indicates that the intensities of radiations of different wavelengths emitted by a hot body depend upon the temperature The curves representing the distribution

of radiation from a black body at different temperatures are shown in Fig 1.13

A black body is one which will absorb completely the incident radiation of wavelength The ideal black body does not reflect any energy, but it does radiate energy

Although no actual body is perfectly black, it is possible

to postulate a condition where radiation is completely enclosed in a space surrounded by thick walls at a con-stant temperature Thus, we may imagine a cavity inside a solid body containing a small hole in one wall whereby the investigator may study the enclosed radiation without alter-ing its character When a beam of light is passed through the hole, it would be absorbed by the inner walls of the enclosure and their temperature increases As soon as the beam was removed, since the enclosed kept at a constant temperature the excess energy emerges through the hole

in the form light This gives a distinctive spectrum, usually

referred to as the black body spectrum If the energy

emit-ted is plotemit-ted against its frequency or wavelength where

n = c / l, the curve shows a maximum, as indicated in Fig 1.13 The graph shows that the energy emitted is

Solved Problem 1

Calculate and compare the energies of two radiations, one

with a wavelength of 8000 Å and the other with 4000 Å

19 2

2.475 104.95 10

E

or 2E1 = E2

Solved Problem 2

A radio station is broadcasting a programme at 100 MHz

frequency If the distance between the radio station and

the receiver set is 300 Km, how long would it take the

sig-nal to reach the receiver set from the radio station? Also

calculate wavelength and wave number of these radio

waves

Solution:

All electromagnetic waves travel in vacuum or in air

with the same speed of 3 × 10−8 m s−1

Fig 1.13 Energy distribution for black body radiation at

different absolute temperatures

and the ball can have any value of energy ing to any point on the ramp Energy in this case in not quantized.

correspond-1.6.3 Photoelectric effect

When a beam of light falls on a clean metal plate in uum, the plate emits electrons This effect was discovered

vac-by Hertz in 1887, and is known as the photoelectric effect

The metal surface emits electrons by the action of light, that can be demonstrated using negatively charged gold leaf electroscope (Fig 1.15) As the light from carbon arc

falls on the metal plate, the diversion of leaves is reduced slowly This shows that electrons come out of all metal plates

greatest at the middle wavelengths in the spectrum and

least at the highest and lowest frequencies If curves are

plotted for a series of temperatures, as given in Fig 1.13

It is found that the maximum moves towards shorter

wave-lengths as the temperature rises

max Planck 1858–1947

A German physicist He received his Ph.D in

theo-retical physics from the University of Munich in

1879 Max Planck was well known for the

Quan-tum Theory which won the Nobel Prize for physics

in 1918 He also made significant contributions to

thermodynamics and other areas of physics

The shapes of the curves could not be explained on

the basis of wave theory of radiation Max Planck in 1900

resolved this discrepancy by postulating the assumption

that the black body radiates energy — not continuously

but discontinuously in the form of energy packets called

quanta The general quantum theory of electromagnetic

radiations can be stated as

(i) When atoms or molecules absorb or emit the radiant

energy, they do so in separate units of waves These

waves are called quanta or photons

(ii) The energy of a quantum or photon is given by

where, v is the frequency of the emitted radiation and

h is the Planck’s constant.

(iii) An atom or molecule emits or absorbs either one

quantum of energy (hv) or any whole number

multiples of this unit

This theory provided the basis for explaining the

photo-electric effect, atomic spectra and also helped in

understand-ing the modern concepts of atomic and molecular structure

Quantization of energy

The restriction of any property to discrete values is called

quantization A quantity cannot vary continuously to have

any arbitrary values but can change only discontinuously

to have some specific values For example, a ball moves

down a staircase (Fig 1.14a) then the energy of the ball

changes discontinuously and it can have only certain

def-inite values of energy corresponding to the energies of

various steps Energy of the ball in this case is quantized

On the other hand, if the ball moves down a ramp (Fig

1.14b), then the energy of the ball changes continuously

mv2 = hυ - hυ0

12hυ

Fig 1.15 (a) Einstein’s explanation of photoelectric

effect, and (b) Experimental device for photoelectric effect

Work function 5 eV = 5 × 1.602 × 10−19 J = 8.01 × 10−19

J (\1eV = 1.602 × 10−19 J)Work function = hv0

9.11 10 kg

h

v v m

c v

The studies have revealed the following facts:

(i) Photoelectric effect is instantaneous, i.e., as soon as

the light rays fall on the metal surface, electrons are

ejected

(ii) Only light of a certain characteristic frequency is

required for expulsion of electrons from a particular

metal surface

(iii) The number of electrons emitted are directly

propor-tional to the intensity of the light

(iv) The gas surrounding the metal plate has no effect on

photoelectric transmission

(v) Kinetic energy of photoelectron depends upon the

nature of metal

(vi) For the same metal kinetic energy of photoelectrons

varies directly with the frequency of light

If the frequency is decreased below a certain value

(Threshold frequency), no electrons are ejected at all

The classical wave theory of light was completely

inadequate to interpret all these facts Einstein applied the

quantum theory of radiation to explain this phenomenon

An electron in a metal is found by a certain amount of

energy The light of any frequency is not able to cause the

emission of electrons from the metal surface There is

cer-tain minimum frequency called the threshold frequency

which can just cause the ejection of electrons Suppose the

threshold frequency of the light required to eject electrons

from the metal is v0 When photon of light of this frequency

strikes the metal surface, it imparts its entire energy (hv0)

to the electron This enables the electrons to break away

from the atom by overcoming the attractive influence of the

nucleus Thus, each photon can eject one electron If the

frequency of the light is less than n0, the electrons are not

ejected If the frequency of the light (say v) is more than

v0, more energy is supplied to the electron The remaining

energy, which will be the energy hv imparted by incident

photon and the energy used up, i.e., hv0, would be given as

kinetic energy to the emitted electron Hence,

2 0

12

where 1

2mv

2 is the kinetic energy of the emitted

electron and hv0 is called the work function.

2mv

This equation is known as Einstein’s photoelectric

equation If the frequency of light is less than the

thresh-old frequency, there will be no ejection of electrons Values

of photoelectric work functions of some metals are given

below

Also according to Einstein’s mass energy equation E

= mc2 where m is the mass of photon From the above two

equations, we get

2

hv m c

This phenomenon is known as dispersion and the pattern

of colours is called spectrum The red colour light radiation

having longest wavelength will be deviated least while the radiation of violet colour having shortest wavelength will be deviated more The spectrum of white light that we can see ranges from violet at 7.50 × 1014 Hz to red at 4 × 1014 Hz

In this spectrum one colour merges into another colour

adjacent to it viz violet merges into blue, blue into green

and so on When electromagnetic radiation interacts with matter, atoms and molecules may absorb energy and reach

to a higher energy state While coming to the ground state (more stable lower energy state) the atoms and molecules emit radiations in various regions of the electromagnetic spectrum

Work function = 4.65 eV

KE = hv − Work function

= 4.887 eV − 4.65 eV = 0.237 eV

1.6.4 Compton effect

According to classical electromagnetic wave theory,

mon-ochromatic light falling upon matter should be scattered

without change in frequency But when X-rays are impinged

on matter of low atomic weight, X-rays of slightly longer

wavelength than those of impinging beam were produced

In other words when a photon collides with an electron, it

may leave with a lower frequency, and the electron thereby

acquires a greater velocity In the collision the energy of the

photon is reduced and the energy of the electron is increased

This is called Compton effect and can be explained as a

result of the impact between two bodies, the photon and the

electron and it is of course additional proof of the

corpus-cular nature of light This impact is elastic, and both kinetic

energy and momentum are conserved After the encounter

each body has a different energy and a different momentum

from the values it had before the contact, but the sum of the

energies and the sum of the momenta are unchanged

1.6.5 Dual Nature of electromagnetic

Radiations

As described, photoelectric effect and Compton effect could be

explained considering that electromagnetic radiations

consist-ing of small packets of energy called quanta or quantum (or

single photon) These packets of energy can be treated as

parti-cles, on the other hand, radiations exhibit phenomena of

inter-ference and diffraction which indicate that they possess wave

nature So, it may be concluded that electromagnetic radiation

possesses the dual nature, i.e., particle nature as well as wave

nature Einstein (1905) even calculated the mass of the photon

associated with a radiation of frequency v as given below:

The energy E of the photon is given as E = hv

Albert einstein 1879–1955

Born in Germany but later shifted to America He

was regarded as one of the two great physicists the

world has known, the other being Isaac Newton He

is well known for his three research papers on special

relativity, Brownian motion and photoelectric effect

These research papers were published in 1905 while

he was working as a technical assistant in a Swiss patent office in Berne He received the Nobel Prize for Physics in 1921 for his explanation of the photo-electric effect His work has influenced the develop-ment of physics in significant manner

“To him who is discoverer in the field (or ence), the products of his imagination appear so nec- essary and natural that he regards them, and would like to have them regarded by others, not as creations

sci-of thought but as given realities”— Albert Einstein

extended throughout the spectrum and are ally obtained from the light sources like mercury, sodium, neon discharge tube, etc.

gener-(c) Band spectrum: This type of spectrum arises

when the emitter in the molecular state is excited

Each molecule emits bands which are istics of the molecule concerned and that is why

character-we call this as molecular spectrum also The

sources of band spectrum are (i) carbon with a metallic salt in its core, (ii) vacuum tube, etc.

In the emission spectra, bright lines on black ground will appear

back-It may be noted that the dark lines in absorption tra appear exactly at the same place where the coloured lines appear in the emission spectra

(ii) Absorption spectra: It is produced when the light

from a source emitting a continuous spectrum is first passed through an absorbing substance and then recorded after passing through a prism in a spectro-scope In that spectrum, it will be found that certain colours are missing which leave dark lines or bands

at their places This type of spectrum is called the absorption spectrum Similar to emission spectra, absorption spectra are also of three types:

(a) Continuous absorption spectrum: This type

of spectrum arises when the absorbing material absorbs a continuous range of wavelengths An interesting example is the one in which red glass absorbs all colours except red and hence, a con- tinuous absorption spectrum will be obtained.

(b) Line absorption spectrum: In this type, sharp

dark lines will be observed when the absorbing substance is a vapour or a gas The spectrum obtained from sun gives Fraunhofer absorption lines corresponding to vapours of different ele- ments which are supposed to be present on the surface of the sun.

(c) Band absorption spectrum: When the

absorp-tion spectrum is in the form of dark bands, this is

known as band absorption spectrum An

inter-esting example is that of an aqueous solution

of KMnO 4 giving five absorption bands in the green region.

The pattern of lines in the spectrum of an element is characteristics of that element and is different from those

of all other elements In other words, each element gives a unique spectrum irrespective of even the form in which it

is present For example, we always get two important lines

589 nm and 589.6 nm in the spectrum of sodium whatever may be its source It is for this reason that the line spectra

are also regarded as the fingerprints of atoms.

1.6.7 Types of Spectra

Spectra are of two types: Emission spectra and Absorption

spectra

(i) Emission spectra: When energy is supplied to a

sample by heating or irradiating it, the atoms or

mol-ecules or ions present in the sample absorb energy

and the atoms, molecules or ions having higher

energy are said to be excited While coming to the

ground (normal) state the excited species emit the

absorbed energy The spectrum of such emitted

radia-tion is called the emission spectrum Emission

spec-tra are further classified according to its appearance

as continuous, line and band spectra

(a) Continuous spectrum: When the source

emitting light is an incandescent solid, liquid or gas at a high pressure, the spectrum so obtained

is continuous In other words, this type of trum is obtained whenever matter in the bulk is heated For example, hot iron and hot charcoal gives the continuous spectra.

spec-(b) Line spectrum: Line spectrum is obtained when

the light emitting substance is in the atomic state

Hence, it is also called as the atomic spectrum

Line spectrum consists of discrete wavelengths

Emission spectrum of Barium

Absorption spectrum of Barium

Fig 1.17 Illustrating emission and absorption spectra

Fig 1.19 Hydrogen spectrum (a) Relative location of the Lyman, Balmer, Paschen, Brackett and Pfund series of hydrogen

spectrum (b) The Balmer-series spectrum of hydrogen

PrismIncreasing wavelength

Source of white light

Continuous emission spectrum of white light/sun

Fig 1.18 Emission and absorption spectra

hydrogen Spectrum

As explained above, each element emits its own teristic line spectrum which is different from that of any other element Since, hydrogen contains only one electron, its spectrum is the simplest to analyze

charac-To get the spectrum of hydrogen, the gas is enclosed in

a discharge tube under low pressure and electric discharge

is passed through it The hydrogen molecules dissociate into atoms and get excited by absorbing the energy: On the principle that what goes up must come down, sooner or later the atoms must loose their energy and fall back to the ground state, by loosing the energy as light The light that

is given out can be measured in a spectrometer and the tern recorded on a photographic paper The pattern is called hydrogen spectrum

pat-The hydrogen spectrum obtained consists of a series

of lines in the visible, ultraviolet and infrared regions

These have been grouped into five series in Fig 1.19 which are named after their discoverers These are given

in Table 1.3 with the year when discovered The Balmer series happened to be the first series of lines in the hydro-gen spectrum This was because the lines were in the visible part of the spectrum and therefore the easiest to observe

Balmer (1885) discovered a relationship between the

wave number and the position of the line in the series

v is the wave number and m is an integer having values

3, 4, 5, 6, etc Ritz (1908) gave a generalization known

as Ritz Combination principle which was nicely

applica-ble to hydrogen spectrum According to this principle, the wave number in any line in a series can be represented as a difference of two square terms, one of which is consistent and the other varies throughout the series Thus,

Since atoms of different elements give

characteris-tic sets of lines of definite frequencies, emission spectra

can be used in chemical analysis to identify and estimate

the elements present in a sample The elements rubidium,

caesium, thallium, gallium and scandium were

discov-ered when their minerals were analyzed by spectroscopic

methods The element helium was discovered in the Sun by

spectroscopic method

1920 he was named as Director of the Institute of Theoretical Physics.

Bohr’s theory of atomic structure, which was presented in 1913, laid a broad foundation for the great atomic progress of recent years

Second only in importance to his celebrated theory are the facts that it was in Bohr’s laboratory that the implications of nuclear fission were first predicted and that he obtained an understanding

of nuclear stability that contributed greatly to the spectacular development of atomic energy

After the First World War, Bohr worked for peaceful uses of atomic energy He received the first Atoms for Peace award in 1957 Bohr was awarded the Nobel Prize for Physics in 1922

“ The very word ‘experiment’ refers to a situation where we can tell others what we have done

and what we have learned” — Neils Bohr.

“Our experiments are questions that we put to

Nature.” — Neils Bohr.

In order to explain the line spectra of hydrogen and the overcome to objections leveled against Rutherford’s model

of the atom, Neils Bohr (1913) proposed his quantum mechanical structure of the atom He made use of quan-tum theory, according to which energy is lost or gained not gradually, but in bundles or quanta The concept of nucleus explained by Rutherford was retained in Bohr’s model

Through this model does not meet the modern quantum mechanics, it is still in use to rationalize many points in the atomic structure and spectra The important postulates of the Bohr Theory are:

1 Electrons move in certain fixed orbits associated with

a definite amount of energy The energy of an electron moving in an orbit remains constant as long as it stays

in the same orbit called stationary state or orbit.

where, RH is Rydberg constant

All the lines appearing in the hydrogen spectrum are

governed by the above equation For,

An electronic transition from M shell (m = 3) to K shell

(n = 1) takes place in a hydrogen atom Find the wave

number and the wavelength of radiation emitted (R =

v

−

Solved Problem 7

In hydrogen atom, an electron jumps from fourth orbit

to first orbit Find the wave number, wavelength and the

energy associated with the emitted radiation

Table 1.3

Series

Region of spectrum Value of n Values of m

Based on the above postulates, Bohr calculated the radii of the various orbits and the energies associated with the electrons present in those orbits The frequen-cies of the spectral lines determined experimentally by Lyman, Balmer and others are in excellent agreement with those calculated by Bohr’s theoretical equations.

1.7.1 bohr’s Theory of the hydrogen Atom

Bohr pictured the hydrogen atom as a system consisting

of a single electron with a charge designed as e, rotating

in a circular orbit of radius r about the nucleus of charge

r

ve_

Tangential velocity of the revolving electron

Calculation of velocity V: The angular momentum of

the electron is defined as the product of the velocity of the electron in its orbit, its mass, and the radius of its orbit The

product is symbolized as mvr According to Bohr’s lates, the angular momentum are whole multiples of h / 2p.

postu-Therefore, according to this quantum restriction, angular momentum may be restricted as

2

nh mvr=π

\ The velocity of the electron in an orbit,

2

nh v mr

=

Radius of an Orbit

According to Coulomb’s law, the electrostatic force of attraction

Fe between the charges may be evaluated mathematically as

2 2

e

Ze F r

By definition, the magnitude of the centrifugal force

Fc for an electron of mass m, with a velocity in its orbit of

V and with an orbit radius r is given as

2

c

mv F r

Hence, a certain fixed amount of energy is associated

with each electron in a particular orbit Thus, stationary

orbits are also known as energy levels or energy shells

Bohr gave number 1, 2, 3, 4 etc, (starting from the nucleus

to these energy levels) The various energy levels are

designated as K, L, M, N etc, and are termed as principal

quantum numbers.

2 Energy is emitted or absorbed only when an electron

jumps from one orbit to another, i.e., form one energy

level (or principal quantum number) to another

Since each level is associated with a definite amount

of energy, the farther the energy level from the nucleus, the

greater is the energy associated with it

3 Permissible orbits are those for which the angular

momentum is an integral multiple of (h/2 p).

The electrons move in such orbits without any loss of

energy Thus, angular momentum

where, n is an integer 1, 2, 3 me is mass of

elec-tron and r is the radius of orbit The n values corresponds

to the principal quantum number

4 When an electron gets sufficient energy from outside,

an electron from an inner orbit of lower energy

state E1 moves to an outer orbit of higher energy

state E2.

The excited state lasts for about 10−8 s The difference

of energy E2 −E1 is thus radiated out During the emission

or absorption of radiant energy to Planck’s, Einstein

equa-tion E = hv is obeyed Thus the frequency of emitted

where, En is the total energy of the electron in the orbit

designated by the quantum number n.

mZe

=π

Since, n = 1 and Z for hydrogen = 1

2

4

h r me

=π

2 27

2

nh V mr

=π

or KJ mol−1 or electron volt (eV)One erg/molecule = 1.44 × 1013 Kcal / mol

In order that the electron’s orbit may remain stable, it

is necessary to assume that the two forces, electrostatic and

centrifugal, are equal and opposed to each other From this

assumption may be deduced the following mathematical

relationship

mv r

Ze r

Equations (1.26) and (1.22) may be equated since they

are equal to each other

22

mZe

=

Consequently, the radius r0 of the smallest orbit (first

orbit) for the hydrogen atom is

2

h r me

=

energy of the electron

By definition the kinetic energy of body is equal to 1 2

2mv

The total energy E of the electron is the sum of its kinetic

and potential energies If the potential energy of an electron

is taken as zero when it is at an infinite distance from the

nucleus, the value at a distance r is given by −Ze2 / r This

value may be obtained by integrating Eq (1.24) between

the limits of r and infinity The negative sign indicates that

work must be performed on the electron to transfer it to

infinity Therefore, the total energy of the electron is

and upon combining Eqs (1.30) and (1.31) the

follow-ing expression may be derived

22

Ze E r

Now if we substitute the value r from Eq (1.28) into

(1.32) the total energy of the electron may be stated as

If the term wave number is used for frequency

v v c

All the terms in the fraction 2p2Z2 e4 m/h3c are

constant For hydrogen atom Z is 1 If we evaluate the

fraction from the constant terms, the following value is obtained:

2 2 4

1 3

equa-in Eq (1.15)

1.7.3 limitations of the bohr’s model

Bohr’s atomic model was a very good improvement over Rutherford’s nuclear model and enabled in calcu-lation of radii and energies of the permissible orbits in the hydrogen atom The calculated values were in good agreement with the experimental values Bohr’s theory could also explain the hydrogen spectra successfully and also the spectra of hydrogen like atoms (He+, Li2+,

Be3+, etc) The theory was, therefore, largely accepted and Bohr was awarded Nobel prize in recognition of

1.7.2 Origin of Spectral lines and the

hydrogen Spectrum

As described by Bohr, energy is radiated when an electron

moves from one orbit of a quantum number of n2 to an

orbit of quantum number n1, where n1 is the inner orbit

Energy is absorbed if the electron moves in opposite

direc-tion, i.e., from inner orbit to outer orbit The energy DE

which is emitted may be represented as a difference in

energy of the two electronic states and can be indicated by

Spectral lines are produced by the radiation of

pho-tons, and the position of the lines on the spectral scale is

determined by the frequency, or frequencies, of the

pho-tons emitted Transitions to the innermost level n1 from

orbits n2, n3, n4 etc, gives rise to the first, second, third etc,

lines of the Lyman series in the UV region Transitions

from the outer most energy levels gives rise to spectral

lines of higher frequencies Transition of electron to n2

level from outer level gives Balmer series of lines in the

visible region Similar transitions from higher orbits to

the third orbit (n = 3) produces Paschen series in the

infra-red region Other series of lines have been discoveinfra-red for

similar shifts in the far infrared region A sketch

indicat-ing the transitions which produces these spectral lines is

given in Fig 1.21

From the Eq (1.34) the difference between the

energies of an electron in the two orbits n2 and n1 may be

Pfundseries

Paschenseries

BrackettseriesBalmer

series

Lymanseries

Balmer seriesPaschen seriesBrackett series

Pfund series

Radiation

of longwavelengthRadiation

with intermediatewavelength

Radiation with shortwavelength

Fig 1.21 Origin of emission spectrum of hydrogen atom

louis de broglie 1892–1987

He was a French physicist Studied history but while working on radio communications during the First World War as an assignment, he developed interest

in science He received his Dr Sc from the sity of Paris in 1924 He was professor of theoretical physics in the University of Paris from 1932 to 1962

Univer-He was awarded Nobel Prize for physics in 1929

By making use of the Einstein’s (E = mc2) and Planck’s

quantum theory, (E = hv) de Broglie deduced a

fundamen-tal relation called the de Broglie equation:

h mv

This equation gives the relationship between the wavelength of the moving particle and its mass In the

Eq (1.39), l is the wavelength of the wave associated with

an electron of mass ‘m’ moving with velocity v.

Eq (1.39) can be written as

In the E.q (1.42) ‘p’ represents the momentum mv of

particle; and l corresponds to the wave character of matter

and p its particle character Thus, the momentum (p) of a moving particle is inversely proportional to the wavelength

of the waves, associated with it.

The revolutionary postulate of de Broglie received direct experimental verification in 1927 by Davisson and Germer, G.P Thomson and later by Stern They showed that heavier particles (H2, He etc.) showed diffraction pat-terns when reflected from the surface of crystals Particu-larly Davisson and Germer found that electron beam was diffracted when striken on a single crystal of nickel, which proves the wave like character of electrons

de broglie’s Relationship Concept and bohr’s Theory

Application of de Broglie’s relationship to a moving tron around a nucleus puts some restrictions on the size of the orbits It means that electron is not a mass particle mov-ing in a circular path but instead a standing wave train (non-energy, radiating motion) extending around the nucleus in the circular path as shown in Fig 1.22 (a) and 1.22 (b)

elec-this work However, Bohr’s model could not explain the

following points:

(i) Bohr’s model could not explain the spectra of atoms

containing more than one electron Also, it could

not explain hydrogen spectrum obtained using high

resolution spectroscopes Each spectral line, on high

resolution was found to consist of two closely spaced

lines

(ii) It was observed that in the presence of a magnetic

field each spectral line gets splitted up into closely

spaced lines This phenomenon, known as Zeeman

effect, could not be explained by the Bohr’s model

Similarly, the splitting of spectral line under the

effect of applied electric field, known as Stark effect,

could not be explained by the Bohr’s model

(iii) It could not explain the spectra of atom of elements

other than hydrogen

(iv) Bohr’s model could not explain the ability of atoms

to form molecules and the geometry and shapes of

molecule

1 8 WAveS AND PARTiCleS

Because of the limitations of Bohr’s atomic model

sev-eral scientists tried to develop a more subtle and

gen-eral model for atoms During that period two important

proposals were contributed significantly for the

mod-ern quantum mechanical model of the atom They are

(i) Dual nature, i.e., wave as well as particle nature of

matter

(ii) Heisenberg’s uncertainty principle

1.8.1 Dual Nature of matter

According to Maxwell’s concept, light, radiation

con-sists of waves while Planck’s quantum theory considers

photons as particles Thus, light, which consists of

elec-tromagnetic radiation, is both a wave and particle Based

on this analogy in 1924, the Frenchman, Prince Louis de

Broglie published an exceedingly complicated account

of the wave-particle duality de Broglie stated that any

form of matter such as electron, proton, atom or

mol-ecules, etc, has a dual character The waves predicted by

de Broglie are known as matter waves These waves are

quite different from electromagnetic waves in following

two respects:

(i) Matter waves cannot radiate through empty space

like the electromagnetic waves

(ii) Speed of matter waves is different from that of

electromagnetic waves

de Broglie’s equation is true for material particles of all sizes and dimensions However, in the case of small micro objects like electrons, the wave character is of significance only In the case of large macro-objects the wave charac-ter is negligible and cannot be measured properly Thus, de Broglie equation is more useful for small particles.

meaning of y and its Significance

y is the wave function or the amplitude of the wave The value of amplitude increases and reaches the maximum which is indicated by peak in the curve This is shown by the upward arrow in the figure The value of the amplitude decreases after reaching the maximum value This is shown

by the downward arrow

Above the X-axis the amplitude is shown as +ve, along the X-axis it is zero and below the X-axis it is nega-tive The intensity of light is proportional to the square of

amplitude (or A2) Therefore, A2 can be taken as a measure

of the intensity of light since light is considered to consist

of photons (corpuscular theory), the density of photons is

considered to be proportional to A2 Thus, as far as light

waves are concerned A2 indicates the density of photons in space or the intensity of light

This concept can be extended even to the y functions moving in the atom since electron moving with high speed

is associated with a wave characteristics Therefore, y2 in the case of electron wave denotes the probability of finding

an electron in the space around the nucleus or the electron density around the nucleus If y2 is maximum, the prob-ability of finding an electron is also maximum

h mv

For the wave to remain continually in phase, the

cir-cumference of the orbit should be an integral multiple of

which is the same as Bohr’s postulate for angular moment

of electron From the E.q (1.45) it can be known that

elec-trons can move only is such orbits for which the angular

momentum must be an integral multiple of h/2p If the

circumference is bigger or smaller than the value as given

by in Fig 1.22 b, the electron wave will be out of phase

Thus, de Broglie relation provides a theoretical basis for

the Bohr’s postulate for angular momentum In Fig 1.22 (a)

the wave is in phase continually

In the summer of 1927 physicists from all over the

world arrived in Brussels at the Solvay Congress At

this congress de Broglie’s concept on the relationship

between waves and particles was totally rejected For

many years to come, a complete different

representa-tion of this relarepresenta-tionship led the way It was

Heisen-berg and Schrodinger who supported and strongly

represented the concept of de Broglie at another

congress and got it accepted

Fig 1.22 Diagrammatic representation of electron orbits

one of which is (a) in phase; and the other (b) out of phase

X

+

–

Fig 1.23

“If an electron is exhibiting the dual nature, i.e., wave and particle, is it possible to know the exact location of the electron in space at some given instant?”

The answer to the above question was given by

Heisenberg in 1927 who stated that

“For a subatomic object like electron, it is impossible

to simultaneously determine its position and velocity at any given instant to an arbitary degree of precision.”

Heisenberg gave mathematical relationship for the uncertainty principle by relating the uncertainty in position (Dx) with uncertainty in momentum (Dp) as

The ≥ in the Eq (1.46) means that the product of Dx and

Dp can be either greater than or equal to but would be never smaller than h / 4p But h / 4p is constant Therefore, it follows

from Eq (1.46) that smaller the uncertainty in locating the exact position (Dx), greater will be the uncertainty in locating the exact momentum (Dp) of the particle and vice versa

As Dp is equal to mDv the Eq (1.46) is equivalent to saying that position and velocity cannot be simultaneously determined to an arbitrary precision

However, in our daily life, these principles have no significance This is because we come across only large objects The position and velocity of these objects can be determined accurately because in these cases the changes that occur due to the impact of light are negligible The microscopic objects suffer a change in position or velocity

as a result of the impact of light For example, to observe

an electron, we have to illuminate it with light or tromagnetic radiation The light must have a wavelength smaller than the wavelength of electron When the photon

elec-of such light strikes the electron, the energy elec-of the electron changes In this process, no doubt, we shall be able to cal-culate the position of the electron, but we would know very little about the velocity of the electron after the collision

1.8.3 Significance of Uncertainty Principle

The most important consequence of the Heisenberg

uncertainty principle is that it rules out existence of nite paths or trajectories of electrons and other similar particles The trajectory of an object is determined by

defi-its location and velocity at various moments At any ticular instant if we know the position of a body and also its velocity and the forces acting on it at that instant we can predict at what position it will present at a particular time This indicates that the position of an object, and its velocity completely determine its trajectory Because, it

par-is not possible simultaneously to determine the position and velocity of an electron at any given instant precisely

it is not possible to talk of the trajectory of an electron

Solved Problem 10

A moving electron has 2.8 × 10−25 J of kinetic energy

Calculate its wavelength (mass of electron = 9.1 × 10−31 kg)

h mv

He received his Ph.D in physics from the

Univer-sity of Munich in 1923 Later he worked with Max

Born at Gottingen and with Niels Bohr at

Copen-hagen He spent about 14 years (1927–1941) as

professor of physics at the University of Leipzig

He led the German team working for atomic bomb

during the Second World War Once the war ended

he became the director of Max Planck Institute for

physics in Gottingen He was awarded Nobel Prize

for physics 1932

The exact sciences also start from the

assump-tion that in the end, it will always be possible to

understand nature, even in every new field of

experi-ence but we make no priori assumptions about the

meaning of the word understand.

W Heisenberg

According to classical mechanics, a moving electron

is considered to be a particle Therefore, its position and

momentum could be determined with a desired accuracy

On the other hand de Broglie also considered a moving

electron to be wave-like Therefore, it becomes impossible

to locate the exact position of the electron on the wave

because it is extending throughout a region of space So,

the following fundamental question arises:

Bohr model In calculating electron orbits precisely, Bohr was violating this fundamental requirement and hence his theory was only partially successful So, a model which can account the wave-particle duality of matter and be consistent with Heisenberg uncertainty principle was in quest This came with the advent of quantum machanics.

Concept of Probability

The consequences of the uncertainty principle are far reaching and probability takes the place of exactness in velocity (which

is related to kinetic energy) of an electron The Bohr concept

of the atom, which regards the electrons as rotating in nite orbits around the nucleus, must be abandoned and should

defi-be replaced by a theory which considers probability of ing the electrons in a particular region of space This means that, it is possible to state the probabilities of the electron to

find-be various distances with respect to the nucleus In the same way, probable values of velocity can also be given It should

be clearly understood, however, that a knowledge of ity for an electron ‘moves’ from one location to another

probabil-1.8.4 Quantum mechanical model of Atom

Classical mechanics based on Newton’s laws of motion cessfully describes the motion of all macroscopic objects since the uncertainties in position and velocity are small enough

suc-to be neglected However, it fails in the case of microscopic objects like electrons, atoms, molecules etc So, while consid-ering the motion of microscopic objects, the concept of dual behaviour of matter and the uncertainty principle are taken into account The branch of science that takes into account

this dual behaviour of matter is called quantum mechanics.

As far as the other dynamical variables of an electron are concerned, we can show that the uncertainty principle leads to the following result.

The total energy of an electron in an atom or ecule has a well-defined (sharp) value The probability distribution as well as the sharp values can be calculated from a function designated as y (x) and called the wave

mol-function or the psi mol-function y (x) is obtained by

solv-ing the Schrodsolv-inger equation which is the fundamental

equation in quantum mechanics in the same manner that Newton’s equation is fundamental in classical mechanics

important Features of the Quantum mechanical model of Atom

Quantum mechanical model of the atom is the outcome

of the application of Schrodinger wave equation to atoms

Its important features are

• Quantization of the energy of electron in atoms, i.e.,

it can have certain discrete values

The effect of Heisenberg’s uncertainty principle is

significant only for motion of microscopic objects and is

negligible for that of macroscopic objects This can be seen

from these illustrated examples

Solved Problem 11

Calculate the uncertainty (Dv) in the velocity of a cricket

ball of mass 1 kg, if certainty (Dx) in its position is of the

A microscope using suitable photons is employed to locate

an electron in an atom within a distance of 0.1 Å What is

the uncertainty involved in the measurement of its velocity?

Solution:

4

h v

Why bohr’s model was a Failure?

Bohr considered the electron as a charged particle

mov-ing in well defined circular orbits around the nucleus So,

the position and the velocity of the electron can be known

exactly at the same time Uncertainty principle shows that

it is impossible to measure these variables simultaneously

because the wave character of electron was not

consid-ered in the Bohr model Calculation of the trajectory of

an electron in an atom or molecule is, therefore, a futile

exercise We can now appreciate the major fault of the

Austrian physicist Erwin Schrödinger who invented a method of showing how the properties of waves could be used to explain the behaviour of electrons in atoms Schro-dinger published his ideas in January 1926 This date repre-sents one of the milestones in the history of Chemistry His work formed the basis of all our present ideas on how atoms bond together The heart of his method was his prediction

of the equation that governed the behaviour of electrons, and the method of solving it Schrodinger’s equation is

where, m = Mass of electron

E = Total energy of the electron (kinetic energy + potential energy)

U = Potential energy

h = Planck’s constant

y = Wave functionThis equation applies to stationary waves as it does not have the time dependence part of the wave function

Schrödinger won the Nobel prize for physics in

1933 The solution of this equation are very complex and you will learn them for different systems in higher classes

For a system (such as an atomic or a molecule whose energy does not change with time), the Schrödinger equa-tion is written as Hˆ y = Ey, where Hˆ is a mathematical

operator called Hamiltonian Schrödinger gave a recipe

of constructing this operator from the expression for the total energy of the system The total energy of the system takes into account the kinetic energies of all the sub-atomic particles (electrons, nuclei) attractive potential between the electrons and nuclei and repulsive potential among the electrons and nuclei individually Solutions of this equation

gives E and y.

1.8.6 The meaning of Wave Function

The wave function could only be used to provide

informa-tion about the probability of finding the electron in a given region of space around the nucleus Max Born, a German

physicist proposed that we must give up ideas of the tron orbiting the nucleus at a precise distance In this respect Bohr was wrong in thinking that the electron in the ground state of the hydrogen atom was always to be found at a dis-tance a0 form the nucleus Rather, it was only most probably

elec-to be found at this distance The electron had a smaller ability of being found at a variety of other distances as well

prob-It is important to realise that we should not try to talk about finding the electron at a given point The reason for this is that there is an infinite number of points around the nucleus So, the probability of finding the electron at any

• The allowed solutions of Schrödinger wave equation

tell about the existence of quantized electronic

energy levels for electrons in atoms having wave like

properties

• As per Heisenberg’s uncertainty principle, both the

position and velocity of an electron cannot be

deter-mined simultaneously and exactly So, the path of the

electron can never be determined accurately Because

of this reason the concept of probability was introduced

for finding the electron at different points in an atom

• The wave function for an electron in an atom is the

atomic orbital Whenever we say about electron by

a wave function it occupies that orbital Because

several wave functions are possible for an electron,

there also as many atomic orbitals These one

elec-tron orbital wave functions or orbital form the basis

for electronic structure of atoms Every orbital

pos-sess certain energy and can accommodate only two

electrons In atoms having several electrons, the

elec-trons are filled in the order of increasing energy In

the multielectron atoms each electron has an orbital

wave function characteristic of the orbital it occupies

All the information about the electron in an atom is

stored in its orbital wave function y and quantum

mechanics makes it possible to extract this

informa-tion out of y

The probability of finding the electron at a point

within an atom is proportional to y2 at that point

Though y is sometimes negative, y2 is always

posi-tive and is known as the probability density The

values of y2 predicts the different points in a region

within an atom at which the electron will be most

probably found

1.8.5 Schrödinger Wave equation

erwin Schrödinger 1887–1966

Erwin Schrödinger was born in Austria He received

his Ph.D in theoretical physics from the University

of Vienna in 1910 At the request of Max Planck,

Schrödinger became his successor at the University

of Berlin in 1927 Because of his opposition to Hitler

and Nazi policies, he left Berlin and returned to

Austria and in 1936, when Austria was occupied by

Germany, he was forcibly removed from his

profes-sorship Then he moved to Dublin, Ireland He shared

the 1933 Nobel Prize for physics with P.A.M Dirac

During 1920 there was a great deal of interest in

wave-particle duality and de Broglie’s matter waves It was the

1.9 QUANTUm NUmbeRS

The mathematical solution of three dimensional Schrödinger

wave equation gives three values of E for acceptable values

of y These values are related to one another through whole

numbers These values are termed as Quantum numbers

and represented with n, l, m — called principal, azimuthal

and magnetic quantum numbers, respectively These

three quantum numbers together with the fourth called spin quantum number describe fully the location and energy of

an electron Thus, quantum numbers are the numbers which determine the energy of electron, the angular momentum, shape of the electron orbital, the orientation of the orbital and spin of the electron Thus, each quantum number is

associated with a particular characteristic of the electron

These quantum numbers are discussed below briefly

1 Principal Quantum Number (n): The number

allot-ted to Bohr’s original stationary states, visualized as circular orbits is called the principal quantum number

The innermost orbit, i.e., that nearest to the nucleus has a principal quantum number 1, the second orbit has

a quantum number of 2, and so on So, the principal

quantum number denoted by n have value 1, 2, 3, 4,

Alternatively, letters are used to characterize the orbits, K, L, M, N, for 1, 2, 3, 4, The choice of letters originates from Mosely’s work on the X-ray spectra of the elements He called group of lines in the spectra the K, L, M, N, groups

The number of electrons in an atom which can have the

same principal quantum number is limited and is given by 2n2

where n is the principal quantum number concerned Thus,

Principal quantum number (n) 1 2 3 4

Maximum number of electrons 2 8 18 32This is the most important quantum number as it determines to a large extent the energy of an electron It also determines the average distance of an electron from

the nucleus As the value of n increases, the electron gets

farther away from the nucleus and its energy increases The

higher the value of n, the higher is the electronic energy

For hydrogen and hydrogen like species, the energy and size of the orbital are determined by the principal quantum number alone

The principle quantum number also identifies the number of allowed orbitals within a shell With the increase

in the value of ‘n’ the number of allowed orbitals increases and are given by n2 All the orbitals of a given value of ‘n’

constitute a single shell of atom

2 Azimuthal Quantum Number (l): It is also known

as orbital angular momentum or subsidiary quantum number It defines the three dimensional shape of the

one of these points is infinitely small, i.e., zero Then to

predict the position of electron, we can imagine taking out

a series of photographs of an atom to give us an

instan-taneous picture of the whereabouts of the electron If we

combine all pictures we would end up with a picture as

shown in Fig 1.24 The separate dots have overlapped to

give regions in which the density of dots is very high and

regions where the density of dots is much lower In the high

density regions, we say that there is a high probability

den-sity The maximum in the density comes at exactly the same

distance a0, as Bohr predicted in his work The circulated

symmetry of the probability density is clear However, we

should remember that atoms exist in three dimensions, so

really the diagram should be shown as a sphere

It is easier to draw circles rather than spheres, so usually

we draw the density diagram as a circle Also, it is common

practice not to include the shading and to agree that when a

circle is drawn, it provides a boundary surface within which,

say, there is a 95 per cent probability of finding the electron

When Schrödinger equation is solved for hydrogen

atom, the solution gives the possible energy levels the

elec-tron can occupy and the corresponding wave function(s) y

of the electron associated with each energy level

Accepted solutions to the wave functions are called

Eigen wave functions The probability of finding an

elec-tron at a point in space whose coordinates are x, y and

z is given by | y (x, y, z)|2dv This three dimensional region

obtained where the probability to find the electron is about

95 per cent, is called an orbital

Because atomic behavor is so unlike ordinary

experi-ence, it is very difficult to get used to and it appears

peculiar and mysterious to everyone, both to the

nov-ice and to the experienced scientist Even the experts

do not understand it the way they would like to, and it

is perfectly reasonable that they should not, because

all of direct human experience and of human intution

applies to large objects We know how large objects

will act, but things on a small scale just do not act

that way So, we have to learn about them in a sort of

abstract or imaginative fashion and not by

connec-tion with our direct experience Feynman Lectures on

Physics Vol l Chapter 37.

a0

Fig 1.24 Electron density

These different orientations are called orbitals The

number of orbitals in a particular sub-shell within a principal energy level is given by the number of values

allowed to m l which in turn depends upon on the value

of l The possible values of m l range from + l through 0

to − l, thus making a total of (2l + 1) values Thus, in a

subshell, the number of orbitals is equal to (2l + 1).

For l = 0 (i.e., s-subshell) m l can have only one value

m l = 0 It means that s-subshell has only one orbital

For l = 1 (i.e., p-subshell) m l can have three values +1,

0 and −1 This implies that p-subshell has three

orbitals.

For l = 2 (i.e., d-subshell) m l can have five values +2,

+1, 0, and −2 It means that d-subshell has five

orbitals.

For l = 3 (i.e., f-subshell) m l can have seven values +3, +2, +1, 0, −1, −2 and −3 It means that

f-subshell has seven orbitals.

The number of orbitals in various types of subshells are as given below

No of orbitals (2l + 1) 1 3 5 7 9The relationship between the principal quantum

number (n), angular momentum quantum number (l ) and magnetic quantum number (m l) is summed up in Table 1.4

4 Spin Quantum Number (ms): This quantum number

which is denoted by ms does not follow from the wave mechanical treatment Electron spin was first postu-lated in 1925 by Uhlenbeck and Goudsmit to account for the splitting of many single spectral lines into double lines when examined under a spectroscope of

sub-shell For each value of the principal quantum

number there are several closely associated orbitals,

so that the principal quantum number represents a

group or shell of orbits In any one shell, having the

same principal quantum number the various

subsidi-ary orbits are denoted as s, p, d, f, sub-shells The

letters originates from the sharp, principal, diffuse and

fundamental series of the lines in spectra.

The number of sub-shells or sub-levels in a principal

shell is equal to the value of n The values of subshells

is represented with l l can have values ranging from

0 to n − 1, i.e., for a given value of n the possible value of

l can be 0, 1, 2 (n−l) For example when n = 1 value of

l is only 0 For n = 2 the possible value of l can be 0 and

1 For n = 3 the possible values are 0, 1 and 2 Sub-shells

corresponding to different values are as follows:

Notation for

subshell

The permissible values for ‘l’ for a given principal

quan-tum number and the corresponding sub-shell notation are

3 Magnetic Quantum Number (m l): This quantum

number which is denoted by m l refers to the different

ori-entations of the electron cloud in a particular subshell

Table 1.4 Relationship amongst the value of n, l and ml

Energy

level

Principal quantum number (n)

In a given energy level

4s 4p 4d 4f

0 +1, 0, –1 +2, +1, 0, –1, –2 +3, +2, +1, 0, –1, –2, –3

1 3 5 7

16

which expresses the amplitude of vibration of a plucked string.

As it is the square of amplitude of the vibrating string which measures the intensity of the wave involved, so |y|2

measures the probability of an electron existing at a point, meaning therefore, that the chance of finding an electron at that point is zero A high value of |y|2 at a point means that there is a high chance of finding an electron at the point

The value of the wave function y, for an tron in an atom is dependent, in general, both on the

elec-radial distance, r, of the electron from the nucleus of

the atom and on its angular direction away from the nucleus

The nucleus, itself, however, cannot provide any sense

of direction until a set of arbitrary chosen axes are imposed If cartesian axes are chosen as in Fig 1.25 (a) then

super-a point in spsuper-ace, P, super-around the nucleus, N, csuper-an be defined

in terms of x−, y− and z-coordinates Such axes do not, however, have any absolute directional significance until

an external magnetic field is applied The axes then have

a definite direction in relation to the direction of the netic field Alternatively, polar coordinates in Fig 1.25 (b) can be used

mag-The mathematical relationship between y and r and the angular direction can be established accurately for a single electron in a hydrogen like atom For more complicated atoms containing more than one electron, the correspond-ing relationships can only be established approximately because of the difficulty involved in solving the mathemat-ical equations

The differences between s−, p− and d-orbitals depend

on the ways in which y and / or y2 vary with r and with the

angular direction, as explained in the following sections

The probability of finding an electron in a given volume of space is represented by radial probabil-ity distribution curves These curves indicate how the probability of finding an electron varies with the radial distance from the nucleus without any reference to its

high resolving power The electron in its motion about

the nucleus also rotates or spins about its own axis In

other words, an electron has, besides charge and mass,

an intrinsic spin angular quantum number

Spin angular momentum of the electron: A vector

quantity, can have two orientations relative to a chosen

axis The spin quantum number can have only two values

i.e., ↑ and the other indicates anti-clockwise spin, generally

represented by an arrow downwards, i.e., ↓ The electrons

that have different ms values (one 1

2

2

− ) are said to have opposite spins An orbital cannot hold

more than two electrons and these two electrons should

have opposite spins

Orbit and Orbital

Orbit and orbitals are different terms Bohr proposed

an orbit as a circular path around the nucleus in

which the electrons moves As per Heisenberg’s

uncertainty principle the precise description is not

possible So, its physical existence cannot be

demon-strated experimentally

An orbital is a three-dimensional region

calcu-lated from the allowed solutions of the Schrödinger’s

equation It is quantum mechanical concept and

refers to one electron wave function y in an atom It

is characterised by three quantum numbers (n, l and

ml) and its value depends upon the coordinates of the

electron y has no physical meaning but its square

|y|2 at any point in an atom gives the value of

prob-ability density at that point Probprob-ability density (|y|2)

is the probability per unit volume and the electron in

that volume |y|2 varies from one region to another

region in the space but its value can be assumed to

be constant within a small volume The total

prob-ability of finding the electron in a given volume can

be calculated by the sum of all products of |y|2 and

the corresponding volume elements It is thus

pos-sible to get the probable distribution of an electron

in an orbital

1.10 ShAPeS OF ORbiTAlS

The nature of the Schrödinger equation is such that the wave

function y may be regarded as an amplitude function This

corresponds, on a three-dimensional scale, to the function

Fig 1.25 The position of a point P relative to point

N can be expressed in terms of (a) Cartesian coordinates

x, y, z or (b) Polar coordinates r, θ and f.

The volume of such a shell viz, dV, will be given by

The probability of finding electron within the small

radial shell of thickness dr around the nucleus, called

radial probability will therefore be given by

= 4pr2 dr |y2n, l | (1.48)

The radial probability distribution between r = 0 to

r = r will be given by the summation of all the probability distributions for concentric radial shells from r = 0 to

In other words, the radial probability

distribu-tion of electron may be obtained by plotting the funcdistribu-tion

4pr2 |y2

n, l | against r, its distance from the nucleus Such

graphs are radial probability distribution curves

The radial probability distribution curves for 2s, 3s, 3p and 3d electrons are shown in Fig 1.29.

direction from the nucleus The radial wave function is

generally written as yn, l Let us now plot the function yn, l

against r for electrons belonging to different orbitals and

try to correlate them with the probability density around

a point at a distance r from the nucleus The plots are as

shown in Fig 1.26

From these plots, it is clear that y n, l cannot be related

with probability density around any point at a distance r

from the nucleus because

(i) y n, l is maximum at r = 0 If y n, l represents

ity density the electron will have maximum

probabil-ity of occurring at the nucleus This cannot be true

as the actual probability of finding an electron at the

nucleus is zero

(ii) y n, l has both positive as well as negative values (if

probability curves for 2s and 3s electrons)

How-ever, probability density cannot have a negative

value.

Objection No (ii) can be removed by considering |yn, l|2

to be related to probability density instead of y n, l Now

even if y n, l is negative at some space its square has to be

positive The plots of y n, l versus r for electrons belonging

to different orbitals are as shown in Fig 1.28

The curves do not exhibit negative value for the

radial wave functions at any distance But even |y n, l|2

can-not be related with probability density because |yn, l|2 is

maximum at r = 0 This means that the probability of

finding electron is maximum on the nucleus which again

cannot be true (as already explained) Thus, neither y

n, l nor y2n, l can be directly related with probability of

finding electron at a point which is at a distance r from

the nucleus

Let us consider the space around the nucleus (taken at

the centre) to be divided into a large number of thin

con-centric spherical shells of thickness dr Consider one of

these shells with the inner radius r (Fig 1.27).

Fig 1.27 Division of space around the nucleus into small

spherical shells of very small thickness

It may be noted that for 1s orbital, the probability

den-sity is maximum at the nucleus and it decreases sharply

as we move away from it On the other hand, for 2s orbital

the probability density first decreases sharply to zero and

again starts increasing After reaching a small maxima

it decreases again and approaches zero as the value of

r increases further The region where this probability

den-sity function reduces to zero is called nodal surfaces or

nodes In general it has been found that ns-orbital has (n−1)

nodes, that is, number of nodes increases with increase of

principal quantum number n In other words, number of

nodes for 2s orbital is one, two for 3s and so on

These probability density variations can be visualized

in terms of charge cloud diagrams (Fig 1.30) In these diagrams, the density of the dots in the region represents electron probability density in that region

1.10.1 boundary Surface Diagrams s-orbitals: The shapes of the orbitals in which constant

probability density for different orbitals can be

represent-ed with boundary surface diagram The value of

prob-ability density |y|2 is constant in this boundary surface drawn in space In principle, many such boundary surfaces

The radial probability function 4pr2|ψn l,|2

written for the sake of simplicity as 4pr2 |yn, l|2 is evidently the product

of two factors While the probability factor y2 decreases

as r increases This gives rise to curves of the type shown

in Fig 1.31 At r = 0, though the factor |y|2 is maximum,

may be possible However, for a given orbital, only that

boundary surface diagram of constant probability density

is taken to be good representation of the shape of the

or-bital which encloses a region or volume in which the

prob-ability of finding the electron is very high, say 90 per cent,

but cannot be 100 per cent because always there will be

some value, however small it may be, at any finite distance

from the nucleus Boundary surface diagram for an ‘s’

or-bital is actually a sphere centred on the nucleus In two

dimensions, this sphere looks like a circle The probability

of finding the electron at any given distance is equal in all

directions It is also observed that the size of the s-orbital

increases in n, that is, 4s > 3s > 2s > 1s and the electron

is located further away from the nucleus as the principal

quantum number increases

p-orbitals: The probability density diagrams for p

orbit-als are very different to those of s-orbitorbit-als If you look

back at quantum numbers you will find that we said that

the magnetic quantum number can tell us the number of

orbitals of a given type The result is that for s-orbitals the

magnetic quantum number, m l, has only one value This

means that there is only one variety of s-orbitals that is

the variety we have already met; the spherically

symmet-ric ones For p-orbitals there are three possible values of m

(+1, 0, −1) — the three types of p-orbitals are called px, py

and pz The boundary surface diagrams are known as

ra-dial probability distribution curves or rara-dial charge

den-sity curves or simply as radial distribution curves Such

curves truly depict the variation of probability density

of electronic charge with respect to r (distance of charge

from the nucleus.)

Radial probability at a distance r is the probability of

finding electron at all points in space which are at a distance

r from the nucleus and the radial probability distribution is the

graph of these probabilities as a function of r.

The radial probability distribution curves for 1s and 2p

electrons are as shown in Fig 1.31

Z

–Z

YX–X

3s orbital

Z

–Z

YX–X

2s orbital

Z

–Z

YX–X

symmetric structure about the z-axis having a ring-like

collar in XY plane The dumbell shaped part of the curve

has a positive geometric sign (because whatever be the sign of z positive or negative, its square is always posi-

tive), ring the collar in the XY plane has a negative

geo-metric sign

The orbital d has a double dumbell shape The quan- xy tity XY will be positive when both x and y are positive or when both are negative However, XY will be negative when either of x or y is negative Thus, the sign of the curve

is positive in first and third quadrants while it is negative in second and fourth quadrants The shape of d d orbirals xz, yzcan be explained in a similar manner

The orbital d x2−y2is also double dumbell shaped but its

lobes lie on X and Y axes The signs of the lobes on X-axis

will always be positive (∵whatever be the sign of X, X 2 will

always be positive) whereas the sign of the lobes on Y axis

will always be negative (∵ whatever be the sign of Y, − Y 2 will

always be negative) The exact shapes of d orbitals are

obtained by taking into consideration the total wave tion Accordingly 3d x2−y2 orbital would be similar in shape to

4d x−y orbital or 5d x2−y2 orbital except for the fact that

1 A 5d orbital would have two, a 4d orbital have one and

a 3d orbital have no radial node.

2 A 5d orbital would occupy more space than a 4d orbital

and a 4d orbital would occupy more space than a 3d

orbital as shown in Fig 1.35

Every d-orbital also has two nodal planes passing through the origin and bisecting the XY plane containing

Z axis These are called angular nodes and are given by l.

For any orbital (i) The number of angular nodes (nodal planes)

is equal to l value.

(ii) The number of radial nodes (nodal surfaces or

nodes) is equal to (n−l−1).

(iii) The total number of radial nodes and angular nodes

for any orbital is equal to (n − l − 1) + l = n − 1.

(Fig 1.28) the factor 4pr2 is zero Similarly, when r is very

large, the factor 4pr2 is no doubt very large but the

prob-ability factor |y|2 is negligible so that the radial probability

is exceedingly small as shown in Fig 1.32

Here, unlike s-orbitals, the boundary surface diagrams

are not spherical Instead each p-orbital consists of two

sec-tions called lobes that are one either side of the plane that

passes through the nucleus The probability density

func-tion is zero on the plane where the two lobes touch each

other This plane is called nodal plane The nodal plane for

px orbital is YZ-plane, for py orbital, the nodal plane is XZ

plane, and for pz orbital nodal plane is XY plane It should

be understood, however, that there is no simple relation

between the values of ml (−1, 0 and +1) and the x, y and z

directions The size, shape and energy of the three orbitals

are identical

Like s-orbitals, p-orbitals increase in size and energy

with increase in the principal quantum number and hence

the order of the energy and size of various p-orbitals is

4p > 3p > 2p Further, like s-orbitals, the probability density

functions for p-orbital also pass through value zero, besides

at zero and infinite distance, as the distance increases The

number of nodes are given by n − 2, that is, number of

radial nodes is 1 for 3p-orbital, two for 4p-orbital three and

so on The shapes of 2px, 3px and 4px orbits with radial

nodes are shown in Fig 1.33

orbitals: The orbitals having l value as 2 are known as

d-orbitals The minimum value of principal quantum number

must be 3 to have l value 2 as the value of l cannot be

greater than n − 1 There are five d-orbitals corresponding

to ml values (−2, −1, 0, +1 and +2) for l = 2 Their boundary

surface diagrams are shown in Fig 1.34

The five d-orbitals are designed as d d d d xy, yz, xz, x2−y2

and d The orbital z2 d has a dumbell shaped curve z2

Nodal plane

+

+ -

+

-

Fig 1.32 The shapes and boundary surface diagrams for

+ X

Z

-Z -Y

-Y -Y

degenerate orbitals In hydrogen atom when electron is

present in 1s orbital, the hydrogen atom is considered in

most stable condition and it is called the ground state

This is because the electron is nearer to the nucleus But

when electron is present in 2s, 2p or higher orbitals in the

hydrogen atom, it is said to be in the excited state

In multielectron atoms, unlike the hydrogen atom, the energy of an electron depends both on its principal quantum number (shell), and the azimuthal quantum number (sub shell) This means that different sub shells

viz., s, p, d, f, in a principal quantum number will

have different energies, because of the mutual repulsion between the electrons In hydrogen atom there is only attractive force between nucleus and electron but no repulsive forces The stability of an electron in a multi-electron atom is because the total attractive interactions are more than the repulsive interactions experienced by every electron with other electrons The attractive force

f-orbitals: The orbitals having l value as 3 are known

as f-orbitals The minimum value of principal quantum

number must be 4 to have l value 3 as the value of l

can-not be greater than n − 1 There are seven f-orbitals

corre-sponding to m l values (−3, −2, −1, 0, +1, +2, +3) for l = 3

Their boundary surface diagrams are shown in Fig 1.36

Fig 1.35 The 3dx -y 2 2, 4dx 2−y 2 and 5dx 2−y 2 orbitals in three

dimensional space showing radial nodes

xy

++

+

++

++

–+

Fig 1.34 Shapes and boundary surface diagrams for 3d ,3d ,3d , dxy yz xz 3 x – y2 2 and 3d orbitals The nodal planes for z 2 dx – y 2 2

are shown Similarly there will be two nodal planes for each d ,d and dxy yz xz orbitals

sub-shells belonging to same principal quantum number have different energies However, in hydrogen atom, these have the same energy In multielectron atoms, the dependence of

energies of the sub shells on ‘n’ and ‘l’ are quite complicated but in simple way is that of combined value of n and l Lower the value n + l for a sub shell, lower is its energy If the two sub-shells have the same n + l value, the orbital having lower

n value have the lower energy The energies of sub-shells in

the same shell decrease with increase in the atomic number (Z*) For example, the energy of 2s orbital of hydrogen atom

is greater than that of 2s orbital of lithium and that of lithium

is greater than that of sodium and so on, i.e., E2s (H) > E2s (Li)

> E2s (Na) > E2s (K)

1.11 FilliNg OF ORbiTAlS

The distribution of electrons in various orbitals is known as

electronic configuration Having derived the energy level

sequence, it is now a simple matter to write the electronic configurations of atoms by making use of Aufbau principle, Pauli’s exclusion principle and the Hund’s rule of maximum multiplicity

Aufbau Principle

Aufbau is a German term meaning “building up” This principle is utilized to deduce the electronic structure of poly-electron atoms by building them up, by filling up of orbitals with electrons The Aufbau principle states that

“in the ground state of the atoms, the orbitals are filled in order of their increasing energies.” In other words, in the

ground state of atom, the orbital with a lower energy is filled

up first before the filling of the orbital with a higher energy commences

The increasing order of energy of various orbitals is

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s,

The above sequence of energy levels can be easily remembered with the help of the graphical representation shown in Fig 1.38

Pauli’s exclusion Principle

The four quantum numbers define completely the position

of electron in an atom It is thus possible to identify an electron in an atom completely by stating the values of

its four quantum numbers Wolfgang Pauli, an Austrian

scientist, put forward an ingenious principle which controls the number of electrons to be filled in various orbitals, and

hence, it is named as the exclusion principle It states that

no two electrons in an atom can have the same set of four quantum numbers.

of nucleus on the electrons increases with increase in

the positive charge (Ze) of nucleus The attractive power

of nucleus on the electrons will be less due to screening

effect of inner electrons The net positive charge

expe-rienced by the electron from the nucleus is known as

effective nuclear charge (Z*)

The attractive and repulsive interactions depend upon

the shell and shape of the orbital in which the electron is

present The spherical s orbital can shield the electrons from

the nuclear attraction more effectively than the p-orbital with

dumbell shape This is because of the spherical nature of the

s-orbital can shield nucleus from all directions equally but the

electron in a p-orbital say px can shield the nucleus in only one

direction, i.e., x-direction Similarly, because of difference in

their shape and more diffused character the d-orbitals have

less shielding power than p-orbitals Further because of their

different shapes the electron in spherical orbital spends more

time close to the nucleus when compared to p-orbital and the

p-electron spends more time in the vicinity of nucleus when

compared to d-electron For this reason the effective nuclear

charge experienced by different sub shell decreases with

increase in the azimuthal quantum number (l) From this we

can easily understand that s-electron is strongly attracted by

the nucleus than the p-electron which in turn will be strongly

attracted than the d-electron and so on Thus, the different

4p 4s 4d

3p 3s 2p 2s 1s 3d

5025

Fig 1.37 The relative energies of the atomic orbitals as a

function of atomic number

hund’s Rule of maximum multiplicity

The orbitals belonging to same sub-shell have same energy

and are called degenerate orbitals Hund’s rule states that the electron pairing in degenerate orbitals of a given sub shell will not take place unless all the available orbitals of

a given shell contains one electron each.

Since there are three p, five d and seven f orbitals, ing of electrons will start in p, d and f orbitals with the entry

pair-Thus, in the same atom any two electrons may have

three quantum numbers identical but not the fourth which

must be different This means that the two electrons can

have the same value of three quantum numbers, n, l and

ml but must have the opposite spin quantum numbers

This restricts that only two electrons may exist in the same

orbital and these electrons must have opposite spins

Pau-li’s exclusion principle helps in calculating the number of

electrons to be present in any sub shell For example, the

1s sub-shell comprises one orbital and thus the maximum

number of electrons present in 1s sub-shell can be two

because of the following two possibilities

n = 1, l = 0, m = 0, s = 1

2+

n = 1, l = 0, m = 0, s = 1

2

−

The p-sub-shell which is having 3 orbitals (px, py and pz)

can accommodate 6 electrons, the d-sub shell which having

5 orbitals can accommodate 10 electrons and the f-sub-shell

having 7 orbitals can accommodate 14 electrons This can

be summed up as the maximum number of electrons in the

shell with principal quantum number n is equal to 2n2

Fig 1.38 The order in which the atomic orbitals are used in building up the electron configuration of many electron

atoms The orbitals are used in sequence from the bottom in accordance with the Aufbau principle, Hund’s rule and Pauli’s

exclusion principle

5s 4s 3s 2s 1s

3p 2p

4p

Here a question arises? Why only two electrons

can present in one orbital? It can be answered

as follows We may recall from our knowledge of

elementary physics that the motion of an electric

charge creates a magnetic field In an orbital the

electron spin may be clockwise or anti-clockwise as shown in Fig 1.39 This results in the formation of two magnets Placing of two electrons together in the same orbital results in considerable repulsion due to same negative charge By overcoming the repulsive forces and to keep the two electrons together in the

same orbital some energy is required, called pairing

energy If the two electrons also have the same

spin there will be magnetic repulsion also, ing the requirement of more pairing energy But, if the two electrons are spinning in opposite directions and forming a pair in an orbital, mutually cancel their magnetic moments Now, we can easily understand that for an electron if pairing energy is less than the energy of next higher energy level the electron will

caus-be paired in the orbital But if the pairing energy is greater than the energy of next higher energy level the electron goes to the next higher energy level

(iv) Electrons and the nucleus are held together by trostatic forces of attraction

elec-Rutherford, therefore, contemplated the dynamical stability and imagined the electrons to be... light and other forms of

radiant energy propagate through space in the form of waves These waves have electric and magnetic fields

associated with them and are, therefore,... amplitude of the wave The value of amplitude increases and reaches the maximum which is indicated by peak in the curve This is shown by the upward arrow in the figure The value of the amplitude