Inorganic chemistry for the medical entrance examinations

To Find the Number of Sigma and Pi Bonds in a C C C C C C C H COORDINATE OR SEMI-POLAR BOND Coordinate bond is a special type of bond which is formed by donation of electron pair from d

The Pearson Guide to

Inorganic Chemistry for the Medical Entrance

Examinations

Atul Singhal

The aim of this publication is to supply information taken from sources believed to be valid and reliable This is not an attempt to render any type of professional advice or analysis, nor is it to be treated as such While much care has been taken

to ensure the veracity and currency of the information presented within, neither the publisher nor its authors bear any responsibility for any damage arising from inadvertent omissions, negligence or inaccuracies (typographical or factual) that may have found their way into this book

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Chapter 2: Classification of Elements and Periodicity Properties 2.1–1.32

Chapter 10: Noble Gases or Zero Group VIIIA – Group elements 10.1–10.21 Chapter 11: Transition Metals Including Lanhanides and Actinides 11.1–11.40

This page is intentionally left blank.

The Pearson Guide to Inorganic Chemistry for the Medical Entrance Examination is an invaluable

book for all the students preparing for the prestigious medical entrance examination It provides

class-tested course material and problems that will supplement any kind of coaching or resource the

students might be using Because of its comprehensive and in-depth approach, it will be especially

helpful for those students who do not have enough time or money to take classroom courses

A careful scrutiny of previous years’ A.I.P.M.T papers and various other competitive PMT

examinations during the last 10 to 12 years was made before writing this book It is strictly

based on the latest AIPMT/STATE P.M.T syllabus (2009–10) recommended by the executive

board It covers the subject in a structured way and familiarizes students with the trends in these

examinations Not many books in the market can stand up to this material when it comes to the

strict alignment with the prescribed syllabus.

It is written in a lucid manner to assist students to understand the concepts without the help of

any guide.

The objective of this book is to provide this vast subject in a structured and useful manner so as

to familiarize the candidates taking the current examinations with the current trends and types

of multiple-choice questions asked.

The multiple-choice questions have been arranged in following categories:

Gear Up I (To Revise the Concepts)

Gear Up II (To Sharpen the Concepts)

Gear Up III (Concept Crackers)

A Peep into the AIPMT

MCQ’s from Recent Entrance exams

Assertion and Reason

This book is written to pass on to another generation, my fascination with descriptive physical

organic chemistry Thus, the comments of the readers, both students and instructors, will be sincerely

appreciated Any suggestions for added or updated additional readings would also be welcome

Dr Atul Singhal singhal.atul50@yahoo.com singhal.atul1974@gmail.com

The contentment and ecstasy that accompany the successful completion of any work would remain essentially incomplete if I fail to mention the people whose constant guidance and support has encouraged me.

I am grateful to all my reverend teachers, especially, the late J K Mishra, Dr D K Rastogi, the late

A K Rastogi and my honourable guide, Dr S K Agarwala Their knowledge and wisdom has tinued to assist me to present this work

con-I am thankful to my colleagues and friends, Deepak Bhatia, Er Vikas Kaushik, Er A R Khan, Vipul Agarwal, Er Ankit Arora, Er Wasim, Manoj Singhal, Vijay Arora (Director, Dronacharya), Anupam Shrivastav (Bansal Classes), Rajiv Jain (MVN, Faridabad), Ajay Verma, Ashutosh Tripathi, Vivek shukla and Gaurav Bansal (C-25Classes) Satish Gupta (Resorance Jaipur), Chandan Kumar (Everonn Toppers).

I am indebted to my father, B K Singhal, mother Usha Singhal, brothers, Amit Singhal and Katar Singh and Sisters, Ambika and Poonam, who have been my motivation at every step Their never- ending affection has provided me with moral support and encouragement while writing this book.

Last but not the least, I wish to express my deepest gratitude to my wife Urmila and my little,—but witty beyond years, daughters, Khushi and Shanvi who always supported me during my work.

Dr Atul Singhal singhal.atul50@yahoo.com singhal.atul1974@gmail.com

Molecular orbital theory involving homounclear diatomic molecules Hydrogen-bonding

3

3 3

VALENCY

Valency is a property of atoms whereby they form chemical

bond among themselves The term valency was introduced

by Frankland and it means ‘power to combine.’ Hence, it is

the power of an atom to combine with another atom Atoms

do so by either giving up or accepting electrons in their

outermost shell Modern or electronic concept of valency

was given by Kossel and Lewis; it was completed by

Langmuir

Valency (V)  No of valence electrons

For instance, the electronic configuration for the group

IA element sodium (Na), is 2, 8, 1 Here, the number of

valence electron is 1 and hence its valency is 1

If the number of valence electrons is more than 4, then

we use the following relationship to determine the valency:

For example, the configuration of nitrogen (N) is 2, 5 ing to the above relationship, its valency will be given as

Accord-V  5  8  3 (negative sign signifies the tendency

to accept electrons)

CHEMICAL BOND

Chemical bond is the force of attraction that binds two

atoms together A chemical bond balances the force of

attraction and force of repulsion at a particular distance

A chemical bond is formed to:

Kattain the octet stateminimize energygain stabilitydecrease reactivityWhen two atoms come close to each other, forces of

attraction and repulsion operate between them The

tance at which the attractive forces overcome repulsive

forces is called bond distance Here, potential energy for

the system is lowest, hence the bond is formed

Types of Bonds

Following are the six types of chemical bonds Here, they

are listed in a decreasing order of their respective bond

6 Van der Waals bond

Metallic bond, hydrogen bond and van der Waals bond

are interactions

Octet Rule

It was introduced by Lewis and Kossel According to

this rule, each atom tries to obtain the octet state, that is,

a state with eight valence electrons

Exceptions to the octet rule

Contraction of octet state

Here central atom is electron deficient or does not

have an octet state For example,

BeX

4

BX6

AIX6

Ge(CH )e

Expansion of octet state

IONIC OR KERNEL BOND

Ionic bond is formed by the complete transfer of valence

electrons from a metal to a non-metal This was first

Maximum number of electrons transferred by a metal

transfers three electrons to F)

During electron transfer, the outermost orbit of metal

is destroyed and the remaining portion is called core or kernel, so this bond is also called kernel bond

Nature of ionic bond is electrostatic or coloumbic force

of attraction

It is a non-directional bond

Conditions for the Formation of an Ionic Bond

The process of bond formation must be exothermic ($H  ve) and for it the essential conditions are

Metal must have low ionization energy

Non-metal must have high electron affinity

Ions must have high lattice energy

Cation should be large with low electronegativity.Anion must be small with high electronegativity

Born–Haber Cycle

The formation of an ionic compound in terms of energy can be shown by Born–Haber cycle It is also used to find lattice energy, ionization energy and electron affinity.For example,

For the formation of an ionic solid, energy must be

released during its formation, that is, %H must be

nega-tive for it

Properties of Ionic Compounds

1 Ionic compounds have solid crystalline structures (flat

surfaces), with definite geometry, due to strong

electro-static force of attraction as constituents are arranged in

a definite pattern

2 These compounds are hard in nature

Hardness t Electrostatic force of attraction

Ionic radius

3 Ionic compounds have high value of boiling point,

melt-ing point and density due to strong electrostatic force of

attraction

Boiling point, melting point о Electrostatic force of

attraction

Electorstatic force of attraction

4 Ionic compounds show isomorphism, that is, they have

same crystalline structure For example, all alums, NaF

and MgO

5 These are conductors in fused, molten or aqueous state

due to presence of free ions In solid state, these are non-

conductors as no free ions are present

6 They show fast ionic reactions as activation energy is

zero for ions

7 They do not show space isomerism due to non-

directional nature of ionic bond

8 Lattice energy (U) is released during the formation of an

ionic solid molecule from its constituent ions

Lattice Energy

Lattice energy is also the energy needed to break an

ionic solid molecule into its constitutent ions It is

denoted by U

U о Charge on ion

Size of ion

Hence, lattice energy for the following compounds

increases in the order shown below:

As charge on a metal atom increases, its size decreases

In case of univalent and bivalent ionic compounds, tice energy decreases as follows:

lat-Bi-bi  Uni-bi or Bi-uni  Uni-uniFor example,

9 Ionic compounds are soluble in polar solvents like water due to the high dielectric constant of these solvents, therefore, force of attraction between ions are destroyed and they dissolve in the solvent

Facts Related to Solubility

If %H (hydration)  Lattice energy then ionic pound is soluble

If %H (hydration)  Lattice energy then ionic pound is insoluble

com-pound is at equilibrium state

Some Solubility Orders

Crystals of high ionic charges are less soluble For

If atoms are same or their electronegativity is same, the

covalent bond between them is non-polar For example,

X–X, O = O, N y N

If atoms are different or have different value of

electro-negativity, the covalent bond formed between them is

polar For example,

H

E

 OE H E, H E XEHere, number of electrons shared or covalent bonds rep-

resent covalency

One atom can share a maximum of three electrons with

the other atom

The nature of covalent bond is explained on the basis of

Heitler–London’s valence bond theory, Pauling–Slater’s

overlapping theory and Hund–Mullikan’s theory

Orbital concept of covalent bond was introduced by

Heitler and London According to this concept,

“Co-valent bond is formed due to half-filled atomic orbitals

having electrons with opposite spin to each other.”

Due to overlapping, the potential energy of system

decreases

The internuclear distance with maximum overlapping

and greater decrease of potential energy is known as

bond length.

Energy consideration of covalent bond When two

force start operatings

Repulsive

Nucleus

Electron cloudAttractive

It is observed that attractive force are more than a

repul-sive forces which results in decreased energy, so the

poten-tial energy of the system decreases

The minimum energy point corresponds to critical

dis-tance between two nuclei, when maximum lowering of

energy takes place This distance is called bond length

e.g., in H–H, bond length is 74 pm

–

Bond length (74pm)

Inter nuclear distance

H H (too far) (too close)

H H HH

Figure 1.2

Features of Covalent Compounds

1 Covalent compounds mostly occur in liquid and gaseousstate, but if molecular weight of the compound is high,they may occur in solid state also For example,

2 ‘Like dissolves like’, that is, non-polar solute dissolves

Boiling point and melting point о Hydrogen bonding

о Molecular weight For example,

4 Covalent compounds are non-conductors due to absence

of free ions, but graphite is a conductor, as in graphite,free electrons are available in its hexagonal sheet likestructure

In case of diamond, the structure is tetrahedral so free electrons are not available It is therefore not a conductor

5 Covalent bond is directional, so these compounds canshow space isomerism

6 When cation and anion are close to each other, the shape

of anion is distorted by the cation, this is called

polar-ization Due to this, covalent nature develops in an ionic

Figure 1.3 Effect of Polarization

Sigma bond is formed by axial or head to head or linear overlapping between two s–s or s–p or p–p orbitals

Sigma( ) bond

bond

S – S

P

bond

Figure 1.4 Formation of Sigma Bond

1 Sigma bond is stronger and therefore less reactive, due to more effective and stronger overlapping than pi bond

2 The minimum and maximum number of sigma bond between two bonded atoms is one

T

5 In sigma bond, free rotation of the atoms is possible

6 Sigma bond determines the shape of molecule

As the covalent nature increases, the intensity of the

Fajan’s rule

Polarization or covalent nature is explained by the

fol-lowing rules:

Charge on cation polarization, covalent nature or

polarizing power of cation t charge on cation That is,

greater the charge on cation, greater will be its

polariz-ing power and more will be covalent nature For

Size of Cation When charge is same and anion is

That is, smaller cation has more polarizing power

For example,

LiCl  NaCl  KCl  RbCl  CsCl

Size of anion This property is taken into account when

the charges are same and the cation is common

Polarization or covalent nature t size of anion

Hence, larger anions are more polarized

For example, LiF LiCl LiBr  LiI

Larger the size of anion easier, will be its

polariza-tion

A cation with 18 valence electrons has more

polar-izing power than a cation with 8 valence electrons

•

•

1 It is a weak or less stable bond, and therefore more

reac-tive, due to less effective overlapping

2 Minimum and maximum number of pi bonds between

two bonded atoms are 0 and 2 respectively

Number of pi bonds

5 In case of a pi bond, free rotation is not possible

6 It does not determine the shape of a molecule but

short-ens bond length

To Find the Number of Sigma and Pi Bonds in a

C C

C

C

C

C C

H

COORDINATE OR SEMI-POLAR BOND

Coordinate bond is a special type of bond which is formed

by donation of electron pair from donor to receiver, that is,

it involves partial transfer or unequal sharing of electrons

It is denoted as (m) from donor to receiver

HH

HN: + BF

Coordinate bond is intermediate between ionic and covalent bonds, but more closely resembles a covalent bond

The properties of coordinate compounds are more close

to covalent compounds For example, Properties of

Sugden or singlet linkage is formed by donation of one

The nature of a hydrogen bond is either dipole–dipole type, ion–dipole type or dipole–induced dipole typeHCl has no H-bonding as chlorine is large in size

H-bond strength for the following order is 10 kCal per

mole, 7 kCal per mole and 2 kCal mole respectively

Hydrogen bonding is of following two types:

Intermolecular H-bonding

Intermolecular H-bonding is formed between two or

more different molecules of the same or different types

Figure 1.6 Intermolecular H-bonding in

Water and Ammonia

Facts Related to Intermolecular

Hydrogen Bonding

One water molecule can form hydrogen bonding with

four other water molecules

Due to hydrogen bonding in water, the water molecules are

closely packed, so water has less volume but more density

than ice where an open cage like structure is observed

Water has maximum density at 4oC as above 4oC some

hydrogen bonds are broken leading to a decrease in the

Effects of Intermolecular H-bonding

Increase in boiling point, melting point, solubility, mal stability, viscosity, surface tension and occurence liquid state is observed as molecules get associated more closely due to inter molecular H-bonding

ther-HF is a liquid and has a higher boiling point than other

HX molecules which are gases at room temperature (Here X  halogens)

Alcohols are highly soluble in water in any proportion and have higher boiling points than others which are very less soluble in water

Glycerol is highly viscous with a high boiling point.Acids have higher boiling point and solubility than their corresponding acid derivatives

In DNA and RNA, the complementary strands are held together by intermolecular H-bonding between the nitrogenous bases of the two strands

Nucleic acid and proteins are held together by hydrogen bonds

pos-sible, due to absence of hydrogen bonding because of large sizes of the halogen atoms

The extent of hydrogen bonding in water is higher than

O C

H 3 C

Figure 1.7 Dimerization of Acetic Acid

Intramolecular H-bonding or Chelation

Intramolecular H-bonding or chelation is formed within a molecule For example,

O H F

+S

–S

O N

H O O

H C O

O O

Other examples are pyridine-2-carbonaldoxime and

o-hydroxybenzoic acid

Effects of Intramolecular H-bonding

Due to this bonding, the boiling point and acidic

nature of the molecule decreases but its volatile nature

increases

O-nitrophenol has a low boiling point and reduced acidic

nature, but is more volatile than p-nitrophenol A mixture

of both these componds can be separated by steam

distil-lation method

METALLIC BONDING

The concept of metallic bonding was introduced by

Drude and Lorenz in the form of electron-sea model.

Metallic bond is the force of interaction between the

mobile electrons and positively charged kernels found

in metal atoms which holds the atom together

Metallic bond strength t Number of valence electrons

or charge on nucleus

Conditions for Formation of Metallic Bond

1 The metal should have low ionization energy

2 The number of vacant orbitals should be enough in the

metal

Properties Related to Metallic Bond

A metal has a bright lusture because of to and fro

oscil-lations of mobile electrons on the surface of metal

Metals are ductile (can be drawn into wires) and male-able

(can be beaten into sheets) as the metallic bond is

non-directional and the atomic kernels of metals are slippery

Metals have high thermal and electrical conductivities

due to the presence of mobile electrons On increasing

temperature, the condustivity decreases, as the increase

of temperature causes vibration of kernels which in turn

pushes the mobile electrons away from the kernels

Boiling point, melting point, hardness, density of metal

B Metallic bond strength

Therefore, alkali metals are soft and can be cut with a

knife due to weak metallic bonding

Hg is liquid possessing the lowest melting point 38.5

(among metals) due to very weak metallic bond

Iridium and osmium have very high densities due to

strong metallic bonding

Demerits of Electron-Sea Model

Electron-Sea model cannot explain heat of atomization, heat of fusion, hardness and melting point in a proper way

It cannot explain why Cu is 50 times more conductive than Bi, and why Na is soft and Os is hard

Melting point of mercury is 234 K and that of tungsten

is 3275K

RESONANCE

When all the properties of a molecule cannot be explained

by a single structural formula, then such molecules are represented by many structural formulas that are canoni-cal structures or contributing or resonating structures

It is observed due to the delocalization of Q electrons.Canonical structures for a given molecule, have the same arrangement of atoms

Position and arrangement of atoms are same in cal structures, they only differ in the distribution of elec-trons

canoni-Canonical structures are depicted by the symbol

Canonical structures should be planar or nearly planar.Total number of paired and unpaired electrons are also same in canonical structures For example,

O–

Resonance changes bond length, for example, in

ben-zene C–C  1.39 Å, which is an intermediate value

between (C–C)  1.54 Å, (C5C)  1.34 Å

Resonance Energy

Resonance energy  Energy of most stable canonical

structure – Resonance hybrid energy

Resonance energy d Number of canonical structure

Resonance energy d Stability

ReactivityResonance energy  Expected heat of hydrogenation

 Actual heat of hydrogenation

Due to high resonance energy, benzene is quite stable

and undergoes electrophilic substitution reactions It

does not undergo addition reactions, although it has

double bonds (due to delocalization of Q electrons or

resonance)

Benzene has 36 kcal/mole of resonance energy

In tautomerism, arrangement of atoms is different for

its different arrangements but in resonance, the

arrange-ment of atoms is same

Stability of Different Canonical Structures

1 A non-polar structure is always more stable than a polar

structure In the following example, the structures are

arranged in a decreasing order of stability

–

–

In the last two structures, the charges are apart so they

are less stable

2 Greater the number of covalent bonds, greater will be

the stability Therefore,

1 Isovalent resonance The canonical structures have

same number of bonds and same type of charges For

2 Heterovalent resonance Here, the canonical

struc-tures have different number of bonds and charges

For example, buta- 1, 3-diene, vinyl cyanide

HYBRIDIZATION

Pauling and Slater introduced this concept to explain the shape of molecules which could not be explained by the valence bond theory

It is the intermixing or re-distribution of energy among two or more half-filled, fully filled, incompletely filled

or empty orbitals of comparable energy, to form same number of hybrid orbitals Hybrids have identical ener-gies and similar shapes

Facts About Hybridization

Number of atomic orbitals taking part in hybridization

is equal to number of hybrids formed

Electrons do not undergo hybridization

A hybrid bond is always a sigma bond

A hybrid bond is always stronger than a non-hybrid bond

Hybridization occurs at the time of bond formation

Hybridization t Overlapping (for enough overlapping, orbitals must be at an approppriate distance from each other, that is, neither very close nor very far)

Hybridization increases stability and decreases

reactiv-ity and energy of a molecules

Hybridization occurs in the central atom in a molecule

respectively

Hybridization does not occur in isolated atoms but in

bonded atoms

Types of Hybridization

1 sp hybridization Here, one s and one p orbitals form

two sp hybrid orbitals after intermixing

Shape of molecule is linear and bond angle is 180o

For example, X–M–X (M  Be, Zn, Hg)

spyspH

Shape of these species is trigonal or coplanar and the

X

B

Shape of the species is tetrahedral and bond angle is

NH3, PH3, H2O, H2S,

H

C

Shape of the species is square planar and bond angle

Shape of the species is trigonal bipyramidal and bond

hybrid orbitals

Shape of the species is octahedral and bond angle is

FF

FFF

F F F F F

F Xe

Rules to Find the Type of Hybridization

For covalent compounds and ions

1 Count the total number of valence electrons and (±)

charge, to find a particular value For example, in the

2 Now divide the total value of electrons to get the

quo-tient X (number of bond pair electrons)

If total value of X is between 2 to 8, divide it by 2

If total value is between 10 to 56 divide it by 8

If total value is 58 or more, divide it by 18

3 If any remainder is left, divide again as above to get

another quotient Y (number of lone pair electrons)

Rule to find the geometry of covalent compounds The

shape or geometry of a molecule or ion can be found out

by finding the type of hybridization, number of bond pairs

and lone pair of electrons using the following relation

Here, P total numbers of pairs of electrons around the

central atom which gives the present hybridization

atom or number of bond pairs of electrons.For example,

Hybridization in complexes Coordination number of

ligands is used to find the hybridization

VSEPR (Valence Shell Electron Pair Repulsion Theory)

Valence shell electron pair repulsion theory was duced by Nyholm and Gillispie to predict the shape of polyatomic molecules and ions

intro-According to this theory, beside hybridization, the nature of electrons around the central atom also decide the shape of molecule

•

•

1

There may be two types of electrons around the central

atom, that is, bond pair or lone pair type

These electrons undergo electron–electron repulsion

and the decreasing order of electronic repulsion is lp–lp

lp–bp  bp–bp

Due to this electronic repulsion, the shape of the

mol-ecule becomes distorted and the bond angle changes

Distortion in shape or change in bond angle B electronic

Here, S atom has two bond pairs and one lone pair of

electron, so lp–bp type of repulsion distorts shape, that is,

it bends and changes the bond angle and the shape becomes

sp3 Hybridization

1 When the central atom has four bond pairs of electrons,

the shape will be normal with normal bond angle that is

2 When the central atom has 3 bond pairs and 1 lone pair

of electron, there will be lp–bp type of repulsion, which distorts shape and changes bond angle, that is, the shape

is pyramidal and the bond angles are less than 1098 28 For example, N

In ammonia, the bond angle is 107

3 When the central atom has 2 lone pair and 2 bond pair

of electrons, there will be lp–lp type of electronic sion, so the shape will be distorted and it will be angular

A

.

4 When the central atom has 3 lone pairs and 1 bond pair

of electron, there will be lp–lp type of electronic sion So shape is highly distorted and it becomes linear

sp3d Hybridization

1 When the central atom has 5 bond pair of electrons, the shape will be normal with normal bond angle, that is, trigonal bipyramidal and bond angle of 90o, 120o As only bp–bp type of electronic repulsion occurs, so there

is no distortion in shape and no change in bond angle

2 When the central atom has 4 bond pair and 1 lone pair of

electrons, the shape will be distorted and it will possess a

A

B B

B B

:

3 When the central atom has 3 bond pairs and 2 lone pair

of electrons, the shape will be distorted and it will be a

A

B B B

:

:

4 When the central atom has 2 bond pair and 3 lone pair of

electrons, the shape will be distorted and the shape will

B

B

A

sp3d2 Hybridization

1 When the central atom has 6 bond pairs of electrons,

the shape will be normal with normal bond angles that

elec-tronic repulsion occurs, so there is no distortion in shape

A

FF

F

FS

2 When the central atom has 5 bond pair and 1 lone pair

of electrons, the shape will be distorted and it will be



3 When the central atom has 4 bond pair and 2 lone pair

of electrons, the shape will be distorted and it will be

A

Hybridization and Shapes of Some Simple Molecules

Pairs

Number of Charge Clouds

Molecular Geometry and Shape

Examples

Dipole Moment

r

Dipole moment is used to measure the polarity in a

mol-ecule It is denoted by M Mathematically, it is given as

tronegative species or less electronegative to more

AX

μ = 0Non-polar

•

Calculation of Resultant Bond Moments

Let AB and AC be two polar bonds inclined at an angle R

when

NRN Nwhen

Number of Charge Clouds

Molecular Geometry and Shape

Examples

Dipole moment t Number of lone pair of electrons.

Fluorine has 3 lone pair, oxygen has 2 lone pair, and

ammonia has 1 lone pair of electron

X X

having normal shapes according to hybridization like

linear, trigonal, tetrahedral, will be non-polar, as for

Cl

P Cl

Cl

Cl Cl Cl

Molecules in which the central atom has lone pair of

electrons or have distorted shapes, like angular,

pyra-midal, sea-saw shapes will have some value of dipole

μNet = 1.62D

N H H

μNet = 1.47D

H

N F F

μNet = 0.24D

F

substrac-tive with μ due to lp electrons

Dipole moment of a cis-alkene is more than trans-

Exception Unsymmetric alkenes with odd number of

carbon has some value of dipole moment For example, trans-2-pentene

μ = +ve (But less than C is)

Specific Cases of Dipole Moment

T dintinguish cis and trans alkenes

MOLECULAR ORBITAL THEORY

Molecular orbital theory was given by Hund and liken

Mul-It is based on LCAO (Linear Combination of Atomic Orbitals) model

Atomic orbitals undergo linear combination to form same number of molecular orbitals, if they fullfil the following conditions:

1 Atomic orbitals must have comparable energies

2 Atomic orbitals must overlap linearly for enough and effective overlapping

3 Atomic orbitals must have same symmetry along with

the major molecular axis, for example, if Z axis is the

Molecular orbitals are formed due to constructive and

destructive interference of atomic orbitals

Constructive interaction of orbitals between orbital

lobes having same wave function Z produces bonding

molecular orbitals like T, Q and % these are HoMOs

(Highest occupied molecular orbitals)

a

a a

Figure 1.8 Depiction of Interactions Involving HoMOs

Destructive interaction between orbitals having

differ-ent sign of Z produces anti-bonding molecular orbitals

or LuMOs (Lowest unoccupied molecular orbitals) For

-

-Anti-Bonding

Destructive interaction

Amplitude = 0 Amplitude = a

Figure 1.9 Interactions Involving LuMOs

•

•

•

Facts Related to HoMOs and LuMOs

Energy: LuMOs  HoMOsWavelength: LuMOs  HoMOsLuMOs have nodal planes while HoMOs may or may not have nodal planes

Electrons contribute force of attraction in HoMOs while they contribute repulsion in LuMOs

The shape of the molecular orbitals formed depend upon shape of atomic orbital from which they are formed.Like atomic orbitals, molecular orbitals also follow:

1 Pauli exclusion principle—Any molecular orbital can have a maximum of two electrons with opposite spin

2 Hund’s rule—In degenerate molecular orbital, before pairing, each molecular orbital must have one electron

3 Aufbau principle—Electrons are filled from lar orbital of lower energy to higher energy

molecu-Formation of Various Molecular Orbital

Figure 1.10 Molecular Orbitals

Order of Filling Electrons in Molecular Orbital

T 1s is the lowest energy molecular orbital while

Due to intermixing of 2s and 2p orbitals in cases where

If unpaired electrons (n  1, 2) are present in a molecule

Bond length is the average distance between the centers

of the nuclei of the two bonded atoms

It is determined by X-ray diffraction and spectroscopic methods

In case of ionic compounds, it is the sum of ionic radius

of cation and anion, while in case of covalent pounds, it is sum of their covalent radius

com-Factors affecting bond length

Bond length t Size of atomFor example, HF HCI HBr HI

As F ...

the other atom

The nature of covalent bond is explained on the basis of

Heitler–London’s valence bond theory, Pauling–Slater’s

overlapping theory and Hund–Mullikan’s theory... contribute force of attraction in HoMOs while they contribute repulsion in LuMOs

The shape of the molecular orbitals formed depend upon shape of atomic orbital from which they are formed.Like... charges are apart so they

are less stable

2 Greater the number of covalent bonds, greater will be

the stability Therefore,

1 Isovalent resonance The canonical structures