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The Structure of an Atom ° An atom consisfts of electrons, positively charged protons, and neutral neutrons ¢ Electrons form chemical bonds ¢ Atomic number: numbers of protons in its

Acids and Bases

Organic Chemistry

° Organic compounds are compounds containing carbon

¢ Carbon neither readily gives up nor readily accepts

electrons

the second row of the periodic table

¢ Carbon shares electrons with other carbon atoms as well as with several different kinds of atoms

The Structure of an Atom

° An atom consisfts of electrons, positively charged protons,

and neutral neutrons

¢ Electrons form chemical bonds

¢ Atomic number: numbers of protons in its nucleus

¢ Mass number: the sum of the protons and neutrons of an atom

¢ lsotopes have the same atomic number but different mass

numbers

¢ The atomic weight: the average weighted mass of its atoms

¢ Molecular weight: the sum of the atomic weights of all the atoms

in the molecule

The Distribution of Electrons in an Atom

¢ Quantum mechanics uses the mathematical equation of wave

motions to characterize the motion of an electron around a

nucleus

¢ Wave functions or orbitals tell us the energy of the electron and the volume of space around the nucleus where an electron is

most likely to be found

¢ The atomic orbital closer to the nucleus has the lowest energy

¢ Degenerate orbitals have the same energy

Table 1.1 Distribution of Electrons in the First Four Shells

That Surround the Nucleus

First shell Second shell Third shell Fourth shell

Number of atomic orbitals l 1,3 1,3,5 1, 3,5, 7

Maximum number of electrons 2 8 18 32

Name of element

Hydrogen

Helium Lithium

Beryllium

Boron Carbon

Nitrogen Oxygen

Fluorine Neon Sodium

¢ The Aufbau principle: electrons occupy the orbitals with the lowest energy first

¢ The Pauli exclusion principle: only two electrons can

occupy one atomic orbital and the two electrons have

opposite spin

¢ Hund's rule: electrons will occupy empty degenerated orbitals before pairing up in the same orbital

¢ lonic compounds are formed when an electropositive element transfers electron(s) to an electronegative element

Covalent Compounds

° Equal sharing of electrons: nonpolar covalent bond (e.g., H,)

¢ Sharing of electrons between atoms of different

electronegativities: polar covalent bond (e.g., HF}

TABLE 1.3 The Electronegativities of Selected Elements?

IA IIA IB IIB WIA | IVA | VA | VIA | VIIA

H

ti Be B C N O F 2 17) | 13 2U | 23) | 20 | 37 | 40 =

®@

Na | Mg Al Si P S Cl 5 3) j I5 1s | 7Ì 5 | 3!) =

Electrostatic Potential Maps

red < orange <_ yellow < green < blue

most negative most positive

electrostatic potential electrostatic potential

A Dipole

¢ A polar bond has a negative end and a positive end

dipole moment (D) =u=exd

(e) : magnitude of the charge on the

a4 distance between the two charges

Table 1.4 The Dipole Moments of Some Commonly Encountered Bonds

Bond Dipole moment (D) Bond Dipole moment (D)

Lewis Structure

Lewis structures

Formal charge =

number of valence electrons —

(number of lone pair electrons +1/2 number of bonding electrons)

Table 1.5 Kekulé and Condensed Structures

Kekul structure Condensed structures

Important Bond Numbers

two bonds

The s Orbitals

1s atomic orbital 2s atomic orbital 2s atomic orbital

node not shown node shown

° Molecular orbitals belong to the whole molecule

° ơ bond: formed by overlapping of fwo s orbitals

° Bond strength/bond dissociation: energy required to break a bond or energy released to form a bond

In-phase overlap forms a bonding MO; out-of-phase

overlap forms an antibonding MO

Sigma bond (o) is formed by end-on overlap of two

Pi bond (x) is formed by sideways overlap of two parallel

Bonding in Methane and Ethane:

The orbitals used in bond formation determine the

bond angles

a

¢ Tetrahedral bond angle: 109.5

¢ Electron pairs spread themselves into space as far from each other as possible

Bonding in Ethene: A Double Bond

side view top view

¢ The bond angle in the sp? carbon is 120°

¢ The sp? carbon is the trigonal planar carbon

Bonding in Ethyne: A Triple Bond

o bond formed by sp-s overlap

¢ A triple bond consists of one o bond and two x bonds

¢ Bond angle of the sp carbon: 180°

Bonding in the Methyl Cation

Bonding in the Methyl Radical

Bonding in the Methyl Anion

methyl anion ball-and-stick model of the methyl anion electrostatic potential map

for the methyl anion

bond is formed by the overlap

of an sp? orbital of oxygen with

the s orbital of hydrogen

\ bond is formed by the overlap

of an sp3 orbital of nitrogen with the s orbital of hydrogen

ammonium ion ball-and-stick model of the ammonium ion electrostatic potential map

for the ammonium ion

Bonding in Hydrogen Halides

Table 1.6 Hydrogen-Halogen Bond Lengths and Bond Strengths

lọ

1.6090 71 298

Summary

¢A x bond is weaker than a ơ bond

° The greater the electron density in the region of orbital overlap, the stronger is the bond

° [he more s character, the shorter and stronger is the bond

° [he more s character, the larger is the bond angle

Molecular Dipole Moment

The vector sum of the magnitude and the direction of the individual bond dipole determines the overall dipole moment of a molecule

Cl

carbon dioxide th u=0D

Bronsted—Lowry Acids and Bases

¢ Acid donates a proton

¢ Base accepts a proton

weaker acid weaker base stronger base stronger acid

¢ Strong reacts to give weak

° The weaker the base, the stronger Is its conjugate acid

¢ Stable bases are weak bases

K,: The acid dissociation constant

The Henderson—Hasselbalch Equation

The pH indicates the concentration of hydrogen tons (H*)

[HA A’ |

° When atoms are very different in size, the stronger acid will have its proton attached to the largest atom

° When atoms are similar In size, the stronger acid wIll

have its proton attached to the more electronegative

Acetic acid is more acidic than ethanol

~—

CH,CO-Lewis Acids and Bases

¢ Lewis acid: non-proton-donating acid; will accept two electrons

¢ Lewis base: electron pair donors

the curved arrow indicates where the pair of electrons starts from and where it ends up

aluminum trichloride dimethyl ether

a Lewis acid a Lewis base

borane ammonia

a Lewis acid a Lewis base