GN Lewis (1875-1946) a professor and Dean of Chemistry at Berkeley University proposed that chemical bonds resulted when two atoms shared a pair of electrons. The Lewis concept allowed for a kind of "electron dot bookkeeping" to show how atoms could share electrons to achieve their quota as a noble gas or an octet of electrons.
Some general rules

No. of valence electrons of an atom = Group number
 Double and even triple bonds are commonly observed for C, N, P, O, and S
 Octet Rule - there is a total of 4 bonding pairs and lone pairs around the central atom.
Those atoms that have more or less than 4 are non octet.
 The central atom is the one with lowest electron affinity. It is never H. Electron affinity is defined as the energy released when a gaseous atom gains a electron. Electron affinity increases across a period and down a group.
 Some common shapes of molecules (more about how these are determined later)
Formula Lewis (electron dot) structure
Structural formula
(Use molymod models make the structure. Draw in the correct shape)
Cl
2
Chlorine
HF
Hydrogen
Fluoride
NH
3
Ammonia
H
2
O
Water
1

O
2
Oxygen
CO
2
Carbon dioxide
N
2 nitrogen
CH
4 methane
C
2
H
6 ethane
CH
3
Cl chloromethane
NF
3
Nitrogen trifluoride
C
2
H
5
OH ethanol
C
2
H
4 ethene
C
2
H
2 ethyne
2

CO
Carbon monoxide
H
2
S
Hydrogen sulphide
CH
3
CHO ethanal
Some covalent molecules are non octet, they do not obey the octet rule. These molecules either have more or less than four pairs of electrons around the central atom or can have an odd number of electrons. E.g. BF
3
, NO
2
BF
3
Boron trifluroide
NO
2
Nitrogen dioxide
SF
4
Sulfur tetrafuoride
When covalent bonds form we usually think of both atoms in the bond contributing an equal number of electrons to the bond. However, it is not uncommon for one atom to provide both electrons for bond formation. Such donor atoms tend to be nitrogen, oxygen and chlorine and these bonds are called coordinate or dative covalent bonds. E.g. SO
2
, O
3
, Al
2
Cl
6
SO
2
Sulfur dioxide can exist as two possible
Lewis Structures that follow the octet rule.
These two equivalent structures are called resonance
structures. The true structure is a hybrid of the two. There is also a non-octet version.
3

O
3 ozone
Al
2
Cl
6
Two AlCl
3
molecules join to form the dimer, Al
2
Cl
6
. Two dative covalent bonds are formed when a non-bonding electron pair on a chlorine atom of one
AlCl
3
molecule is shared with the aluminium atom of the second AlCl
3 molecule in the dimer.
More advanced Lewis Structures
HCOOH
Methanoic acid
CH
3
OCH
3
Methoxy methane
CH
3
COCH
3 propanone
Two possible structures
N
3
H
Hydrozoic acid
4

N
2
H
4
Dinitrogen tetrahydride
N
2
H
2
Dinitrogen dihydride
CH
3
N
2
Element Electron Affinity (kJ/mol) Electron Configuration
H 72.8 1s 1
He
Table of Electron Affinities of common Elements
<0 1s 2
Li
Be
B
C
N
O
F
Ne
59.8
<0
27
122.3
<0
141.1
328.0
<0
[He] 2s 1
[He] 2s 2
[He] 2s 2 2p 1
[He] 2s 2 2p 2
[He] 2s 2 2p 3
[He] 2s 2 2p 4
[He] 2s 2 2p 5
[He] 2s 2 2p 6
5

Octet obeying structure of SO
2
. These two equivalent structures are called resonance
structures. The true structure is a hybrid of the two.
6

7