Page 13 - Mechanism and Theory in Organic Chemistry

plete but each of which serves its special purpose in helping us understand some
aspect of the physical reality.

The Electron Pair Bond-Lewis Structures
The familiar Lewis structure is the simplest bonding model in common use in
organic chemistry. It is based on the idea that, at the simplest level, the ionic
bonding force arises from the electrostatic attraction between ions of opposite
charge, and the covalent bonding force arises from sharing of electron pairs be-
tween atoms.
The starting point for the Lewis structure is a notation for an atom and its
valence electrons. The element symbol represents the core, that is, the nucleus and
all the inner-shell electrons. The core carries a number of positive char~es equal
to the number of valence electrons. This is calle~~e.
V.3hmxkctrons are shown explicitly. For elements in the third and later rows
ofthe periodic table, the d electrons in atoms of Main Groups 111, IV, V, VI, and
VII are counted as part of the core. Thus :
. .
:Br: :Se: :I:
Ions are obtained by adding or removing electrons. The charge on an ion is
given by
charge = core charge - number of electrons shown exvlicidy
An ionic compound is indicated by writing the Lewis structures for the two ions.
A covalent bond model is constructed by allowing atoms to share pairs of
electrons. Ordinarily, a shared pair is designated by a line:
H-H
All valence electrons of all atoms in the structure must be shown explicitly. Those
electrons not in shared covalent bonds are indicated as dots, for example:

If an ion contains two or more atoms covalently bonded to each other, the
total charge on the ion must equal the total core charge less the total number of
electrons, shared and unshared :
. .
(H-0 :) -
. .
H core = + 1
0 core = +6

total core = + 7
number of electrons = - 8
-
total charge = - 1

In order to write-correct Lewis structures, two more concepts are needed.
First, consider the total number of electrons in the immediate neighborhood of
each atom. This number is called the valence-shell occupancy of the atom, and to
find it, all unshared electrons around the atom and all electrons in bonds leading