Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

Example 5.6

Determine the formal charges on the atoms in SO

. 2

The Lewis structure of SO

: 2

O-S=O

1. The oxygen atom that is shown with t

he double bond has four nonbonding electrons

plus one-half of the four bonding electrons

. Thus, the Lewis structure puts a total of

six valence electrons around the oxygen. T

he number of valence electrons in a free

oxygen atom is also six, so this oxygen atom has no formal charge.

2. The oxygen atom that is shown with

a single bond has six nonbonding electrons and

one-half of the two bonding electrons, for a tota

l of seven valence electrons. This is

one more electron than the free atom, so th

is oxygen carries a -1 formal charge.

3. The sulfur atom contains two nonbonding

electrons and one-half of six bonding

electrons, for a total of five valence elec

trons. A free sulfur atom has six valence

electrons. Thus, the number of valenc

e electrons around a sulfur atom in SO

is one 2

less than around a free atom, so sulfur carries a +1 formal charge.

Nonzero formal charges should always be indicated in a Lewis structure by showing the formal charge with a circle around it. The Lewi

s structures of the tw

o resonance forms of

SO

are shown in Figure 5.11. 2

When more than one Lewis structure can be

drawn for a molecule, formal charge can

be used to help determine the preferred structure because energy is required to separate positive and negative charge (Coulomb’s law). C

onsequently, a Lewis structure that shows

no formal charge is energetically favored ove

r one that does. In addition, electrons are

more likely to reside on electronegative atoms. We conclude the following:

The Lewis structure that is pref

erred is the one in which the

formal charges of all atoms are

closest to zero. If non-zero formal charge must be assigned, negative formal charge should reside on the more electronegative atoms. An atom has zero formal charge in a molecule if the number of bonds in which it is

involved is equal to the number of unpaired el

ectrons in its Lewis symbol (Figure 5.5).

Table 5.4 summarizes this fact for the at

oms encountered most frequently. Because

structures with zero formal charge are preferred, the numbers of bonds shown in Table 5.3 are the ones most frequently drawn for these

atoms. However, there are many cases in

which the formal charges are not zero,* and the

number of bonds to the atom is different

from the number shown in the table.

OSO

OSO

• 
  


• 
  

+

+

Figure 5.11 Formal charge in SO

(^2)
Lewis structures of sulfur di
oxide that show all non-zero
formal charge. Recall that the line with an arrow at each end is used to indicate resonance. Table 5.4
The number of bonds to an atom
that results in zero formal charge.
Atom
Number of bonds
C
4
N
3
O
2
F, Cl, Br, I
1

• Carbon has zero formal charge in all of its compounds, so
  four
  bonds are always drawn to carbon atoms in their compounds
  ! Carbon is the basis of organic chemistry, and the
  number of compounds that contain carbon is infinite, so this is a very important rule that will aid us in drawing many Lewis structures.
  © by
  North
  Carolina
  State
  University