The Foundations of Chemistry

Experiments show that the CXO bonds in CO 32 are neitherdouble nor single bonds,

but are intermediate in bond length and strength. Based on measurements in many

compounds, the typical CXO single bond length is 1.43 Å, and the typical CUO double

bond length is 1.22 Å. The CXO bond length for each bond in the CO 32 ion is inter-

mediate at 1.29 Å. Another way to represent this situation is by delocalizationof bonding

electrons:

The dashed lines include that some of the electrons shared between C and O atoms are

delocalizedamong all four atoms; that is, the four pairs of shared electrons are equally

distributed among three CXO bonds.

EXAMPLE 7-4 Lewis Formulas, Resonance

Draw two resonance structures for the sulfur dioxide molecule, SO 2.

Plan

The stepwise procedure (Section 7-5) can be used to write each resonance structure.

Solution

SO

qq

N1(8)2(8) 24 e

A1(6)2(6) 18 e

S NA  6 eshared

The resonance structures are

We could show delocalization of electrons as follows:

Remember that Lewis formulasdo not necessarily show shapes.SO 2 molecules are angular, not

linear.

You should now work Exercises 50 and 52.

OOS (lone pairs on O not shown)

OOS O SO or OOS OOS

O

2 

O

C

O

(lone pairs on O atoms not shown)

When electrons are shared among
more than two atoms, the electrons are
said to be delocalized.The concept of
delocalization is important in
molecular orbital theory (Chapter 9).

288 CHAPTER 7: Chemical Bonding

Problem-Solving Tip:Some Guidelines for Resonance Structures

Resonance involves several different acceptable Lewis formulas with the same arrange-

ment of atoms. They differ only in the arrangements of electron pairs, never in atom

positions. The actual structure of such a molecule or ion is the average, or composite,

of its resonance structures, but this does not mean that the electrons are moving from

one place to another. This average structure is more stable than any of the individual

resonance structures.

The rose on the left is in an
atmosphere of sulfur dioxide, SO 2.
Gaseous SO 2 and its aqueous
solutions are used as bleaching
agents. A similar process is used to
bleach wood pulp before it is
converted to paper.