5 Steps to a 5 AP Chemistry 2019

158 ❯ STEP 4. Review the Knowledge You Need to Score High

Resonance

Sometimes when writing the Lewis structure of a compound, more than one possible struc-

ture is generated for a given molecule. The nitrate ion, NO 3 - , is a good example. Three

possible Lewis structures can be written for this polyatomic anion, differing in which oxygen

is double bonded to the nitrogen. None truly represents the actual structure of the nitrate

ion; that would require an average of all three Lewis structures. Resonance theory is used to

describe this situation. Resonance occurs when more than one Lewis structure can be written

for a molecule. The individual structures are called resonance structures (or forms) and are

written with a two-headed arrow (↔) between them. Figure 11.11 shows the three resonance

forms of the nitrate ion.

Increasing energy

Atomic

orbital

of O

Atomic

orbital

of O

2 p

2 s 2 s

2 p

Molecular

orbital

of O 2

ππ

π∗ π∗

Figure 11.10 Molecular orbital diagram of valence-shell electrons of O 2.

TIP