Resonance

Sometimes one Lewis structure is not enough to describe a molecule. Ozone, O₃, can be drawn with a double bond on the left and a single bond on the right, or the other way round. But experiments show both O–O bonds are the same length, 128 pm: between a normal O–O single bond (148 pm) and an O=O double bond (121 pm).

The answer is resonance. We draw several canonical structures (resonance structures), all with the same positions of the nuclei and differing only in where the electrons are. The real molecule is a resonance hybrid of them all: a single structure that is a kind of average, not exactly like any one of them.

Other examples: the carbonate ion, CO₃²⁻, can be drawn in three ways, with the double bond on each oxygen in turn, so its three C–O bonds are all equal. In CO₂, the C–O bond length is 115 pm, between a C=O (121 pm) and a C≡O (110 pm).

Key facts about resonance:

• The resonance hybrid has lower energy (is more stable) than any single canonical form.
• Resonance averages the bond characteristics.
• The canonical forms are only drawings: they do not exist on their own, and the molecule does not flip back and forth between them. A mule is a cross between a horse and a donkey; it is not a horse one moment and a donkey the next.
• Only electrons move from one canonical form to another; the atoms never move.