Illustrated Glossary of Organic Chemistry

Resonance: (1) A condition in which molecular structure cannot be adequately represented by a single Lewis structure; two or more Lewis structures are required.

Empirical evidence reveals the two C-O bonds of acetate ion have equal bond length. A single Lewis structure implies the C-O bond lengths are unequal:

The resonance hybrid (derived from several resonance contributors) explains the equal C-O bond lengths:

Resonance contributors

Resonance hybrid

(2) A system in oscillation due to some external force. This is the resonance in nuclear magnetic resonance, where the oscillation is between the ground and excited nuclear spin states.

Orientation of
applied magnetic
field (B0)

Add energy

Release energy

I = + 1/2
Lowest energy
Ground state

I = - 1/2
Higher energy
Excited state
Oscillation (resonance) between the ground and excited nuclear spin states, caused by absorption and release of energy (radio wave photons).