resonance

Meaning: Resonance structures are Lewis structures of one species with the same atom positions; only π electrons and lone pairs differ, tracked with curved arrows. The real molecule is a resonance hybrid, a weighted average of the contributors. Delocalizing electrons lowers energy, so the hybrid is more stable than any single contributor. Bond lengths and charges end up intermediate: the C–C bonds in benzene are all equal, and the two C–O bonds in a carboxylate are equal. More stable contributors count for more: those with full octets, fewer formal charges, and negative charge on the more electronegative atom.

On the MCAT: Expect questions that ask for the major contributor or for the number of equivalent structures. The most common use is ranking acidity or basicity. A conjugate base stabilized by resonance means a stronger acid, so carboxylic acids are more acidic than alcohols, and phenol is more acidic than cyclohexanol. An amine lone pair delocalized into a ring or carbonyl is less basic. Amide C–N bonds have partial double-bond character, which restricts rotation. Resonance is also the basis of aromatic stability.

Common confusion: Resonance structures are not isomers, and the molecule does not flip between them. It is a single structure, and the double-headed arrow ↔ is used instead of the equilibrium arrow ⇌. Atoms do not move. Keto and enol forms are tautomers, which are different compounds in equilibrium, not resonance forms.