resonance structure

Meaning: Resonance structures have the same atoms in the same positions and the same total electrons, but different placements of π bonds, lone pairs, and formal charges. Curved arrows show the electron movement between them. The actual molecule is a resonance hybrid, a weighted average of its contributors. Electrons are delocalized over several atoms, which lowers the energy (resonance stabilization). More important contributors have full octets, fewer formal charges, more bonds, and negative charge on the more electronegative atom. Examples: benzene, carboxylate, nitrate, amides, allylic cations.

On the MCAT: Expect questions that use resonance to rank stability or acidity. A carboxylic acid is more acidic than an alcohol because its conjugate base, the carboxylate, spreads the negative charge over two oxygens. Allylic and benzylic carbocations are more stable than similar non-delocalized ones. An amide C–N bond has partial double-bond character, so rotation is restricted and the peptide bond is planar. Aniline is a weaker base than an alkylamine because its lone pair is delocalized into the ring. Hybrid bond lengths fall between single and double.

Common confusion: Resonance structures are not isomers, and the molecule does not flip back and forth between them. Only the hybrid exists. Atoms never move, so tautomers (which differ in H position) are not resonance forms. Use a double-headed arrow ↔, not the equilibrium arrows ⇌.