Resonance

Resonance represents a molecule or ion whose electron distribution cannot be captured by one Lewis structure as a weighted combination of contributing structures. The real species is a single resonance hybrid, not a structure switching among contributors.

Resonance hybrid: A molecular representation whose electron distribution reflects contributions from multiple Lewis structures. It is the model of the real species, rather than any single contributor.
Lewis structure: A diagram showing valence electrons as bonds and lone pairs around atoms. Resonance starts from Lewis structures that differ only in electron placement.
Benzene: An aromatic hydrocarbon with six carbon atoms arranged in a planar ring and six delocalized pi electrons. Its equivalent carbon–carbon bonds cannot be represented by either alternating-bond Lewis structure alone.
Tautomerism: The interconversion of constitutional isomers by movement of a proton and a shift in bonding. Unlike resonance, tautomerism changes atom positions and describes distinct chemical species.
Resonance contributor: One Lewis structure included in a resonance description of a molecule or ion. Contributors differ in electron placement while retaining the same atom arrangement.
Octet rule: The chemical principle that many main-group atoms form bonds to acquire eight valence-shell electrons. Valid contributors usually preserve suitable valence-shell electron counts.
Carbonate ion: The polyatomic ion CO₃²⁻, with three oxygen atoms bonded to one carbon atom. Three equivalent contributors describe its equivalent carbon–oxygen bonds.
Isomerism: The existence of compounds with the same molecular formula but different arrangements of atoms or stereochemistry. Resonance contributors are not isomers because their atoms retain the same connectivity and positions.
Formal charge: An atom's assigned charge in a Lewis structure, calculated by equally dividing bonding electrons between bonded atoms. Comparing formal charges helps assess the relative importance of contributors.
Valence electron: An electron in an atom's outer shell that can participate in bonding or remain as a lone pair. Contributors rearrange valence electrons while keeping the total electron count fixed.

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