Resonance structure

A resonance structure is one of several Lewis structures used to represent a molecule whose electrons are delocalized. The molecule is not switching among these drawings; its actual electronic structure is described by their combination.

Resonance hybrid: A representation of a molecule’s actual electron distribution as a combination of its resonance structures. It captures the shared electronic structure that the individual drawings approximate.
Lewis structure: A diagram showing valence electrons as bonds and lone pairs around atoms. Resonance structures are alternative Lewis diagrams of the same atomic arrangement.
Benzene: An aromatic hydrocarbon with six carbon atoms arranged in a planar ring and formula C₆H₆. Its two Kekulé structures depict equivalent placements of alternating double bonds.
Tautomer: One of two isomers that interconvert by moving a proton and shifting a double bond. Tautomers are distinct compounds in equilibrium, unlike resonance structures of one compound.
Electron delocalization: The spreading of electrons across multiple atoms rather than confinement to a single bond or atom. Delocalization is why a molecule may need more than one Lewis structure.
Valence electron: An electron in an atom’s outer shell that can participate in chemical bonding. Only valence-electron placement changes between resonance structures.
Carbonate ion: A polyatomic ion, CO₃²⁻, with one carbon atom bonded to three oxygen atoms. Its three equivalent resonance structures distribute the negative charge among oxygen atoms.
Conformational isomerism: The existence of different molecular conformations produced by rotation around single bonds. Conformers differ in atom positions; resonance drawings keep the atomic framework fixed.
Curved-arrow notation: A notation showing the movement of electron pairs during a chemical transformation or between resonance structures. Curved arrows show how electron placement changes from one drawing to another.
Octet rule: The guideline that many main-group atoms form bonds to achieve eight valence-shell electrons. It constrains which electron arrangements are plausible Lewis structures.

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