Illustrated Glossary of Organic Chemistry

Resonance contributor (resonance structure; canonical form; canonical structure): One of a set of Lewis structures that differ only in the distribution of electrons in covalent bonds and lone pairs. The total number of electrons and spatial position of the atoms are identical, but the formal charges, number of lone pairs, and number and type of covalent bonds may differ. The weighted average of resonance contributors gives the resonance hybrid. Resonance contributors are separated by a single-shafted, double-headed arrow. Electron reassignment can optionally be indicated with single-shafted, single-headed curved arrows.

Acetate ion resonance contributors. The first two contributors are equally significant (they don't violate any of the resonance contributor preference rules). The third contributor is not significant because it violates three resonance contributor preference rules. Non-significant resonance contributors are usually not included in the resonance hybrid (shown below).

Acetate ion resonance hybrid.