Q.C. Buta-1,3-diene

Occasionally, honestly applying the Lewis-structure rules to a single molecule or ion still produces MORE than one equally valid structure — not because any of them is wrong, but because no single dot-and-line diagram can capture the molecule's true electronic structure. The carbonate ion, CO₃²⁻, is the standard example: three Lewis structures can be drawn, identical in the relative positions of the atoms, differing only in WHICH of the three oxygens carries the C=O double bond. Such structures are called resonance structures (or canonical structures), and the phenomenon is resonance.

Experimentally, all three C-O bonds in carbonate are found to be perfectly EQUIVALENT — a length intermediate between a single and a double bond. The ion's real structure is therefore a resonance hybrid: a weighted average of the contributing canonical structures, not an actual physical flickering between them (the ion does not switch from one canonical form to another; the hybrid IS the molecule, at all times). No single Lewis structure can depict the hybrid exactly, but a picture with fractional (partial) bonds to all three oxygens and a fractional −2/3 charge on each gives a fair qualitative sense of it. …