resonance

Sometimes a single Lewis structure just will not do the molecule justice. Take the nitrate ion, NO3-: you would draw one nitrogen-oxygen double bond and two single bonds, but experiment shows all three N-O bonds are identical. Resonance is the idea that when several equally good Lewis structures can be drawn, the real molecule is a blend — a hybrid — of all of them, not any one alone, and certainly not flipping rapidly between them.

Each individual drawing is called a resonance structure or contributor; they differ only in where the electrons (especially double bonds and lone pairs) are placed, never in where the atoms sit. The true molecule is a single, fixed thing whose electron distribution is the weighted average of the contributors — we say the electrons are delocalised, spread out over several atoms rather than pinned between two. In nitrate, the one double bond is really shared out over all three oxygens, so each N-O bond has a bond order of about one and one-third, intermediate between a single and a double bond, exactly as the equal bond lengths show.

Resonance is the everyday repair for the limitation of a single Lewis structure, and it appears all over inorganic chemistry: carbonate CO3 2-, sulfate, the oxoacids, ozone O3, and the benzene ring. It is honest to remember that resonance is a feature of the Lewis model, not a real motion: the molecule does not oscillate between forms. The deeper, quantitative account of the same delocalisation is given by molecular-orbital theory, with its delocalised pi systems.