Sometimes several valid Lewis structures have the same atomic connectivity but differ in where electrons are placed. These are resonance structures.

For the nitrate ion, $NO_3^-$, one Lewis structure can place the $N=O$ double bond to any one of the three oxygen atoms. The real ion does not rapidly switch among three different molecules. Instead, its electron distribution is delocalized over the equivalent bonds.

Resonance structures are therefore alternative bookkeeping representations of one electronic structure. They are connected by a double-headed resonance arrow and differ only in electron placement, not in atom positions.

A useful resonance description obeys three rules:

• the nuclei keep the same connectivity;
• the total number of valence electrons and overall charge remain unchanged;
• each contributor must itself be a valid Lewis structure.

Contributors with more favorable formal-charge patterns usually have greater importance, but equivalent contributors contribute equally.

Delocalization often makes real bonds intermediate between the single- and multiple-bond patterns suggested by any one contributor. Resonance thus explains why a single localized Lewis structure can be useful yet incomplete.