Several valid Lewis structures can often be drawn for one molecule or ion. Which criterion selects the most important contributor to the resonance hybrid?

Core Concept

Formal charge bookkeeping compares assigned electrons with free-atom valence, estimating how much charge each drawing forces onto atoms. Structures concentrating less charge sit lower in energy, and when charge is unavoidable, electronegativity decides where it rests most comfortably: negative charge belongs on the atom best able to hold electron density. Full octets come first, then minimal formal charges, then sensible charge placement.