Resonance
The description of a molecule whose true structure is a weighted average of two or more valid Lewis structures.
Formula / equation
A ↔ B ↔ C
Unit
Dimensionless
In depth
Resonance structures differ only in the placement of electrons, never of nuclei, and none of them exists on its own — the real species is a single hybrid intermediate between them. Delocalising electrons over more atoms always lowers the energy, so resonance-stabilised species are unusually stable.
Examples in the real world
The carbonate ion's three equivalent C–O bonds, each of order 1.33; benzene's equal 139 pm C–C bonds.