A resonance may be defined as a phenomenon in which a single compound is supposed to be existing as a hybrid of two or more compounds differing in the distribution of electrons and not of atoms. These different structures of the molecule are known contributing structures or resonating structures or canonical forms. The actual structure that is intermediate between all the contributing structures is called resonance hybrid. Different contributing structures are written by putting a double head arrow ( ↔ ) between them.
Resonance in benzene: Benzene ring has three double bonds in it and is expected to be quite reactive. But benzene is extremely stable. The stability of benzene is explained in terms of resonance. Benzene molecule is a resonance hybrid of the following two main contributing structures: