For other uses, see Resonance (disambiguation).
Two of the contributing structures of nitrogen dioxide
In chemistry, resonance or mesomerism  is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula. A molecule or ion with such delocalized electrons is represented by several contributing structures  (also called resonance structures or canonical forms).
Each contributing structure can be represented by a Lewis structure, with only an integer number of covalent bonds between each pair of atoms within the structure. Several Lewis structures are used collectively to describe the actual molecular structure. However these individual contributors cannot be observed in the actual resonance-stabilized molecule; the molecule does not oscillate back and forth between the contributing structures, as might be assumed from the word "resonance". The actual structure is an approximate intermediate between the canonical forms, but its overall energy is lower than each of the contributors. This intermediate form between different contributing structures is called a resonance hybrid. Contributing structures differ only in the position of electrons, not in the position of nuclei.
Resonance is a key component of valence bond theory.
Electron delocalization lowers the potential energy of the substance and thus makes it more stable than any of the contributing structures. The difference between the potential energy of the actual structure and that of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy.