In chemistry, resonance or mesomerism ^[1] 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 ^[2] (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.^[3] 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.^[4] 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^[5] or delocalization energy.

Resonance is distinguished from tautomerism and conformational isomerism, which involve the formation of isomers, thus the rearrangement of the nuclear positions.

General characteristics of resonance

Molecules and ions with resonance (also called mesomerism) have the following basic characteristics:

Contributing structures of the carbonate ion

• They can be represented by several correct Lewis formulas, called "contributing structures", "resonance structures" or "canonical forms". However, the real structure is not a rapid interconversion of contributing structures. Several Lewis structures are used together, because none of them exactly represents the actual structure. To represent the intermediate, a resonance hybrid is used instead.
• The contributing structures are not isomers. They differ only in the position of electrons, not in the position of nuclei.
• Each Lewis formula must have the same number of valence electrons (and thus the same total charge), and the same number of unpaired electrons, if any.^[6]
• Bonds that have different bond orders in different contributing structures do not have typical bond lengths. Measurements reveal intermediate bond lengths.
• The real structure has a lower total potential energy than each of the contributing structures would have. This means that it is more stable than each separate contributing structure would be.