Resonance in chemistry is a method of describing the electronic structure of a molecule or polyatomic ion when one localized bonding diagram is insufficient. Two or more contributing structures, usually drawn as Lewis structures, collectively describe a resonance hybrid. The contributors are not separate species between which the substance switches: they are alternative representations used to describe one electronic state. Resonance is closely associated with electron delocalization and forms an important part of valence bond theory. The term mesomerism is essentially synonymous with resonance. (goldbook.iupac.org)
Contributing structures and notation
A Lewis structure assigns electrons to individual chemical bonds or nonbonding positions. This is useful bookkeeping, but some electronic distributions extend across several atoms. Resonance structures retain the same nuclear arrangement while placing bonding electrons, lone pairs, and formal charges differently. The resulting hybrid may have equivalent bonds where an individual drawing suggests alternating single and double bonds. (openstax.org)
Contributors are connected by a double-headed arrow, ↔. This notation does not indicate chemical equilibrium between distinct substances. Curved arrows within the drawings show how electron pairs are reassigned to generate another contributor. They begin at a lone pair or bond and end at an atom or bonding position. In a resonance diagram, these arrows describe a change in representation, not a sequence of events in a chemical reaction. (openstax.org)
Rules and relative importance
Valid resonance contributors preserve atomic connectivity, total electron number, and overall charge. In common organic examples, they differ in the placement of π-bond electrons and lone pairs while retaining the σ-bond framework. Moving an atom or transferring a hydrogen nucleus produces a different structure rather than another resonance contributor. Normal valence restrictions remain applicable; second-period atoms cannot be given more than eight valence-shell electrons in an ordinary Lewis representation. (openstax.org)
Contributors need not be equally important. Common guidelines favor complete octets where possible, smaller formal charges, and negative charge on more electronegative atoms. Formal charge is a bookkeeping assignment based on equal sharing of bonding electrons, not a direct measurement of local charge. Equivalent contributors have equal importance by symmetry; nonequivalent contributors generally contribute unequally. Consequently, the hybrid is not necessarily a simple arithmetic average of its drawings. (openstax.org)
These guidelines require context rather than mechanical application. A charge-separated contributor may remain chemically informative even when a neutral contributor dominates. Counting drawings alone does not quantify stabilization: contributors differ in electronic character and relative importance. (openstax.org)
Quantum-mechanical interpretation
In quantum mechanics, resonance corresponds to combining the wave functions associated with different valence-bond descriptions. Schematically, the electronic wave function can be written as a linear combination:
where represents a contributing electronic description and is its coefficient. The mixture describes the molecular state; it is not a temporal alternation among classical bonding patterns. This interpretation gives the resonance concept a basis beyond the conventions of structural drawing. (goldbook.iupac.org)
Resonance energy denotes the difference in potential energy between the actual molecular entity and its lowest-energy contributing structure. Because an isolated contributor is not an observable molecular entity, this difference cannot be measured directly and must be estimated using an appropriate reference description. Experimental energetic comparisons can reveal stabilization, but the numerical estimate depends on the comparison chosen. (goldbook.iupac.org)
Representative examples
Acetate. The acetate ion is commonly represented by two equivalent contributors:
Each drawing assigns a double bond to one oxygen and a single bond to the other. The actual ion has two equivalent carbon–oxygen bonds, with the electronic distribution extending over the carboxylate group. It does not contain a double bond that repeatedly jumps between oxygen atoms. (openstax.org)
Carbonate. The carbonate ion, , has three equivalent principal contributors, each placing the carbon–oxygen double bond at a different oxygen. Its three carbon–oxygen bonds are equivalent. Averaging these conventional drawings gives a formal bond order of for each bond, illustrating why integer bond orders in individual Lewis structures need not describe the hybrid adequately. (openstax.org)
Benzene. The two familiar Kekulé contributors of benzene show alternating single and double bonds in different positions. Benzene instead has six equivalent carbon–carbon bonds and a delocalized six-electron π system. Each carbon supplies a perpendicular p orbital that overlaps with neighboring orbitals around the ring. This cyclic electronic arrangement is central to aromaticity, which requires more than merely drawing multiple contributors. (openstax.org)
Amides. An amide can be represented by a neutral carbonyl contributor and a charge-separated contributor in which nitrogen donates its lone pair toward the carbonyl carbon, leaving negative formal charge on oxygen and positive formal charge on nitrogen. This delocalization helps explain why amide nitrogen is much less basic than nitrogen in an ordinary amine: nitrogen protonation disrupts the stabilizing electronic interaction. (openstax.org)
Chemical consequences and scope
Resonance provides a practical framework for interpreting bond equivalence, charge distribution, stability, and reactivity. A substituent whose p or π orbitals overlap with those of the remaining molecular framework can extend delocalization and alter reaction rates or ionization equilibria. This influence is called the mesomeric effect or resonance effect. Its operation depends on orbital overlap, not simply on the presence of a formal multiple bond in a drawing. (goldbook.iupac.org)