When one Lewis structure is inadequate, the real molecule is a hybrid of several contributing structures.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
Resonance occurs when a molecule or ion can be described by two or more valid Lewis structures that differ only in the placement of electrons, not in the positions of the nuclei. No single structure describes the real molecule, which is a hybrid of all its contributors. Electrons are delocalized over several atoms, which lowers the energy and adds stability compared with any one contributor. Resonance structures are connected by double-headed arrows, which mean that they are contributing forms rather than species in equilibrium. The real bond lengths are intermediate between those predicted by the individual structures.
Key ideas: To draw resonance forms, move only electrons (lone pairs and pi bonds), never atoms, and keep the total charge and electron count constant. Count the contributors to get the average bond order: nitrate has three forms, so each N-O bond has order 4/3. Equivalent contributors imply equal bond lengths.
Common mistakes and connections: The most frequent misconception is that the molecule flips between forms, or that it spends time as one form then another. Another is drawing the arrow of an equilibrium between them. The real structure is one species. Links to the resonance hybrid, delocalization, aromaticity and the acidity of carboxylic acids.
Ozone is described by two Lewis structures that differ only in where the double bond sits. Neither alone is real; the molecule is a blend of both. The interactive lesson shows this as a live animation or chart.
In ozone both O-O bonds are 128 pm long, between the expected single and double bond lengths.
When one Lewis structure is inadequate, the real molecule is a hybrid of several contributing structures.
Resonance is a intermediate topic in Bonding & Molecular Structure. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Bonding & Molecular Structure first: Single Double and Triple Bonds, Lone Pairs, Formal Charge.
The next topics in Bonding & Molecular Structure are Resonance Hybrid and Expanded Octet.
Yes. The interactive lesson includes a self-check question on Resonance, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Resonance is topic 12 of 104 in Bonding & Molecular Structure.