According to the theory of resonance, if two or more Lewis structures with the same arrangement of atoms can be written for a molecule, ion, or radical, the actual distribution of electrons is an average of that shown by the various Lewis structures.

Resonance Structures and Resonance Hybrids

The Lewis structure of a nitrite anion (NO₂⁻) may actually be drawn in two different ways, distinguished by the locations of the N–O and N=O bonds.

If nitrite ionscontain a single and a double bond, the two bond lengths are expected to be different. A double bond between two atoms is shorter (and stronger) than a single bond between the same two atoms. However, experiments show that both N–O bonds in NO₂⁻ have the same strength and length, and are identical in all other properties. It is not possible to write a single Lewis structure for NO₂⁻ in which nitrogen has an octet, and both bonds are equivalent.

Instead, the concept of resonance is used: The actual distribution of electrons in each of the nitrogen–oxygen bonds in NO₂⁻ is the average of a double bond and a single bond.

The individual Lewis structures are called resonance structures. The actual electronic structure of the molecule (the average of the resonance forms) is called a resonance hybrid of the individual resonance forms. A double-headed arrow between Lewis structures indicates that they are resonance forms.

The carbonate anion, CO₃²⁻, provides a second example of resonance.

• One oxygen atom must have a double bond to carbon to complete the octet on the central atom.
• All oxygen atoms, however, are equivalent, and the double bond could form from any one of the three atoms. This gives rise to three resonance forms of the carbonate ion.
• Since three identical resonance structures can be written, the actual arrangement of electrons in the carbonate ion is known to be the average of the three structures.
• Again, experiments show that all three C–O bonds are exactly the same.

A molecule described as a resonance hybrid never possesses an electronic structure described by either resonance form. It does not fluctuate between resonance forms; rather, the actual electronic structure is always the average of that shown by all resonance forms.

This text is adapted fromOpenstax, Chemistry 2e, Chapter 7.4 Formal Charges and Resonance.