Resonance
It has been found that the experimentally observed properties of certain compounds cannot be satisfactorily explained by writing a single Lewis structure.
For example, the electronic structure of ozone (O3) can be written by two Lewis structures I and II in which each oxygen atom has an octet of electrons.
If we consider any of these structures I and II, the O=O bond (121 pm) should be shorter than O-O (148 pm) bond. Thus, the molecule should exhibit two bond lengths. But experiments show that both the oxygen bonds are equal and the bond length (128 pm) is intermediate of single and double bonds.
In order to explain the structure of such types of molecules the concept of resonance was introduced. Whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and lone pairs of electrons are taken as the canonical structures of the hybrid which describes the molecule accurately. So, the actual structure of ozone is intermediate of structures I and II and is called resonance hybrid. The structure I and II are called resonating structures or canonical forms. The resonance between canonical forms can be represented by a double headed arrow (↔ ). Although it is not possible to draw the Lewis structures for the resonance hybrid, but structure III represents the structure of O3 more accurately.
It may be borne in mind that actual structure does not oscillate between the resonating forms and has its individual identity.
Characteristics of Resonance:
1. Resonance structures are hypothetical and do not have real existence.
2. The actual molecule has a single structure which is resonance hybrid of various canonical forms and as such it cannot be depicted by a single Lewis structure.
3. Bond lengths in hybrid structures are intermediate of the bond lengths in various resonating forms. For example, C-C bond length in benzene is 139 pm which is intermediate of C-C (154 pm) and C=C (134 pm).
4. Resonance hybrid is most stable, i.e., least energetic than any of the resonating forms. Hence, higher is resonance energy, more will be the stability of the molecule.