The phenomenon of resonance was put forward by Heisenberg to explain the properties of certain molecules.
In case of certain molecules, a single Lewis structure cannot explain all the properties of the molecule. The molecule is then supposed to have many structures, each of which can explain most of the properties of the molecule but none can explain all the properties of the molecule. The actual structure is in between of all these contributing structures and is called resonance hybrid and the different individual structures are called resonating structures or canonical forms. This phenomenon is called resonance.
To illustrate this, consider a molecule of ozone . Its structure can be written as
As a resonance hybrid of above two structures (a) and (b). For simplicity, ozone may be represented by structure (c), which shows the resonance hybrid having equal bonds between single and double.
Resonance is shown by benzene, toluene, O3, allenes (>C = C = C<), CO, CO2, , SO3, NO, NO2 while it is not shown by H2O2, H2O, NH3, CH4, SiO2.
As a result of resonance, the bond lengths of single and double bond in a molecule become equal e.g. O–O bond lengths in ozone or C–O bond lengths in ion.
The resonance hybrid has lower energy and hence greater stability than any of the contributing structures.
Greater is the number of canonical forms especially with nearly same energy, greater is the stability of the molecule.
Difference between the energy of resonance hybrid and that of the most stable of the resonating structures (having least energy) is called resonance energy. Thus,
Resonance energy = Energy of resonance hybrid – Energy of the most stable of resonating structure.
In the case of molecules or ions having resonance, the bond order changes and is calculated as follows,