What is resonance ? give significance of resonance and resonance energy.

What is resonance ?

When a molecule can not be completely represented by a single structure but its characteristic properties can be described by two or more different structures , then the true structure is said to a resonance hybrid of these structure and this phenomenon is called resonance.

              The various structures are called resonating structures.

                          For example, the resonating structure of benzene.

Significance of resonance.

As a result of resonance, the energy of the molecule becomes lower than the the energy of any of the resonating structures. The molecule with a lower energy has higher stability and also higher bond energy. 

Thus, resonance gives the extra stability to the molecule, carbocation or carbanion.
Foe example, resonance hybrid of butadiene and cyclo butadiene.

What is resonance energy ?

The resonance hybrid of an entity is a more stable structure than any one of the resonating structures contributing to it. The resonance is a measure of the extra stability of the resonance hybrid.

The resonance energy is defined as the difference in energy between the actual structure of the entity and the most stable of the hypothetical structures.

Calculation of resonance energy.

Let us assume that E1,E2 and E3 are the energies of the three resonating structures of naphthalene molecule and among these E3 is the lowest energy corresponding to the most stable contributing structure. 

If Eо is the actual energy of the naphthalene,then resonance energy of naphthalene molecule is E3 - Eо . It has been found that the experimentally calculated value of resonance energy of naphthaleme molecule is 255.8 Kcal /mol

Summary: 

What is resonance ? give significance of resonance energy.
Calculation of resonance energy .

Why C - O bond length of phenol is less than methanol ? Click here