Describe resonance effect with one example.

Explanation

It is not possible to explain the properties of a molecule by giving a single structure to It. Then several possible structures are to be proposed. Each structure will explain some properties of the molecule. All structures put together explain all the properties. This phenomenon is known as resonance. All the structures thus proposed are called resonance or canonical structures.
Ex: Urea `CO(NH_(2))_(2)` has the following resonance (or canonical) structures.

Sallent features of resonance :
1) Resonance structures are interconvertable.
2) Resonance Involves only displacement of electrons without disturbing the positions of atoms.
3) All atoms in the molecule should lie in the same plane.
4) The no. of paired and unpaired electrons should be the same in all the structures.
5) The canonical structures should have nearly equal energy.
6) More stable resonance structure contributes more to the actual structure of the molecule.
7) More the delocalisation of electrons, more is the stability.
8) More the covalent bonds, more is the stability and charge separation gives less stability.
Resonance effect: It is the polarity produced in a molecule by the interactions of two `pi` bonds or between a `pi` bond and a lone pair of electrons present on adjacent atoms. This effect is transmitted throughout the chain.
If the transfer of electrons is away from the atoms of substituent groups attached to the conjugated system, then the molecule gets high electron density in some of its positions as in aniline and it is given (+R). If the shift of electrons is towards the atom or substituent groups, it is (-R) as in nitrobenzene.

Resonance Energy: The energy dilfference between the real structure and the most stable resonance structure is called resonance stabilisation energy or resonance energy.