What are the nesscery conditions for …

In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure.Contributing structures differ only in the position of electrons, not in the position of nuclei.

Electrons have no fixed position in atoms, compounds and molecules , but have probabilities of being found in certain spaces (orbitals). Resonance forms illustrate areas of higher probabilities (electron densities). The term Resonance is applied when  there are two or more possibilities available. Resonance structures do not change the relative positions of the atoms For a compound to show resonating structures , it should

1) have a planar structure

2) follow Hackel's rule of electron numbers

3) hybridization remains same and fixed

4) no of lone pairs remain same .