Question

Explain the conditions for resonance.

Answer 1

Resonance is defined as the phenomenon of existence of a molecule in many structures due to delocalization of electrons (usually pie electrons). As a result of the delocalization of the electrons, a molecule can be represented by more than one structure, which differ from each other with respect to the distribution of the electrons.

For example, in the ozone molecule, there is one sigma bond and one pie bond. The electrons of the pie bond are delocalized, as a result of which ozone molecule can be represented by any of the two following structures:
(Check this attachment)

So for resonance, double bond or lone pairs of electrons must be present in the compound in alternate position.

   

Answer 2

Resonance occurs for stability of molecule. maxm books don't provide a clear idea about the conditions of reso. So during my jee preparation days i created a list which i still remember. following are the cases of conjugated system where resonance occur.
same occurs for triple bond. just replace the double bond by triple