Rules for valid resonance structures:

1. Same atomic connectivity (only electrons move, not atoms)
2. Same number of total electrons (paired + unpaired)
3. Same charge distribution (total formal charge conserved)
4. All atoms must obey octet rule (except for elements like B, empty d-orbitals for S, P)
5. More covalent bonds = more stable contributing structure

Resonance energy: Difference between actual energy and energy of the most stable contributing structure. Benzene: ~150 kJ/mol resonance energy.

JEE tip: When comparing resonance structures, one with charge on more electronegative atom is more stable. Structures with separated charges are less stable than neutral ones.