CBSE Class 11-science Answered

All resonance structures for the same molecule must have the same sigma framework (sigma bonds form from the "head on" overlap of hybridized orbitals). While drawing resonance structures for a molecule we are only allowed to move electrons. The positions of all nuclei must remain the same.

Equivalent resonance structures all contribute equally to the resonance hybrid. Nonequivalent resonance structures do not contribute equally; their relative stability determines how much they contribute. More stable structures contribute more; in other words, the molecule "looks more" like the more stable structure(s).

The hybrid structure is defined as the superposition of the resonance structures. A benzene ring is often shown with a circle inside a hexagon (in American texts) rather than alternating double bonds — the latter example misrepresents the electronic structure. Bonds with broken bond orders are often displayed as double bonds with one solid and one dashed line.

A molecule which exists as a mixture of resonance hybrids is more stable (has lower energy) than any one of the individual resonance structures and is often more stable than a molecule which cannot exist as a mixture of resonance structures.