Question

what is the difference between isomerism and resonance ???

Answer 1

Resonance structures represent the same compound while isomers are chemically distinct. Converting from one isomer to another requires breaking σ (sigma) bonds and forming new σ bonds. Resonance structures are different approximate representations of the same molecule; structural drawings of isomers represent separate molecules. We say that a molecule has resonance when one single Lewis structure cannot account for its chemical and physical properties. Different resonance forms of a compound are often not energetically equivalent. Frequently one resonance form is more stable than another in this case the more stable resonance form(s) more closely represent the true nature of the molecule. In the case of benzene the resonance forms are all equivalent and we find that the true nature of the bonding in benzene is roughly a hybrid of double and single bonds between each carbon atom. But consider a molecule like diazomethane: