Delocalization is systems of electron in which bonding electrons are not localised between two bonding atoms forming a single bond but are spread (delocalized) over the whole group. These electrons are called delocalized electron.
Delocalization generally (not always) occurs with pi – electron present in pi – bond.
Resonance structures are the structures of the molecules which have pi– electrons delocalized. These molecules can not be represented by one structure and are represented by combination of two or more structure called resonance structure. The characteristic properties of the molecule are represented by average properties of all the resonating structure.
Resonance energy is the difference in energy of real molecule and the energy of most stable resonating structure of the molecule. The energy of real molecule is always lower than the energy of any resonating structure. The energy of all resonating structure is not same but differs from each other. Real molecule is different from any of the resonating structure. It is not one of the resonating structures but has average properties of all contributing structure.