↪ There are molecules and ions for which drawing a single Lewis structure is not possible.
For example, we can write two structures of O3.
↪ In (A) the oxygen-oxygen bond on the left is a double bond and the oxygen-oxygen bond on the right is a single bond.
↪ In B the situation is just the opposite. The experiment shows, however, that the two bonds are identical.
↪ Therefore neither structure A nor B can be correct.
↪ One of the bonding pairs in ozone is spread over the region of all three atoms rather than localized on a particular oxygen-oxygen bond.
↪ This delocalized bonding is a type of chemical bonding in which bonding pair of electrons are spread over a number of atoms rather than localized between two.
↪ Structures (A) and (B) are called resonating or canonical structures and (C) is the resonance hybrid.
↪ So, in many compounds and ions (e.g., O3, C6H6, CO2, NO etc.), a single valence bond structure cannot explain all the properties of the molecule or ion.
↪ Bonding in such compounds/ions can be explained by the concept of resonance.
↪ The phenomenon in which a molecule or ion cannot be represented by a single structure rather it should be represented by more than one structures to explain its properties, is called resonance.
↪ More than one such structures of the same molecule or ion are called resonating structures.
↪ None of these structures correctly represent the actual structure of molecule or ion.
↪ The real structure of the molecule or ion, is hybrid of these structures, called Resonance hybrid.
↪ The resonating structures are formed due to the delocalization of π-bond or lone pair of electrons or odd electrons.
Some other examples
The difference in the energies of the canonical forms and resonance hybrid is called resonance stabilization energy.
Characteristics of Resonance
↪ Canonical forms have no real existence. Only resonance hybrid has the real existence.
↪ Resonance hybrid has lower energy than any contributing structure. So it is more stable.
↪ Greater is the number of canonical forms with nearly same energy, greater is the stability of the species.
↪ Greater the resonance energy, greater is the stability of the molecules.