When a single structural formula of a compound could not satisfactorily explain all the properties of the molecule then two or more hypothetical structure (arrived by redistribution of valence electrons) may be drawn and each of which separately accounts mass of its properties. These structures are called resonating structures or canonical structures.
The resonance hybrid may be defined as the actual structure of all the different possible structure that can be written for the molecule without changing the relative position of its atom and without violating rules of co-valence maxima for the atoms. Resonance hybrid is more stable than any other of resonating structure. The difference in energy between the hybrid and the most stable canonical structure is referred to as the resonance energy of the molecule.
The resonance energy of a resonance hybrid is the difference between the theoretical and experimental enthalpy of hydrogenation of the compound. The more the resonance energy the more stable is molecule.
1. Necessary conditions for Resonance
(i) All resonating structures must have the same arrangement of atomic nuclei.
(ii) The resonating structure must have the same number of paired and unpaired electrons, however they differ in the way of distribution of electrons.
(iii) the energies of the various resonating structures must be same or nearly same.
(iv) Resonating structure must be planar.
2. Contribution of resonating structures
(i) Non polar structures are more stable than the dipolar.
(ii) Stability is decreased if there is isolation of charges or increase in charge separation.
(iii) If all structures have formal charge the most stable one is that in which the negative and positive charges reside on the most electronegative and most electropositive atoms, respectively.
(iv) Resonating structures with electron-deficient or positively charged atoms have very high energy and hence are unstable.
(v) More the number of covalent bonds in a resonating structure more is the stability.