Resonance and Resonating Structure: In the case of some compounds, all the observed properties cannot be explained by a single structure so we use more than one structures of the compound. These different structures are called resonance structures or canonical structures or contributing structures. The phenomenon is known as resonance. The contributing structures are written by keeping the following points is mind.
(i) In each contributing structure, the position of atoms should be same, only the arrangement of electrons may change.
(ii) Each structure must have the same number of unpaired electrons.
(iii) The contributing structures should have energies close to each other. The contribution of a less stable contributing structure is also less.
(iv) A more stable contributing structure is one in which the negative charge resides on most electronegative atom and positive charge on most electropositive atom.
(v) Various contributing structures are written with a double headed arrow between them.
· Characteristics of Resonance:
i) Resonance stabilizes the molecule. As the number of resonating structures increases, the stability also increases.
ii) Resonance changes the bond length.
iii) The difference in energy between the actual structure and the most stable canonical structure is called resonance energy. Greater the resonance energy, stabler will be the molecule.
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