Resonance

The phenomenon of resonance was put given by Heisenberg to explain properties of the some of the molecules.

In case of the certain molecules, a single Lewis structure cannot explain all properties of molecule. The molecule is then assumed to have several structures, each of which can explain almost all the properties of the molecule but none can explain all properties of the molecule. The actual structure of it is in between of all these structures which are contributing and is known as resonance hybrid and the different individual structures are called as resonating structures or the canonical forms. This phenomenon is termed as resonance.

         To Explain this, consider a molecule of the ozone O₃                                                   . 

As the resonance hybrid of the above two structures (a) and (b). For simplicity, ozone can be represented by structure (c), which shows resonance hybrid having equal bonds between the single and double.

         Resonance is generally shown by the structures of benzene, toluene, O₃, allenes (>C = C = C<), CO, CO₂, , SO₃, NO, NO₂ while it is not shown by the structures of H₂O₂, H₂O, NH_­3, CH₄, SiO₂.

         As a result of the resonance, the bond lengths of single and double bond in the molecule become equal for example O-O bond lengths in the molecule of ozone or C-O bond lengths in     ion.

         The resonance hybrid has the lower energy and therefore greater stability than any of contributing structures.

         Greater is number of canonical forms especially with the nearly same energy, greater is stability of the molecule.

         The difference among the energy of resonance hybrid and that of the most stable of resonating structures (possesing the least energy) is called resonance energy. Therefore,

                 The Resonance energy = Energy of resonance hybrid - Energy of the most stable of resonating structure.

In case of the molecules or ions having resonance, the bond order changes and which can be calculated as follows,

In Carbonate ion

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