chemical bonding

Resonance Theory |Example of resonance in chemistry | Benzene | Ozone

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What is resonance theory?

Resonance Theory:

According to resonance theory some molecules are represented by two or more than two structures and the actual structure is weighed average of these and is called resonance hybrid.

There are certain molecules which cannot be represented by single structure, so we write various possible structures for such molecules. These possible structures are called resonating structures or canonical structures. The real structure is weight average of these resonating structures and is called resonance hybrid. The resonance hybrid is imaginary structure which is obtained by mental combination of resonating structures.

Each resonating structure contributes to real structure. Some structures contribute more (Major contributors) and other contributes less (Minor contributors). More is the stability of a resonating structure more is its contribution to resonance hybrid (real structure).

Who discovered resonance theory?

Ingold in 1933 called this phenomenon as mesomerism, later; Heisenberg explained it theoretically and called it as resonance.

Necessary conditions for resonance are:

  • The position of nuclei of atoms in each structure must be the same.
  • The structures differ only in position of electrons.
  • The number of unpaired electrons in each structure must be the same.

The resonance hybrid is always lower in energy than resonating structures, therefore, it is stable. The difference in energy of resonance hybrid and that of most stable resonating structures is called resonance energy. Higher is the resonance energy, more will be stability of the compound.

Example of Resonance in Chemistry

Ozone O3 Resonating Structures:

Lewis gave two resonating structures for Ozone, which are following:

                  ..     ..    ..                ..    ..    ..     

                  O = O – O׃      ↔      ׃ O – O = O   

                  ··          ··                ··          ··

But actual structure of ozone is weight average of these resonating structures and is called resonance hybrid. The resonance hybrid is imaginary structure which is obtained by mental combination of resonating structures.

            Benzene C6H6 Resonating Structures :

             There are several resonating structures for benzene which are following:

But actual structure of Benzene is weight average of these resonating structures and is called resonance hybrid. The resonance hybrid is imaginary structure which is obtained by mental combination of resonating structures.

Kekule structures are main contributing structures their contribution to resonance hybrid is 80% that of Dewar structures is 19% while other structures contribute only 1%.

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