4.6 Hybridisation

In order to explain the characteristic geometrical shapes of polyatomic molecules like CH4, NH3 and H2O etc., Pauling introduced the concept of hybridisation. According to him the atomic orbitals combine to form new set of equivalent orbitals known as hybrid orbitals. Unlike pure orbitals, the hybrid orbitals are used in bond formation. The phenomenon is known as hybridisation which can be defined as the process of intermixing of the orbitals of slightly different energies so as to redistribute their energies, resulting in the formation of new set of orbitals of equivalent energies and shape. For example when one 2s and three 2p-orbitals of carbon hybridise, there is the formation of four new sp3 hybrid orbitals.

Salient features of hybridization: The main features of hybridisation are as under :

  1. The number of hybrid orbitals is equal to the number of the atomic orbitals that get hybridised.
  2. The hybridised orbitals are always equivalent in energy and shape.
  3. The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.
  4. These hybrid orbitals are directed in space in some preferred direction to have minimum repulsion between electron pairs and thus a stable arrangement. Therefore, the type of hybridisation indicates the geometry of the molecules.

Important conditions for hybridisation

  • The orbitals present in the valence shell of the atom are hybridised.
  • The orbitals undergoing hybridisation should have almost equal energy.
  • Promotion of electron is not essential condition prior to hybridisation.
  • It is not necessary that only half filled orbitals participate in hybridisation. In some cases, even filled orbitals of valence shell take part in hybridisation.

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