Hybridisation 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.
Salient features of hybridisation : The main features of hybridisation are as under :
(i) The number of hybrid orbitals is equal to the number of the atomic orbitals that undergo hybridisation.
(ii) The hybrid orbitals are always equivalent in energy and shape.
(iii) The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.
(iv) These hybrid orbitals are directed in space in some preferred directions to have minimum repulsion between electron pairs and thus have a stable arrangement. Therefore, the type of hybridisation indicates the geometry of the molecules.
Important conditions for hybridisation :
(i) Only those orbitals that are present in the valence shell of the atom undergo hybridisation.
(ii) The orbitals undergoing hybridisation should have almost equal energy.
(iii) Promotion of electron is not essential condition prior to hybridisation.
(iv) 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.
Shapes of hybrid orbitals :
(i) sp-hybridisation : One ‘s’ and one ‘p’ orbital of an atom intermix giving two sp-hybrid orbitals making an angle of 180° with each other.
Each hybrid orbital has 50% s and 50% p-orbital character. The molecules having such type of hybridisation in their central atom have linear geometry with bond angles 180°.
Examples : BeCl2,C2H2,CO2 , HgCl2,CS2,N2O,BeH2 , etc.

(ii) sp2 - hybridisation : One ‘s’ and two ‘p’ orbitals of an atom intermix giving three sp2 - hybrid orbitals, which are directed towards the corners of an equilateral triangle making an angle of 120° with each other. Each hybrid orbital has 33.3% s and 66.7% p-orbital character. The molecules having such type of hybridisation in their central atom are expected to have triangular planar geometry with bond angles 120°.
Examples : BCl3,BF3,SO3 , C2H4 , Graphite, SO2 , etc.

(iii) sp3 - hybridisation : One ‘s’ and three ‘p’ orbitals of an atom intermix giving four sp3 - hybrid orbitals, which are directed towards the corners of a regular tetrahedron making an angle of 109°28′. Each hybrid orbital has 25% s- and 75% p-orbital character. The molecules having such type of hybridisation in their central atom are expected to have tetrahedral geometry with bond angles 109°28′.
Examples : CH4,C2H6,CCl4,H2O,NH3 , diamond, etc.