Hybridisation

Hybridisation is the phenomenon of redistribution of energies of the orbitals of slightly different energies so as to give a new set of orbitals of equivalent energies. The new orbitals are called hybrid or hybridised orbitals. The number of hybridized orbitals formed is equal to the number of atomic orbitals taking part in hybridisation.This phenomenon is more predominant in carbon containing compounds and so to understand this concept, a study of the electronic structure of carbon is essential.

Tetravalency of Carbon

Carbon forms a large number of fascinating variety of compounds. This is because of its unusual property of catenation in which one carbon unites with the other to form long chains and rings. It is this property, which is responsible for the existence of millions of compounds of carbon. This feature can be explained on the basis of tetravalency of carbon.

The electronic configuration of carbon is 1s² 2s² 2p_x¹ 2p_y¹ 2p_z⁰. In the box notation this is represented as:

As there are two half filled orbitals in the valence shell of carbon, its bonding capacity should be two. However, in actual practice carbon exhibits a bonding capacity of four and forms molecules of the type CH₄, CCl₄ etc. In order to explain this tetravalency, it is proposed that one of the electrons from the '2s' filled orbital is promoted to the empty '2p' orbital (2pz), which is in a higher energy state. In this way four half filled orbitals are formed in the valence shell which can account for the bonding capacity of four bonds of carbon. This state is known as the excited state in which the electronic configuration of carbon is:

From the above configuration it is clear that all the four bonds of carbon will not be identical. This is because one bond will be formed by the overlap of '2s' orbital which will have more of 's' character. The other three bonds will be formed by the overlap of the '2p' orbitals, which will have more of 'p' character. Therefore all the four bonds will not be equivalent.

But in practice, most of the carbon compounds have all the four bonds equal. This behaviour can be explained in terms of Hybridisation.

Characterisitics of hybridisation

• The hybridised orbitals are always equivalent in energy and shape.

• The number of hybridised orbitals formed is equal to the number of orbitals that undergo hybridisation.

• Hydridised orbitals form more stable bonds.

• Hydridised orbitals orient themselves in preferred directions in space and so give a fixed geometry or shape to the molecules.

Conditions for hybridisation

• Only the valence shell orbitals of the atom are hybridised.

• Orbitals undergoing hybridisation should have only a small difference in their energies.

• It is not necessary that only half filled orbitals participate in hybridisation. Even filled orbitals of the valence shell can take part in hybridisation.

• Rearrangement by way of promotion to different orbitals is not an essential condition for hybridisation.