State and explain hybridisation. 

The phenomenon of intermixing of atomic orbitals of slightly different energies of an atom so as to redistribute their energies to form the same number of new orbitals of equivalent energies and identical shapes is called hybridization. The new orbitals, thus formed are called hybrid orbitals or hybridised orbitals.
Explanation: In order to understand hybridisation, let us take an example of carbon (Z = 6). Its ground state electronic configuration is

Since it has two filled orbitals, therefore, the valency of the carbon atom should be 2. But actually, carbon atom always exhibits a valency of four (tetravalent). To achieve this, an electron is promoted from 2s filled orbital to the vacant higher energy 2p_zorbital. This is called excited state of a carbon atom.

In the excited state of carbon and p orbitals have different energies. Consequently, four bonds of carbon must be of two types. Three of the bonds should be of one type (s-p bonds) while the fourth bond should be of a different type (s-s bond). However, experimental evidence indicates that all the four bonds in case of CH₄ (methane) are equivalent. To explain the equivalence of all the four bonds in case of methane, the concept of hybridisation is used i.e. all the four orbitals in the valence shell of carbon may get mixed, redistribute energies and give orbitals of new energy and shape. These equivalent orbitals are called hybrid orbitals.