State and explain hybridisation.
The phenomenon of intermixing of atomic orbitals of slightly different enthalpies of an atom so as to redistribute their enthalpies to form the same number of new orbitals of equivalent enthalpies 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 half 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 enthalpy 2p orbital. This is called excited state of a carbon atom.
In the excited state of carbon s and p, orbitals have different enthalpies. Consequently, four bonds of carbon must be of two types. Three of the bonds should be of one type (s - p bonds) while fourth bond should be 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 enthalpies and give orbitals of new enthalpy and shape. These equivalent orbitals are called hybrid orbitals.
