CBSE Class 12-science Answered

The valence shell electron-pair repulsion (VESPR) theory was devised to account for molecular shapes. In this model, atoms and pairs of electrons are arranged to minimize the repulsion of these atoms and pairs of electrons. 

 Lets take example of NH3 molecule.

Since, the non-bonded electron pairs are held somewhat closer to the nucleus, nitrogen, than the attached hydrogen atoms, they tend to crowd the hydrogen atoms. Thus, ammonia exists as a distorted tetrahedron (trigonal pyramidal) rather than a trigonal plane and water also exists as a distorted tetrahedron (bent) rather than a linear molecule with the hydrogen atoms at a 180^o bond angle.

This concept proposes that since the attached groups are not at the angles of the p orbitals and their atomic orbitals would not have maximum overlap (to form strong bonds)the s and p orbitals will be hybridized to match the bond angles of the attached groups.

The number of these new hybrid orbitals must be equal to the numbers of atoms and non-bonded electron pairs surrounding the central atom!

 When bonds are formed between atoms in the formation of molecules, there is a change in the nature of the Atomic Orbitals in each atom.

So, in hybridisation new molecular orbitals are formed and the electrons in these orbitals are those of the molecule as a whole. The alteration of the structure of the atomic orbitals is called hydridisation, as it involves combining a number of orbitals to create an equal number of new orbitals, where each of the new hybrid orbitals have properties which are an average of those of the orbitals from which they were created.