In ammonia (NH₃ ) molecule, there are three bond pairs in covalent bonds (3 N – H) around the nucleus of the nitrogen atom and one lone pair. Lone pair – bond pair repulsion is greater than bond pair – bond pair repulsion. Therefore, NH₃ which is expected to be tetrahedral with four electron pairs in the valence shell and HNH = 1090 28 / , it has HNH = 1070 48/ due to the more repulsion by lone pair on the bond pairs

The shape of the NH₃ molecule is triagonal pyramidal with N at the apex of the pyramid.

4.v) If there are two bond pairs and two lone pairs of electrons around the nucleus of the central atom in its valence shell, lone pair – lone pair repulsion is greater than lone pair – bond pair repulsion. Therefore, the angle between bond pairs further decreases.

Example: Water

In water molecule, (H₂ O) there are four electron pairs around the nucleus of oxygen atom, but, two of them are lone pairs and two bond pairs. Therefore, H₂ O molecule gets ‘V’ shape or bent shape or angular instead of tetrahedral shape as that of CH4 due to lone pair – lone pair and lone pair – bond pair repulsions. HOH is 10^{^}40 31/.

• What is the bond angle in a molecule?

It is the angle subtended by two imaginary lines that pass from the nuclei of two atoms which form the covalent bonds with the central atom through the nucleus of the central atom at the central atom. (see figure ‘α’ is the bond angle).

Valence Shell Electron Pair Repulsion Theory (VSEPRT) mainly fails in explaining the strengths of the bonds. This is because; the theory still depends on the Lewis concept of covalent bond formation. It could not say anything extra about the electronic nature of covalent bonds.