why bond angle in water is less than that of ammonia

Although geometries of NH₃ and H₂O molecules are distorted tetrahedral, bond angle in water is less than that of ammonia. Discuss.

Answer
The molecular geometry of NH₃ and H₂O can be shown as:

The central atom (N) in NH₃ has one lone pair and there are three bond pairs. In H₂O, there are two lone pairs and two bond pairs.
The two lone pairs present in the oxygen atom of H₂O molecule repels the two bond pairs. This repulsion is stronger than the repulsion between the lone pair and the three bond pairs on the nitrogen atom.
Since the repulsions on the bond pairs in H₂O molecule are greater than that in NH₃, the bond angle in water is less than that of ammonia.