Question

the bond angle of H2O is less than bond angle of NH3

Answer 1

because of repulsive forces by electrons in h20

Answer 2

the structure of both the molecules is tetrahedral but nitrogen in nh3 molecule has only one lone pair but oxygen has two lonepairs and lone pairs are more electronegative so the lone pair in the oxygen atom repells the electrons present in the hydrogen atom more and the other lone pair is situated at the topmost position and does not have majior impact and in the case of nh3 molecule the the lone pair is present at the topmost position and the other three are hydrogen molecules so they repel each other equally therefore the bond angle of nh3 is more than the bond angle of oxygen.

it is basically due to the nature of a molecule or compound to attain most stable configuration.