Question

The decreasing values of bond angles from nh3 (106°) to sbh3 (101°) down group-15 of the periodic table is due to

Answer 1

It is basically due to decrease in electronegativity down the group and decrease in bond pair - bond pair repulsion.

This can also be explained using drago rule.

According to drago rule

when atoms of group 14, 15 and 16 are attached to atoms having electronegativity less than 2.5, then is that case no hybridisation is observed. That is why bond angle of NH3 is 107.8 whereas the bond angle of PH3, AsH3, SbH3, and BiH3 is very close to 90.