The H-M-H bond angles for the hydrides of group-15 elements are as follows:
NH3 = 107o
PH3 = 92o
AsH3 = 91o
SbH3 = 90o
The above trend in the H−M−H bond angle can be explained on the basis of the electronegativity of the central atom in the respective compounds. Since nitrogen has a higher electronegativity, there is a higher electron density around nitrogen. This causes greater repulsion among the electron pairs around nitrogen atom, resulting in maximum bond angle. As it is known that electronegativity decreases on moving down a group, so the repulsive interactions between the electron pairs decreases, and thereby decreasing the H−M−H bond angle down the group.