Hydride `NH_(3)` `PH_(3)` `AsH_(3)` `SbH_(3)`
H−M−H angle `107^(@)` `92^(@)` `91^(@)` `90^(@)`
The above trend in the H−M−H bond angle can be explained on the basis of the electronegativity of the central atom. Since nitrogen is highly electronegative, there is high electron density around nitrogen. This causes greater repulsion between the electron pairs around nitrogen, resulting in maximum bond angle. We know that electronegativity decreases on moving down a group. Consequently, the repulsive interactions between the electron pairs decrease, thereby decreasing the H−M−H bond angle.
