Question

Assign a reason for the following:

In group 15, the bond angle \[\ce{H - M - H}\] decreases in the following order: NH₃ (107.8°), PH₃ (93.6°), AsH₃ (91.8°).

Answer 1

1. In group 15 hydrides like NH₃, PH₃, and AsH₃, the central atom has three bond pairs and one lone pair of electrons, forming a pyramidal shape.
2. The bond angle decreases down the group because the size of the central atom increases.
3. In NH₃, nitrogen is small and highly electronegative, causing strong repulsion between electron pairs and a larger bond angle (107.8°).
4. In PH₃ and AsH₃, larger atomic size and lower electronegativity reduce electron density and lone pair-bond pair repulsion, decreasing the bond angle to 93.6° and 91.8°, respectively.

Thus, the bond angle decreases from NH₃ to AsH₃ due to increasing atomic size and decreasing electron pair repulsion.