Question

Giving suitable reasons, arrange the hydrides (MH₃) of group 15 elements in the decreasing order of \[\ce{H - M - H}\] bond angle.

Answer 1

1. The hydrides (MH₃) of group 15 elements have a pyramidal shape with one lone pair and three bond pairs on the central atom.
2. The \[\ce{H - M - H}\] bond angle order is NH₃ (107.8°) > PH₃ (93.6°) > AsH₃ (91.8°) > SbH₃ (91.3°) > BiH₃ (90°).
3. The bond angle decreases down the group because the size of M increases and electronegativity decreases, reducing electron pair repulsion.
4. In NH₃, a smaller N atom with higher electronegativity causes greater repulsion between bond pairs, hence a larger bond angle.
5. In heavier hydrides, less repulsion from bond pairs leads to smaller bond angles.

Thus, the bond angle decreases from NH₃ to BiH₃ due to increasing atom size and decreasing electronegativity, lowering electron pair repulsion.