Question

The HNH angle value is higher than HPH, HAsH and HSbH angles. Why?

Answer 1

1. In NH₃, nitrogen is small and highly electronegative, leading to a higher electron density around nitrogen.
2. This causes greater lone pair-bond pair repulsion, which pushes the hydrogen atoms further apart.
3. As a result, the H-N-H bond angle in NH₃ is about 107.8°.
4. In PH₃, AsH₃, and SbH₃, the central atom is larger with lower electronegativity, causing less electron density and weaker lone pair repulsion.
5. Therefore, the H-M-H angles decrease down the group (PH₃ ≈ 93.6°, AsH₃ ≈ 91.8°, SbH₃ ≈ 91.3°).
6. The bond angles reduce mainly due to the increased size of the central atom and decreased repulsion among electron pairs.

Thus, the small size and high electronegativity of nitrogen in NH₃ cause a larger bond angle compared to other group 15 hydrides.