Question

Why does PH₃ possess a smaller bond angle than that for NH₃?

Answer 1

1. Both NH₃ and PH₃ have a pyramidal shape formed by sp³ hybridisation with one lone pair and three bond pairs.
2. Nitrogen is smaller and more electronegative than phosphorus, so in NH₃, bonding electron pairs are held closer and repel more strongly.
3. Phosphorus is larger with less electronegativity, so bonding pairs in PH₃ are farther apart, repelling less.
4. Lone pair-bond pair repulsion is maximum in NH₃, causing a bond angle of 107.8°.
5. In PH₃, weaker repulsion and larger atom size reduce the bond angle to about 93.6°.

Thus, the PH₃ bond angle is smaller due to larger atomic size and lesser electron pair repulsion than NH₃.