Why bond angle of Phosphine `(PH_3)` is less than Ammonia `(NH_3)` ?

Explanation

In `NH_3`,The nitrogen atom is `sp^3`. hybridised. Three of four `sp^3`- orbitals from three `N - H, sigma` bonds while the fourth contains the lone pair of electron.
Since lone pair-bond pair repulsions are stronger than the bond pair-Bond pair repulsions, therefore in `NH_3`, the bond angle decrease from `109.5^@` to `107.8^@`. As we move from N to sb the electronagitivity of the centrial atom gas on decreasing and hence the bond pair of electron lie away and away from the central atom. In the other words, bond pair-bond pair repulsion is Maximum. In `NH_3` and Minimum `SbH_3`. The bond angle is shown as.