Bond Parameters

The reason why bond angle is larger in ${\mathrm{N}\mathrm{H}}_3$ than  ${\mathrm{P}\mathrm{H}}_3$ are given below.

• In ${\mathrm{N}\mathrm{H}}_3$ , there is one lone pair on the nitrogen atom increases repulsion, while the lone pair on phosphorus is in a higher energy orbital and causes less repulsion.
• Nitrogen is a small and electronegative atom whereas phosphorus is larger and less electronegative than nitrogen.
• In  ${\mathrm{P}\mathrm{H}}_3$, the bonding orbitals are nearly pure p-orbitals, which are less directionals–p hybridization, on the other hand, the bond pairs remain fairly directed, leading to a larger bond angle.

Hence, the bond angle in ${\mathrm{N}\mathrm{H}}_3$ is about 107° ( less than the ideal tetrahedral 109.5°) due to lone pair presence and highly electronegative atom, and the bond angle in PH? ( 93.5°) is much smaller due to less repulsive lone pair, and less electronegative atom.

Hence, the bond angle in ${\mathrm{N}\mathrm{H}}_3$  (107°) is larger than in ${\mathrm{P}\mathrm{H}}_3$ (93.5°).