The actual bond angles are $\begin{array}{c c c c c c c}\mathrm{NH}_3 & & \mathrm{PH}_3 & & \mathrm{AsH}_3 & & \mathrm{SbH}_3 \\ 106.5^{\circ} & & 93.5^{\circ} & & 91.5^{\circ} & & 91.3^{\circ} \end{array}$ The decreased bond angle in other hydrides can be explained bythe fact that the $\mathrm{sp}^{3}$ hybridisation becomes less and less distinct withincreasing size of the central atom i.e., pure p-orbitals are utilised in M-Hbonding.