Bond angle of hydride of group-15 on moving down the group :

Down the group, electronegativity of elements decreases, therefore the tendency of atoms to attract shared electron pars also decreases. As a result, the shared electron pair is more far way from the central atom which results in decrease in bond pair-bond pair repulsion. Therefore, bond angle of group-15 hydrides decreases on moving down the group.

The trend of electronegativity of group-15 elements is:

$\mathrm{N}>\mathrm{P}>\mathrm{As}>\mathrm{Sb}>\mathrm{Bi}$

The trend of bond angle of group-15 hydrides:

${\mathrm{NH}}_3 > {\mathrm{PH}}_3 > {\mathrm{AsH}}_3 > {\mathrm{SbH}}_3 > {\mathrm{BiH}}_3$