Why is the bond angle in ${\mathrm{PH}}_3$ is lesser than that in ${\mathrm{NH}}_3$ molecule?
 

Arrange the following hydrides in the decreasing order of their basicities:

${\mathrm{NH}}_3, {\mathrm{AsH}}_3, {\mathrm{SbH}}_3, {\mathrm{BiH}}_3 \mathrm{and} {\mathrm{PH}}_3$