Arrange the following in the order of property indicated for set :
`NH_3, PH_3 , AsH_3, SbH_3, BiH_3` - increasing base strength .

Explanation

Due to the presence of a lone pair of electrons on the central atom in `NH_3, PH_3, AsH_3, SbH_3 and BiH_3` all behave as Lewis bases. As we move from `NH_3 " to " BiH_3` the size of the atom increases. As a result, the lone pair of electrons occupies a larger volume. Therefore, the electron density on the central atom decreases and hence the basic strength decreases as we move from `NH_3 " to " BiH_3`. Increasing order of basic strength can be written as:
`BiH_3 lt SbH_3 lt AsH_3 lt PH_3 lt NH_3`