Explain why `NH_(3)` is basic while `BiH_(3)` is only feebly basic ?

Explanation

`NH_(3)` is distinctly basic while `BiH_(3)` is feebly basic.
Nitrogen has a small size due to which the lone pair of electrons is concentrated in a small region. This means that the charge density per unit volume is high. On moving down a group, the size of the central atom increases and the charge gets distributed over a large area decreasing the electron density. Hence, the electron donating capacity of group 15 element hydrides decreases on moving down the group.