Explain why `NH_3` is basic while `BiH_3` is only feebly basic.

Explanation

Both N and Bi have a lone pair of electrons in `NH_3` and `BiH_3` respectively. They can denote the electron pair and therefore behave as Lewis base. In `NH_3` , N has small size and the lone pair is concentrated on a small region and electron density on it is maximum. Consequently, it has greater electron releasing tendency. But the size of Bi is large and the electron density of the lone pair is less. As a result, it has lesser tendency to donate electron pair. Hence, `NH_3` is basic while `BiH_3` is only feebly basic.