# 7.11     Explain why $NH_{3}$ is basic while  $BiH_{3}$ is only feebly basic.

Nitrogen has a small size because of which the lone pair of electrons are concentrated in a small region. As we go down a group, the size of the central atom increases and the charge gets distributed over a large area which results in decreasing the electron density. Hence, the electron donating capacity(Basicity) of group 15 element hydrides decreases on moving down the group. And that's why electron releasing tendency(basicity) of $\dpi{100} BiH_{3}$ is less than ammonia.