Explain: `NH_(3)` is basic while `BiH_(3)` is only feebly basic.

Explanation

The presence of a lone pair of electrons on the central atom, viz. N in `NH_(3)` and Bi in `BiH_(3)` , makes these compounds behave as Lewis bases. But `NH_(3)` is more basic than `BiH_(3)` , which can be explained on the basis of electron density on the central atom. Atomic size of N (70 pm) is much smaller than that of Bi (148 pm). Hence, N has a higher electron density than Bi. Therefore N in `NH_(3)` has a much higher tendency to donate its electron-pair than Bi in `BiH_(3)` . As a result, `NH_(3)` is more basic than `BiH_(3)` .