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

Explanation

Both N in `NH_(3)` and Bi in `BiH_(3)` have a lene pair of electrons on the central atom and hence should behave as Lewis bases. But `NH_(3)` is much more basic than `BiH_(3)`. This can be explained on the basis of electron density on the central atom. Since the atomic size of N (70 pm) is much smaller than that of Bi (148 pm), therefore, electron density on the N-atom is much higher than that on Bi-atom. Consequently, the tendency of N in `NH_(3)` to donate its pair of electrons is much higher than that of Bi in `BH_(3)`. Thus, `NH_(3)` is much more basic than `BH_(3)`.