Question

Explain why $N{H}_3$ is basic while $Bi{H}_3$ is only feebly basic.

Answer 1

Both $N{H}_3$ and $Bi{H}_3$ have lone pair of electrons on the central atom and hence should behave as Lewis bases. But $N{H}_3$ is much more basic than $Bi{H}_3$. This can be explained on the basis of electron density on the central atom. Atomic size of $N\left(70pm\right)$ is much smaller than that of $Bi\left(148pm\right)$. Therefore the electron density on $N-atom$ is much higher than that on $Bi-atom$. Consequently, the tendency of $N$ in $N{H}_3$ to donote its pair of electrons is much higher than that of $Bi$ in $Bi{H}_3$. Thus, $N{H}_3$ is much more basic than $Bi{H}_3$.