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

(N/A) $NH_3$ is distinctly basic,whereas $BiH_3$ is only feebly basic.
Nitrogen has a small atomic size,which causes the lone pair of electrons to be concentrated in a small region,resulting in high electron density.
As we move down the group from $N$ to $Bi$,the size of the central atom increases.
This increase in size causes the lone pair of electrons to be distributed over a larger volume,which significantly decreases the electron density.
Consequently,the electron-donating capacity (Lewis basicity) of the group $15$ hydrides decreases as we move down the group.