Explain why NH3 is basic while BiH3 is only feebly basic.
Explain why NH3 is basic while BiH3 is only feebly basic.

NH3​ is distinctly basic while BiH3​ is feebly basic as nitrogen has a smaller size due to which the lone pair of electrons are concentrated in a small area. This means that the charge density per unit volume is high in nitrogen. While moving down a group, the size of the central atom increases and the same charge gets distributed over a larger area, thus, decreasing the electron density. Hence, the electron-donating capacity (or simply the basic character) of group 15 element hydrides decrease on moving down the group.