Question

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

Answer 1

Nitrogen has a small size, due to which the lone pair of electrons is concentrated in a small region. This means that the charge density per unit volume is high. On moving down a group, the size of the central atom increases, and the charge gets distributed over a large area, decreasing the electron density. Hence, the electron-donating capacity of group 15 element hydrides decreases on moving down the group.

Answer 2

1. NH₃ is more basic than BiH₃, in spite of the fact that both N in NH₃ and Bi in BiH₃ contain a single pair of electrons, should operate as Lewis bases, and should have similar basic strengths.
2. The reason for this is that the N atom is smaller (70 pm) than the Bi atom (148 pm).
3. The N atom in NH₃ has a significantly greater electron density than the Bi atom in BiH₃ because of its smaller size.
4. Because of this, NH₃ is more basic and more effective at donating a single pair of electrons.