Question

Why does the basic strength of the hydrides of group 15 elements decrease on moving down the group?

Answer 1

1. Basic strength depends on the ability to donate lone pair electrons.
2. Nitrogen is small, so NH₃ has a concentrated lone pair with high electron density.
3. As the size of the central atom increases down the group, the lone pair electrons are spread over a larger area.
4. This reduces electron density on the atom, decreasing its tendency to donate lone pairs.
5. Hence, basic strength decreases from NH₃ to BiH₃, where BiH₃ shows almost no basic character.

Thus, NH₃ is the strongest base among group 15 hydrides, and the basicity decreases down the group due to increased atomic size and decreased electron density on the lone pair.