Solution

Both $$NH_{3}$$ and $$BiH_{3}$$ have lone pair of electrons on the central atom and hence should behave as Lewis bases. But $$NH_{3}$$ is much more basic than $$BiH_{3}$$. This can be explained on the basis of electron density on the central atom. Atomic size of $$N(70\ pm)$$ is much smaller than that of $$Bi(148\ pm)$$. Therefore the electron density on $$N-atom$$ is much higher than that on $$Bi-atom$$. Consequently, the tendency of $$N$$ in $$NH_{3}$$ to donote its pair of electrons is much higher than that of $$Bi$$ in $$BiH_{3}$$. Thus, $$NH_{3}$$ is much more basic than $$BiH_{3}$$.