Explain why `NH_(3)` is basic while `BiH_(3)` is only feebly basic ?

Explanation

The atomic size of nitrogen (atomic radius = 70 pm) is less than that of bismuth (atomic radius = 148 pm). As a result, electron density on the nitrogen atom is greater than that on bismuth atom . It means that electron releasing tendency of ammonia is more than `BiH_(3)`. Therefore, `NH_(3)` is basic while `BiH_(3)` is only feebly basic.
Note : Atomic size will be considered while comparing the basicity of `NH_(3)` with other hydrides of group - 15 elements.