Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements?
As we move down a group, the atomic size increases and the stability of the hydrides of group 15 elements decreases. Since the stability of hydrides decreases on moving from NH3 to BiH3, the reducing character of the hydrides increases on moving from NH3 to BiH3.
This is because as we move down the group, the size increases, as a result, length of E-H bond increases and its strength decreases, so that the bond can be broken easily to release H2 gas. Hence, BiH3 is the strongest reducing agent.