Question

Why is BiH₃ the strongest reducing agent amongst all the hydrides of Group 15 elements?

Answer 1

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.

Answer 2

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.