As we move down the group -15, the size of the element, increases and therefore the length of the E – H bond increases and its strength decreases. In other words, as we move down the group, the bond with hydrogen can break more easily to evolve H2 gas. Which acts as the reducing agent. Thus Bi-H bond is the weakest amongst the hydrides of elements of group -15, and hence, BiH3 is strong reducing agent.
