Reducing power of hydrides of group-15 elements increase down the group. explain.
Reducing power increases in the following order: NH3 < PH3 < AsH3. This is associated with stability of E-H bond, which also decreases down the group. It is conditioned by the increase in atomic radii from N to As. Since bonding electrons are attracted weaker with nucleus for element with larger radius than for that with smaller one, the bond energy decreases. The low dissociation energy for E-H enhances reactivity of compounds and facilitates transfer of electrons from hydride to oxidizing agent. Therefore, in the 15-th group of elements, the highest reducing power is found for AsH3 and the lowest one is exhibited by NH3.