Why is BiH2 the strongest reducing agent among all the hydrides of group 15 elements?
The 15 group or nitrogen family consists of nitrogen, phosphorus, arsenic, antimony and bismuth. As you move down the nitrogen family: atomic radius increases, ionic radius increases, ionization energy decreases, and electronegativity decreases. Nitrogen and phosphorus are nonmetals. Arsenic and antimony are metalloids. Bismuth is a metal and this is cause the main difference in behavior in comparison to nitrogen, phosphorus, arsenic and antimony. BiH2 has ionic bond between H and Bi, while other elements have covalent, where interaction forces are higher. It means that BiH2 is capable to breaking bond more easy than other and that's why BiH2 is the strongest reducing agent in nitrogen family.