Explain the nature of hydride compounds of group-15 elements.

Explanation

All the elements of group-15 form hydrides of type EH3 where E = N, P, As, Sb or Bi.
On moving down the group, the stability of hydrides decreases i.e.,` NH_3` is most stable while `BiH_(3)` is least stable. This is due to decrease in the bond dissociation enthalpy of E-H bond as a result of increase in atomic size of the element. Consequently reducing character of hydrides increases.
Reducing strength:
`NH_(3) lt PH_(3) lt AsH_(3) lt SbH_(3) lt BiH_(3)`
Thermal stability:
Thus ammonia is mild reducing agent while `BiH_3` is strong reducing agent.
The basicity of the hydrides decreases down the group. `BiH_3` is least basic while `NH_3` is most basic. The high basicity of `NH_3` is due to high electronegativity and small size of nitrogen. Order of basicity :
`NH_(3) gt PH_(3) gt AsH_(3) gt SbH_(3) ge BiH_(3)`
Ammonia (`NH_3`) exhibits hydrogen bonding both in solid and liquid state. Because of this it has higher melting and boiling points than that of `PH_3`.
Order of boiling points :
`BiH_(3) gt SbH_(3) gt NH_(3) gt AsH_(3) gt PH_(3)`