Question

What happens to the following properties of group 15 hydrides as we move down the group?
(i) Basic character
(ii) Thermal stability
(iii) Reducing nature
(iv) Dipole moment
(v) Bond angle

Answer 1

Hint: The properties like basic character, thermal stability are inversely proportional to the size of the central atom. Reducing power is inversely proportional to thermal stability.

Complete step by step answer:
As we know the hydrides are basic in nature. Lone pairs of electrons are available on the central atom. So they can act as Lewis bases. The basic character decreases down the group with the increase in the size of the central atom.
 Can you say how the size of the central atom affects the basic character?
 The electron density on the central atom decreases with the increase in the size of the central atom. Hence its ability to donate a pair of electrons decreases thus reducing the basic character.
Thermal stability is inversely proportional to size. So, as the size of the central atom increases, the stability decreases.
Thus the thermal stability decreases down the group.
Reducing power increases with the decrease in the stability of the hydride. The hydrides of group 15 are strong reducing agents. Bismuth is the strongest reducing agent among all the other hydrides.
Dipole moment is proportional to electronegativity of the central atom. So it decreases down the group.
These hydrides are pyramidal in shape with a lone pair of electrons in one of the orbits. The bond angle gradually decreases down the group due to the decrease in bond pair- bond pair repulsions.

Note: Let us see the trend for boiling points and melting points too. The boiling point decreases from ammonia to phosphine, and then increases from phosphine to bismuthine. The same is observed for their melting points.