Question

Among the hydrides of Group-15 elements, which have the
(a) lowest boiling point?
(b) maximum basic character?
(c) highest bond angle?
(d) maximum reducing character?

Answer 1

(a) $P{H}_3$ : Phosphine has a lower boiling point than ammonia because of the presence of hydrogen bonding in the latter. Down the group, the molecular weight increases and therefore the energy required to break the bond is higher than in phosphine.
(b) $N{H}_3$: All the trihydrides have a lone pair on them. The size of the atom increases down the group and subsequently the electron density decreases. Thus the first member of the group i.e, ammonia, has the highest basic strength.
(c) $N{H}_3$ : As we move down the group, the size of the atom increases and the electronegativity decreases. Thus the force of repulsion between the bonded pairs decreases and the bond angle also decreases. Thus ammonia has the highest bond angle.
d) $Bi{H}_3$ Reducing character increases down the group and so $Bi{H}_3$ has the highest reducing character.