Question

Among the hydrides of Group-15 elements, which have the: -
(A) Lowest boiling point - phosphine.
(B) Maximum basic character - ammonia.
(C) Highest bond angle - ammonia.
(D) All are correct.

Answer 1

Hint: Consider the effect of various intermolecular forces (van der waals forces and hydrogen bonds) on the boiling point. Also correlate the size with electron density for basic nature. Also correlate the electron density with repulsion between the bond pair and lone pair of electrons.

Complete step by step answer:
> On moving from top to bottom in group 15, the atomic radii increase. With increase in the atomic radii, the size of hydrides also increases. With increase in the size, the extent of van der Waals forces also increase. With increase in van der Waals forces, the boiling point increases. Thus, when you move down the group 15, the boiling points of its hydrides will increase. By following this trend, you expect that ammonia has the lowest boiling point among the group 15 hydrides. However, ammonia has higher boiling point than phosphine. This is due to the presence of intermolecular hydrogen bonding interactions in ammonia molecules. These interactions are not present in phosphine molecules. Intermolecular hydrogen bonds lead to molecular association and increase in the boiling point. Hence, among group 15 hydrides, phosphine has the lowest boiling point. So, the option (A) is the correct option.
> Among group 15 hydrides, ammonia has maximum basic character. This is because a small sized nitrogen atom has the highest electron density. With increase in the atomic size on moving down the group, the electron density decreases. So nitrogen can easily donate a pair of electrons and act as a base. Hence, the option (B) is the correct option.
> Among group 15 hydrides, ammonia has the highest bond angle. The small sized nitrogen atom in ammonia has the highest electron density. So ammonia has maximum repulsion between lone pair and bond pair of electrons. Hence, the option (C) is also the correct option.
Thus, all the options are correct options.

Hence, the correct option is the option (D).

Note: A hydrogen bond is formed when hydrogen atom is attached to electronegative nitrogen, oxygen or fluorine atom. When a liquid contains intermolecular hydrogen bonds, energy must be supplied to break these hydrogen bonds. This increases the boiling point of the liquid.