Question

$N{{H}_{3}}$ has a much higher boiling point than $P{{H}_{3}}$ because:
(A) $N{{H}_{3}}$ has a larger molecular weight
(B) $N{{H}_{3}}$ undergoes umbrella inversion
(C) $N{{H}_{3}}$ forms hydrogen bond
(D) $N{{H}_{3}}$ contains ionic bonds whereas $P{{H}_{3}}$ contains covalent bond

Answer 1

Hint : The name of the given compounds are ammonia ( $N{{H}_{3}}$ ) and phosphine ( $P{{H}_{3}}$ ), both the compounds contains hydrogen in it, and both are a type of covalent compound. Although they are a part of the group 15, the electronegativity of these elements can be accounted for as an exception.

Complete Step By Step Answer:
The correct answer to this question is option C, $N{{H}_{3}}$ forms a hydrogen bond in its compound. Specifically, intermolecular hydrogen bonding is formed in ammonia, that is $N{{H}_{3}}$ .
Intermolecular hydrogen bonding occurs between separate molecules in the same or different substance. For example, in ammonia ( $N{{H}_{3}}$ ) intermolecular hydrogen bonding occurs between a hydrogen of one ammonia molecule to a nitrogen of another ammonia molecule.
The reason behind the occurrence of intermolecular hydrogen bonding in ammonia ( $N{{H}_{3}}$ ) and not in phosphine ( $P{{H}_{3}}$ ) is that the electronegativity of nitrogen is more than that of phosphorus.
An atom of nitrogen is smaller than that of phosphorus, and thus the outermost orbit of nitrogen is smaller than that of phosphorus. Nitrogen’s outermost orbit contains five electrons, while phosphorus’ outermost orbit contains six electrons.
Even then, the outermost orbit of nitrogen is much smaller, thus the electronegativity of nitrogen is larger than that of phosphorus.
And thus, hydrogen bonding occurs in ammonia ( $N{{H}_{3}}$ ) and not in phosphine ( $P{{H}_{3}}$ ). And due to the compact size of ammonia molecules, which occurs due to intermolecular hydrogen bonding, the boiling point of ammonia ( $N{{H}_{3}}$ ) is higher than that of phosphine ( $P{{H}_{3}}$ ).
Thus, the correct answer is option C.

Note :
Intermolecular hydrogen bonding occurs in between molecules of ammonia in this question, but it can also occur between two different molecules in the same solution. For example, intermolecular hydrogen bonding can occur between one ammonia and one water molecule. Also, intramolecular hydrogen bonding is a bond formed with hydrogen within the same molecule.