Question

What is the decreasing order of boiling points of the \[{\rm{V}}\] group of hydrides?
A.\[N{H_3} > P{H_3} > As{H_3} > Sb{H_3}\]
B.\[Sb{H_3} > As{H_3} > P{H_3} > N{H_3}\]
C.\[P{H_3} > N{H_3} > As{H_3} > Sb{H_3}\]
D.\[Sb{H_3} > N{H_3} > As{H_3} > P{H_3}\]

Answer 1

Hint: Group \[{\rm{V}}\] in the periodic table is known as the nitrogen family and main elements of this group are nitrogen, phosphorus, arsenic, antimony and bismuth. As there are three electrons present in the valence subshell of each element therefore the elements of the nitrogen family form hydrides of types \[A{H_3}\].

Complete answer:
As we know, on moving down the group the atomic size of the elements increases therefore the Vander Waal forces acting between the atoms also increases which leads to increase in the bond strength of molecules. As the bond strength of a molecule increases, the molecule will require more heat to break its bonds. Therefore, the boiling points of hydrides \[{\rm{V}}\] group increases down the group.
The boiling point of ammonia is exceptionally higher than the boiling points of hydrides of arsenic and phosphorus because of hydrogen bonding that exists between the atoms of nitrogen and hydrogen.
Therefore, the correct decreasing order of boiling points of \[{\rm{V}}\] group of hydrides is \[Sb{H_3} > N{H_3} > As{H_3} > P{H_3}\]
So, option (D) is the correct answer.

Note:
Hydrogen bonding: It is a special type of dipole-dipole interactions between the highly electronegative elements (fluorine, oxygen and nitrogen) and the hydrogen atom via covalent bonding. There are two types of hydrogen bonding:
Intermolecular hydrogen bonding- These are the interactions between the atoms of one molecule with the atoms of another molecule. Example: hydrogen fluoride, water, etc.
Intramolecular hydrogen bonding- These are the interactions between two atoms within the same molecule. Example: ethylene glycol.