Elements of group- 15, 16, and 17 form compounds with hydrogen with general formula \(MH_3\), \(H_2M\), and \(HX\) (called hydrogen halides). The stability of these hydrides, along with \(M-H\) bond strength and basic nature decreases down the group. Boiling point of these hydrides mainly depends on the strength of their intermolecular forces.

The decreasing order of boiling point of group 15 hydrides is:

A. \(NH_3\)

B. \(PH_3\)

C. \(AsH_3\)

D. \(SbH_3\)

E. \(BiH_3\)

Choose the correct answer from the options given below:

\(BiH_3 > SbH_3 > NH_3 > AsH_3  >  PH_3\)

The correct answer is option 4. \(BiH_3 > SbH_3 > NH_3 > AsH_3  >  PH_3\).

The boiling points of hydrides generally depend on the strength of intermolecular forces, particularly London dispersion forces and hydrogen bonding, where applicable.

1. \(BiH_3\) (Bismuthine):

Bismuthine has the highest boiling point among the options. This is due to the larger atomic size of bismuth compared to the other elements in Group 15. Larger atoms have more electrons, leading to stronger London dispersion forces.

2. \(SbH_3\) (Stibine):

Stibine follows with a slightly lower boiling point than \(BiH_3\). While antimony is smaller than bismuth, it still exhibits significant London dispersion forces.

3. \(NH_3\) (Ammonia):

Ammonia has a relatively high boiling point due to the presence of hydrogen bonding. Hydrogen bonding occurs between the hydrogen atom in NH₃ and the lone pair on the nitrogen atom, leading to stronger intermolecular forces.

4. \(AsH_3\) (Arsine):

Arsine has a lower boiling point compared to ammonia due to weaker hydrogen bonding. The larger atomic size of arsenic compared to nitrogen results in less effective hydrogen bonding.

5. \(PH_3\) (Phosphine):

Phosphine has the lowest boiling point among the options. The strength of London dispersion forces in phosphine is lower compared to the other hydrides, and it does not exhibit hydrogen bonding.

In summary, the order of decreasing boiling points is primarily determined by the strength of intermolecular forces, which is influenced by factors such as molecular size and the presence of hydrogen bonding. The larger the atom, the stronger the London dispersion forces, and the higher the boiling point. Hydrogen bonding, when present, further enhances the boiling point.