The correct order of boiling point is

Solution:

The boiling points of hydrides of Group 15 elements (NH₃, PH₃, AsH₃, SbH₃) are influenced by two main factors: hydrogen bonding and van der Waals forces.

1. Hydrogen Bonding: NH₃ exhibits strong intermolecular hydrogen bonding due to the high electronegativity of nitrogen and the presence of hydrogen atoms directly bonded to it. This leads to an abnormally high boiling point for NH₃ compared to PH₃, AsH₃, and even some heavier hydrides in the group.

2. Van der Waals Forces: For PH₃, AsH₃, and SbH₃, hydrogen bonding is negligible. Their boiling points are primarily determined by the strength of van der Waals forces (London dispersion forces). These forces increase with increasing molecular size and mass. As we move down the group from P to As to Sb, the molecular mass increases, leading to stronger van der Waals forces and thus higher boiling points.

Considering both factors:

• PH₃ has the lowest boiling point because it has the smallest molecular mass among PH₃, AsH₃, and SbH₃, and no significant hydrogen bonding.

• AsH₃ has a higher boiling point than PH₃ due to its larger molecular mass.

• SbH₃ has the highest boiling point among PH₃, AsH₃, and SbH₃ due to its largest molecular mass.

• NH₃, despite having a smaller molecular mass than AsH₃ and SbH₃, has a significantly higher boiling point than PH₃ and AsH₃ due to strong hydrogen bonding. However, the van der Waals forces in SbH₃ are strong enough to make its boiling point higher than that of NH₃.

Therefore, the order of boiling points is:

SbH₃ > NH₃ > AsH₃ > PH₃

Boiling points (approximate values):

PH₃: -87.7 °C

AsH₃: -62.5 °C

NH₃: -33.3 °C

SbH₃: -17.1 °C

Thus, the correct order is SbH₃ > NH₃ > AsH₃ > PH₃.