Question

The CORRECT order of melting points of group 15 trifluorides is

• A. PF3 < AsF3 < SbF3 < BiF3
• B. BiF3 < SbF3 < PF3 < AsF3
• C. PF3 < SbF3 < AsF3 < BiF3
• D. BiF3 < AsF3 < SbF3 < PF3

Hint:

When determining the order of melting points, remember that larger atoms typically have weaker bonds, resulting in lower melting points.

Answer 1

The Correct Option is A

To determine the correct order of melting points of group 15 trifluorides, we need to consider the following factors:

• Molecular weight and size: Among the group 15 elements, as you move down the group from phosphorus (P) to bismuth (Bi), both the molecular weight and the atomic size increase. This generally leads to stronger Van der Waals forces due to the larger electron cloud.
• Polarizability: Larger atoms tend to be more polarizable. More polarizable atoms can induce stronger dipole-dipole interactions, contributing to higher melting points.
• Bonding and structure: The type and strength of bonding can significantly affect melting points. In group 15 trifluorides, the crystal structure and the nature of bonding also play an important role. For example, BiF₃ has a significant ionic character compared to PF₃, which is predominantly covalent.

Based on these factors, the melting points of the trifluorides should increase as we go down the group. The correct order of melting points from lowest to highest is:

• PF₃: It is the smallest molecule with the least polarizable P atom, thus having relatively weak intermolecular forces.
• AsF₃: Larger than PF₃ and more polarizable, hence it has a higher melting point.
• SbF₃: Further down the group, SbF₃ is larger with stronger intermolecular attractions.
• BiF₃: Highest melting point due to the largest ionic character and strongest interatomic forces in the group.

Therefore, the correct order of melting points is:

Order	Trifluorides
1	PF3
2	AsF3
3	SbF3
4	BiF3

This explains why the correct answer to the question is PF₃ < AsF₃ < SbF₃ < BiF₃.