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The CORRECT order of melting points of group 15 trifluorides is

  • A.\(\text{PF}_{3} \lt \text{AsF}_{3} \lt \text{SbF}_{3} \lt \text{BiF}_{3}\)
  • B.\(\text{BiF}_{3} \lt \text{SbF}_{3} \lt \text{PF}_{3} \lt \text{AsF}_{3}\)
  • C.\(\text{PF}_{3} \lt \text{SbF}_{3} \lt \text{AsF}_{3} \lt \text{BiF}_{3}\)
  • D.\(\text{BiF}_{3} \lt \text{AsF}_{3} \lt \text{SbF}_{3} \lt \text{PF}_{3}\)

Answer: A

The correct order is PF3 < AsF3 < SbF3 < BiF3.

Down group 15 the trifluorides change from covalent molecules to ionic solids. PF3 is a gas at room temperature and AsF3 a volatile liquid, both held only by weak intermolecular forces. SbF3 is a solid that melts near 290 °C, and BiF3, the most ionic, is a high-melting solid.

The rise follows the increasing electropositive (metallic) character of the central atom, which increases the ionic character of the M-F bond. Every other option puts a heavier fluoride below a lighter one, against this trend.

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