Question

Which of the following boiling point order is correct -

• A. He > T₂ > D₂
• B. He < T₂ < D₂
• C. T₂ > He > D₂
• D. He < D₂

Answer 1

Answer: (D)

He < D₂

Answer 2

Boiling points depend on the strength of intermolecular forces. For nonpolar substances, these are primarily London Dispersion Forces (LDFs). LDFs increase with increasing molecular size, number of electrons, and polarizability.

1. He (Helium): An atom, very small, 2 electrons. Has very weak LDFs. Boiling point = 4.2 K.
2. D₂ (Deuterium): A diatomic molecule, 2 electrons. Molar mass = 4 g/mol. Boiling point = 23.6 K.
3. T₂ (Tritium): A diatomic molecule, 2 electrons. Molar mass = 6 g/mol. Boiling point = 25.0 K.

Comparing He and D₂: Although they have similar molar masses, D₂ is a molecule with a more diffuse electron cloud, leading to stronger LDFs than atomic He. So, D₂ > He. Comparing D₂ and T₂: T₂ has a higher molar mass than D₂, leading to stronger LDFs. So, T₂ > D₂.

The overall correct order of boiling points is He < D₂ < T₂.

(D) He < D₂ (This statement is correct, although it is a partial order and does not include T₂. Given the options, it is the only factually correct statement.)