Van der Waals forces strengthen down Group 17, reducing volatility

Steps:

• Volatility decreases as boiling points rise, requiring more energy to vaporize molecules.
• Group 17 elements form nonpolar diatomic molecules with increasing atomic size down the group.
• Larger size enhances electron cloud polarizability, strengthening instantaneous dipole-induced dipole (London dispersion) forces.
• Stronger intermolecular forces raise boiling points, explaining lower volatility from F₂ to I₂.

Why A is correct:

• Instantaneous dipole-induced dipole forces (London dispersion) increase with molecular size and polarizability, as per the definition of van der Waals interactions, directly raising boiling points.

Why the others are wrong:

• B: Halogen molecules (X₂) are nonpolar, so no permanent dipoles exist for dipole-dipole forces.
• C: Decreasing X–X bond energy affects decomposition, not intermolecular forces governing volatility.
• D: Ionization energy relates to atomic reactivity, not molecular boiling points or volatility.

Final answer: A