Van der Waals forces increase down Group 17, reducing volatility

Steps:

• Volatility decreases down Group 17 as boiling points of halogens rise from F2 to I2.
• Halogens form nonpolar X2 molecules, so volatility depends on intermolecular forces.
• Atomic size and electron count increase down the group, boosting London dispersion forces.
• Stronger van der Waals forces raise boiling points, lowering volatility.

Why D is correct:

• Van der Waals forces strengthen due to greater polarizability from larger electron clouds, per London dispersion force theory.

Why the others are wrong:

• A: Van der Waals forces increase, not decrease, explaining higher boiling points.
• B: Covalent bond strength decreases (longer bonds), but volatility trends from intermolecular, not intramolecular, forces.
• C: Covalent bond strength weakens down the group, opposite of this claim.

Final answer: D