Volatility decreases down group 17 due to stronger London dispersion forces

Steps:

• Volatility inversely relates to boiling point; boiling points increase from Cl₂ to I₂.
• Group 17 elements form nonpolar X₂ molecules, so intermolecular forces are instantaneous dipole-induced dipole (London dispersion).
• Atomic size and electron count increase down the group, raising polarizability.
• Stronger dispersion forces require more energy to vaporize, reducing volatility.

Why A is correct:

• London dispersion forces strengthen with larger, more polarizable molecules, as temporary dipoles induce larger opposing dipoles per the definition of induced dipole interactions.

Why the others are wrong:

• B: X₂ molecules are nonpolar, lacking permanent dipoles for permanent dipole forces.
• C: X-X bond energy decreases, but volatility depends on intermolecular (not intramolecular) forces.
• D: Ionization energy decrease affects ion formation, irrelevant to neutral X₂ volatility.

Final answer: A