Effective nuclear charge drives higher ionization energy for chlorine

Steps:

• Identify positions: Chlorine (group 17) and sulfur (group 16) are in period 3, so they have the same number of electron shells.
• Recall trend: Ionization energy increases across a period due to rising nuclear charge with constant shielding.
• Compare atomic numbers: Chlorine (Z=17) has one more proton than sulfur (Z=16), increasing attraction on valence electrons.
• Conclude factor: Higher effective nuclear charge (Zeff) in chlorine requires more energy to remove an electron.

Why D is correct:

• D states valence electrons in chlorine experience greater effective nuclear charge (Zeff = Z - shielding), per Slater's rules, as both have identical core electrons but chlorine's extra proton boosts pull without added shielding.

Why the others are wrong:

• A: More protons alone ignores similar shielding; it's the net Zeff that matters.
• B: Shielding is nearly identical (same inner shells); valence electrons don't shield each other effectively.
• C: Bond strengths in molecules (Cl2 vs. S8) are irrelevant; ionization energy measures isolated gaseous atoms.

Final answer: D