Effective nuclear charge explains higher ionization energy of Cl vs S

Steps:

• Ionization energy trends increase across a period due to rising nuclear charge with constant shielding.
• Sulfur (atomic number 16) and chlorine (17) share the same 3s²3p configuration shell structure.
• Chlorine's extra proton increases nuclear attraction on valence electrons without added shielding.
• Thus, removing a 3p electron from Cl requires more energy than from S.

Why D is correct:

• D states that Cl experiences higher effective nuclear charge (Z_eff = Z - σ, where σ is shielding constant); same inner electrons mean minimal shielding difference, amplifying proton effect per Slater's rules.

Why the others are wrong:

• A: More protons alone ignores similar shielding; both contribute but not fully explanatory.
• B: Shielding is nearly identical (same core electrons); Cl has less, not more, effective shielding.
• C: Ionization energy measures atomic gaseous ions, not molecular bond strengths.

Final answer: D