Comparing atomic and ionic radii in periodic trends

Steps:

• Atomic radii decrease across a period due to increasing effective nuclear charge; sodium (period 3, group 1) has a larger atomic radius than sulfur (period 3, group 16).
• In Na₂S, sodium forms Na⁺ by losing an electron, shrinking its radius compared to the atom; sulfur forms S²⁻ by gaining electrons, expanding its radius.
• Standard values: atomic radius Na ≈ 186 pm > S ≈ 104 pm; ionic radius Na⁺ ≈ 102 pm < S²⁻ ≈ 184 pm.
• Thus, atomic radius: Na > S; ionic radius: Na < S.

Why B is correct:

• It matches periodic trends and ionic size changes: metals form smaller cations, nonmetals larger anions, per Fajans' rules and electron configuration.

Why the others are wrong:

• A: Wrong on atomic radii; Na > S, not less.
• C: Wrong on atomic radii; Na > S, and ionic Na < S, not greater.
• D: Wrong on ionic radii; Na⁺ < S²⁻ due to anion expansion.

Final answer: B