Electron Shell Differences After First Ionization

Steps:

• Sodium (Z=11) configuration: [Ne] 3s¹; first ionization yields Na⁺ ([Ne], n=2 shell filled).
• Magnesium (Z=12) configuration: [Ne] 3s²; first ionization yields Mg⁺ ([Ne] 3s¹, n=3 shell).
• Second ionization of Na removes a 2p electron (n=2, closer to nucleus).
• Second ionization of Mg removes a 3s electron (n=3, farther from nucleus), requiring less energy due to weaker attraction.

Why A is correct:

• In Na⁺, the outer electron is in the n=2 shell, experiencing stronger nuclear attraction per Coulomb's law (F ∝ 1/r²) compared to the n=3 electron in Mg⁺.

Why the others are wrong:

• B: Na nuclear charge (Z=11) is less than Mg (Z=12), so this reverses the trend.
• C: The 2p electron in Na⁺ has less shielding (inner shell) than the 3s electron in Mg⁺ (shielded by n=2 electrons).
• D: Na⁺ outer electron is in 2p orbital (n=2); Mg⁺ is in 3s (n=3), different principal quantum numbers.

Final answer: A