Across Period 3 of the Periodic Table, why is the first ionization energy of aluminum (Z = 13) slightly lower than that of magnesium (Z = 12)?

Answer and explanation

Correct answer: A. the 3p valence electron of aluminum is shielded by the filled 3s² subshell and higher in energy

  1. Step 1: Magnesium has valence configuration 3s² (fully filled s-subshell, stable). Step 2. Aluminum has valence configuration 3s² 3p¹. The single 3p electron occupies a higher energy subshell and is shielded by the 3s² pair. Step 3. Less energy is required to remove this 3p electron. Answer: the 3p valence electron of aluminum is shielded by the filled 3s² subshell and higher in energy. Why not the others: Aluminum has a higher nuclear charge (Z = 13 vs 12); aluminum valence shell is not filled; atomic radius decreases from Mg to Al.

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