The first I.E. of Na, Mg, Al and Si are in the order : — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Step 1: Generally, first ionization energy increases from left to right across a period (Na < Mg < Al < Si) due to increasing effective nuclear charge. Step 2: However, an anomaly occurs between Mg (1s^2 2s^2 2p^6 3s^2) and Al (1s^2 2s^2 2p^6 3s^2 3p^1). Mg has a stable, fully-filled 3s^2 configuration with greater penetration than the 3p orbital of Al, making it harder to ionize Mg than Al. Step 3: Accounting for this exception, the first ionization energy increases in the order of Na < Al < Mg < Si.