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NEET Chemistry QUESTION #7411
Question 1
If the first ionisation enthalpies of Na, Mg, and Si are 496, 737, and 776 kJ/mol respectively, then the first ionisation enthalpy of Al (kJ/mol) is:
  • Greater than 766
  • Between 496 and 737✔️
  • Between 737 and 766
  • Greater than 496
Correct Answer Explanation
Al lies between Mg and Si. However, Al (3p¹) has a lower IE₁ than Mg (3s², fully filled s-subshell) due to the 3p electron being easier to remove. Al also has lower IE₁ than Si (3p²).

Therefore: IE₁(Al) lies between Na and Mg → between 496 and 737 kJ/mol (approximately 577 kJ/mol). This is the well-known anomaly where Al < Mg in ionisation energy despite being to the right of Mg.