JEE Main 2021 · 27 July · evening · Single correct · +4 / −1
The correct order of first ionisation enthalpy is :
- (a) Mg < S < Al < P
- (b) Mg < Al < S < P
- (c) Al < Mg < S < P
- (d) Mg < Al < P < S
Answer and solution
Answer (c) Al < Mg < S < P
Solution
Chemistry ma’am · 4 steps
Across period 3 the trend rises, but it dips twice: at Al after Mg, and at S after P.
- General trend. Across a period the nuclear charge increases and the atom shrinks, so first ionisation enthalpy usually rises from left to right. Two exceptions in period 3 decide this question.
- Mg versus Al. Mg (3s²) loses an electron from a full, more penetrating 3s subshell. Al (3s² 3p¹) loses its lone 3p electron, which is higher in energy and partly shielded by the 3s pair. So Al is lower than Mg.
- P versus S. P (3p³) has a stable half-filled 3p subshell. In S (3p⁴) one 3p orbital holds a paired electron, and the repulsion between that pair makes one electron easier to remove. So S is lower than P.
Element Valence configuration First IE (approx., kJ mol⁻¹) Mg 3s², filled subshell 738 Al 3s² 3p¹, lone 3p electron 577, lowest P 3s² 3p³, half-filled 1012, highest S 3s² 3p⁴, one paired 3p 1000 - So. The order is Al < Mg < S < P, which is option (c).
The idea to keep. In periods 2 and 3, the IE trend dips at group 13 (the first p electron) and at group 16 (the first paired p electron). Check those two pairs first.
Answer from the NTA final answer key. Solution by PrepWiser.