JEE Main 2021 · 24 February · morning · Single correct · +4 / −1
Consider the elements Mg, Al, S, P and Si, the correct increasing order of their first ionization enthalpy is :
- (a) Al < Mg < Si < S < P
- (b) Al < Mg < S < Si < P
- (c) Mg < Al < Si < S < P
- (d) Mg < Al < Si < P < S
Answer and solution
Answer (a) Al < Mg < Si < S < P
Solution
Chemistry ma’am · 4 steps
Across period 3, IE rises, but Al dips below Mg and S dips below P. Spot those two breaks.
- General trend. Across period 3, nuclear charge increases and atomic size shrinks, so the first ionisation enthalpy generally rises from Mg to S. Two elements break this trend.
- Break one. Al (3s² 3p¹) loses a 3p electron. A 3p electron is higher in energy and is shielded by the filled 3s pair, so it leaves more easily than a 3s electron of Mg (3s²). That makes Al < Mg.
- Break two. P (3p³) has a stable half-filled 3p subshell. In S (3p⁴), one 3p orbital holds a pair, and electron repulsion makes that electron easier to remove. That makes S < P.
Element Valence configuration First IE (kJ mol⁻¹) Al 3s² 3p¹ 577 Mg 3s² 737 Si 3s² 3p² 786 S 3s² 3p⁴ 999 P 3s² 3p³ 1011 - So. The increasing order is Al < Mg < Si < S < P, which is option (a).
The idea to keep. In every period, group 13 sits below group 2 and group 16 sits below group 15. Filled s² and half-filled p³ shells are hard to break into.
Answer from the NTA final answer key. Solution by PrepWiser.