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JEE Main · 2021 · 24 Feb 2021 · Shift 1 · Classification of Elements and Periodicity

Consider the elements Mg, Al, S, P and Si, the correct increasing order of their first ionization enthalpy is:

(A) $Al < Mg < Si < S < P$ ✓ Correct
(B) $Al < Mg < S < Si < P$
(C) $Mg < Al < Si < S < P$
(D) $Mg < Al < Si < P < S$

Answer: (A)

Solution

The first ionization enthalpy trend in the third period is:

Na < Al < Mg < Si < S < P < Cl < Ar

Key points:

Al < Mg: Although Al comes after Mg in the period, Al has lower ionization enthalpy because:

Mg has a completely filled 3s² orbital (stable configuration)
Al has 3s²3p¹ configuration, and removing the single 3p electron is easier

S < P: Although S comes after P in the period, S has lower ionization enthalpy because:

P has a half-filled 3p³ configuration (extra stability)
S has 3p⁴ configuration with one paired electron that experiences repulsion

General trend: Ionization enthalpy increases across a period due to:

Increasing nuclear charge
Decreasing atomic size
Stronger hold on valence electrons

For the given elements: Al < Mg < Si < S < P

This accounts for the anomalies at Mg (full 3s²) and P (half-filled 3p³).

JEE Main · 2021 · 24 Feb 2021 · Shift 1 · Single correct option

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