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Chemistry Question 59 – JEE-MAIN 2026

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Given below are two statements : Statement (I) : The first ionisation enthalpy of the elements Na, Mg, Cl and Ar follows the order Na > Mg > Cl > Ar. Statement (II) : Among Ca, Al, Fe and B, the third ionisation enthalpy is very high for Ca. In the light of the above statements, choose the correct answer from the options given below :

Recall the general trends of ionization enthalpy across a period and down a group, and the exceptions related to stable electron configurations (half-filled or fully-filled orbitals).

Step 1: Analyze Statement (I) - First Ionization Enthalpy Trend✦ Active

The elements Na, Mg, Cl, and Ar are in Period 3 (Ar is a noble gas at the end of Period 3). The general trend for first ionization enthalpy (IE1) across a period is an increase due to increasing effective nuclear charge and decreasing atomic radius. The actual order of IE1 is Na < Mg < Cl < Ar. The given order Na > Mg > Cl > Ar is incorrect. Thus, Statement (I) is false.

Step 2: Analyze Statement (II) - Third Ionization Enthalpy○ Expand

Consider the electron configurations and the removal of the third electron for Ca, Al, Fe, and B. For Ca ([Ar]4s2), the first two electrons are removed from the 4s orbital. The third electron must be removed from the stable noble gas core ([Ar] configuration), which requires an exceptionally high amount of energy. For Al ([Ne]3s23p1), B ([He]2s22p1), and Fe ([Ar]3d64s2), the third electron is removed from a valence shell or sub-valence shell, not a noble gas core. Therefore, the third ionization enthalpy is very high for Ca compared to Al, Fe, and B. Thus, Statement (II) is true.

Step 3: Conclusion○ Expand

Based on the analysis, Statement (I) is false and Statement (II) is true. This corresponds to option 4.

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