Chemistry Q57 – JEE-MAIN 2026

Home Chemistry Periodicity & ElementsQuestion #506

The

1^{st}

ionization enthalpy for Mg is

+ 737

kJ/mol. The most probable estimated value of the

2^{nd}

ionization enthalpy of Mg is _______.

Clue 1 of 3: Nature of Ionization

Ionization is an endothermic process, meaning energy is absorbed, so ionization enthalpy values are always positive.

🧠 Full Solution Path

Step 1: Identify the Nature of Ionization Enthalpy✦ Active

Ionization enthalpy is the energy required to remove an electron from a gaseous atom or ion. This process is always endothermic, meaning energy is absorbed, so the enthalpy change is positive.

💡 Teacher's Secret Hint

Remember that energy must be supplied to remove an electron, hence the positive sign.

Step 2: Analyze the Trend in Successive Ionization Enthalpies○ Expand

For any element, the second ionization enthalpy ( $I E_{2}$ ) is always greater than the first ionization enthalpy ( $I E_{1}$ ). This is because removing an an electron from a positively charged ion ( $M g^{+}$ ) requires more energy than removing it from a neutral atom ( $M g$ ) due to a higher effective nuclear charge and stronger electrostatic attraction.

M g (g) \overset{I E_{1}}{\to} M g^{+} (g) + e^{-}

{Mg}^{+} (g) \overset{I E_{2}}{\to} M g^{2 +} (g) + e^{-}

I E_{2} > I E_{1}

💡 Teacher's Secret Hint

The $M g^{+}$ ion has a smaller radius and a stronger pull on its remaining electrons compared to the neutral $M g$ atom.

Step 3: Evaluate the Given Options○ Expand

Given $I E_{1} = + 737$ kJ/mol. Based on the principles from Step 1 and Step 2, $I E_{2}$ must be a positive value and greater than $+ 737$ kJ/mol. Let's examine the options:

1. $- 906$ kJ/mol: Incorrect, as ionization enthalpy must be positive.

2. $- 856$ kJ/mol: Incorrect, as ionization enthalpy must be positive.

3. $+ 1450$ kJ/mol: Correct, as it is positive and significantly greater than $+ 737$ kJ/mol.

4. $+ 590$ kJ/mol: Incorrect, as it is positive but less than $+ 737$ kJ/mol, which contradicts the trend that $I E_{2} > I E_{1}$ .

💡 Teacher's Secret Hint

Always check both the sign and the magnitude relative to the previous ionization enthalpy.

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Chemistry Question 57 – JEE-MAIN 2026

Clue 1 of 3: Nature of Ionization

🧠 Full Solution Path