JEE Main 2026 — Classification of Elements and Periodicity in Properties Question with Solution

The first ionisation enthalpy generally increases across a period from left to right due to an increase in effective nuclear charge. For the third period elements $\text{Na}$, $\text{Mg}$, $\text{Cl}$, and $\text{Ar}$, the correct order of first ionisation enthalpy is $\text{Ar}>\text{Cl}>\text{Mg}>\text{Na}$. Thus, Statement I is false.

The electronic configuration of $\text{Ca}$ is $[\text{Ar}]4s^2$. After the removal of two electrons, ${\text{Ca}}^{2+}$ attains the highly stable noble gas configuration of Argon $[\text{Ar}]$. Removing a third electron requires breaking this stable noble gas core, which requires a very large amount of energy. Therefore, the third ionisation enthalpy of $\text{Ca}$ is exceptionally high compared to $\text{Al}$, $\text{Fe}$, and $\text{B}$. Thus, Statement II is true.

Answer: Statement I is false but Statement II is true