Q9 / 75 · JEE Main 2026 (02 April Shift 1) · Chemistry

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 :

  • A
    Both Statement I and Statement II are true
  • B
    Both Statement I and Statement II are false
  • C
    Statement I is true but Statement II is false
  • Correct answer
    D
    Statement I is false but Statement II is true

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

Hence, the correct answer is option: 'D'.

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