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 :

The ionisation enthalpy (IE) is the energy required to remove an electron from a gaseous atom/ion. Across a period: nuclear charge increases, atomic size decreases ⇒ IE usually increases. Down a group: atomic size increases, shielding increases ⇒ IE usually decreases. Extra-stable configurations (filled or half-filled subshells, noble-gas cores) raise IE significantly.

Checking Statement (I)
Elements considered: Na (Z = 11), Mg (12), Cl (17), Ar (18). Across the third period the general trend is $$\text{IE}(\text{Na}) \lt \text{IE}(\text{Mg}) \lt \dots \lt \text{IE}(\text{Cl}) \lt \text{IE}(\text{Ar})$$ because:

• Mg has higher IE than Na due to greater nuclear charge and similar shielding.
• Ar, a noble gas with completely filled subshells, has the highest IE in the period.
• Cl lies just before Ar, so its IE is lower than that of Ar but higher than those of Mg and Na.

Thus the correct order is $$\text{Na} \lt \text{Mg} \lt \text{Cl} \lt \text{Ar}$$ whereas Statement (I) claims the reverse order $$\text{Na} \gt \text{Mg} \gt \text{Cl} \gt \text{Ar}$$. Hence Statement (I) is false.

Checking Statement (II)
Third ionisation enthalpy, IE₃, means the energy to remove the third electron, i.e. $$\text{(X}^{2+}) \rightarrow \text{X}^{3+}+e^-$$

Electronic configurations after removal of two electrons:

• Ca (Z = 20): $$\text{Ca}^{2+}:[\text{Ar}]$$ (noble-gas core). Removing a third electron would have to disturb the filled 3p core, so IE₃ becomes extremely large.

• Al (Z = 13): $$\text{Al}^{2+}:[\text{Ne}]\,3s^1$$. The third electron is still a valence 3s electron; removal is easier than breaking into the core.

• Fe (Z = 26): $$\text{Fe}^{2+}:[\text{Ar}]\,3d^6$$. The third electron comes from the 3d subshell, also part of the valence shell, so IE₃ is moderate.

• B (Z = 5): $$\text{B}^{2+}:[\text{He}]\,2s^1$$. Again, the electron removed in IE₃ is a valence 2s electron, not a core electron.

Therefore IE₃ shows a very large jump only for Ca because it requires removing a core (3p) electron after achieving the stable [Ar] configuration. Hence Statement (II) is true.

Summary: Statement (I) is false, Statement (II) is true. Correct option: Option D which is: Statement I is false but Statement II is true.