JEE Main 2026 — 23 January, Evening Shift — Question 53

Statement I: $\mathrm{Na} = [\mathrm{Ne}]\,3s^1$

After first ionization, we have

$\mathrm{Na^+} = [\mathrm{Ne}]$ (noble gas configuration)

Second ionization removes electron from stable core, so energy required is very large.

$\mathrm{Mg} = [\mathrm{Ne}]\,3s^2$

After first ionization, $\mathrm{Mg^+} = [\mathrm{Ne}]\,3s^1$

Second ionization removes a $3s$ electron, not from noble gas core. Thus second ionisation enthalpy of $\mathrm{Na}$ is larger than that of $\mathrm{Mg}$. Thus, the statement I is true.

Statement II: $\mathrm{O^{2-}}$ and $\mathrm{F^-}$ are isoelectronic (10 electrons). In an isoelectronic series, radius decreases with increase in nuclear charge. Since $Z_{\mathrm{O}} = 8$ and $Z_{\mathrm{F}} = 9$, $\mathrm{O^{2-}}$ has larger radius than $\mathrm{F^-}$. Thus, the statement II is true.