JEE Main · 2025 · 29 Jan 2025 · Shift 1 · Classification of Elements and Periodicity

Statement I: ${\mathrm{Mg}}^{2+}$, ${\mathrm{Na}}^{+}$, ${\mathrm{F}}^{-}$ and ${\mathrm{O}}^{2-}$ are isoelectronic (all 10 electrons), so their radii are set purely by nuclear charge: ${\mathrm{Mg}}^{2+}$ (Z=12) pulls its electron cloud in hardest and is smallest, while ${\mathrm{O}}^{2-}$ (Z=8) is held loosest and is largest. The stated order ${\mathrm{Mg}}^{2+}<{\mathrm{Na}}^{+}<{\mathrm{F}}^{-}<{\mathrm{O}}^{2-}$ is correct.

Statement II: Among the halogens, chlorine has the largest magnitude of electron gain enthalpy overall — fluorine's very small atomic size crowds the incoming electron into a compact 2p subshell with strong electron–electron repulsion, making its electron gain enthalpy slightly less negative than chlorine's; the values then fall steadily on descending the group as atomic size increases and the added electron feels weaker effective nuclear attraction. The order $\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}$ is correct.

Both statements are correct.