JEE Main · 2004 · 24 Apr 2004 · Offline · Classification of Elements and Periodicity

Group these by electron count and the pull of the nucleus. $\mathrm{Li}{}^{+}$ and $\mathrm{B}{}^{3+}$ each keep only two electrons, while $\mathrm{F}{}^{-}$ and $\mathrm{O}{}^{2-}$ each carry ten and are isoelectronic with neon. Among isoelectronic species the one with the smallest nuclear charge grips its electron cloud most loosely and so swells the most. Oxygen has only eight protons tugging on those ten electrons, fewer than fluorine's nine, so $\mathrm{O}{}^{2-}$ expands to the largest ionic radius of the four.