Chemistry - PERIODIC TABLE Question with Solution | TestHub

For $(F^{-},C{l}^{-})$ and $(N{a}^{+},R{b}^{+})$: Moving down a group adds new electron shells, so ionic size increases ($C{l}^{-}>F^{-}$ and $R{b}^{+}>N{a}^{+}$).

For $(O^{2-},N^{3-})$ and $(C{a}^{2+},K^{+})$: For isoelectronic species, a lesser nuclear charge (fewer protons) leads to a weaker pull on electrons and a larger ionic radius ($N^{3-}>O^{2-}$ and $K^{+}>C{a}^{2+}$).

For $(F{e}^{2+},F{e}^{3+})$: A higher positive charge increases the effective nuclear charge, making the ion smaller ($F{e}^{2+}>F{e}^{3+}$).

For $(I^{-},H^{-})$: Iodine has significantly more electron shells than hydrogen, making it much larger ($I^{-}>H^{-}$).

Therefore, there are exactly 4 such pairs where the second species is larger than the first.