JEE Main · 2019 · 08 Apr 2019 · Shift 1 · Classification of Elements and Periodicity

The species ${\mathrm{Cl}}^{-}$, $\mathrm{Ar}$ and ${\mathrm{Ca}}^{2+}$ are isoelectronic, all carrying $18$ electrons in the same shells, so their sizes cannot differ by electron count or quantum numbers. What differs is the number of protons pulling on those electrons, $17$ in ${\mathrm{Cl}}^{-}$, $18$ in $\mathrm{Ar}$ and $20$ in ${\mathrm{Ca}}^{2+}$. A greater nuclear charge draws the cloud in more tightly, so it is the nuclear charge that governs the size of these isoelectronic species.