Consider the isoelectronic species ${\mathrm{K}}^{+},{\mathrm{S}}^{-2},{\mathrm{Cl}}^{-},{\mathrm{Ca}}^{+2},$the radii of the ions decrease as

Isoelectronic species contain same no. of electrons with different charges. The size of ion depends on total no. of electrons to proton ratio.

For ${\mathrm{S}}^{-2}\mathrm{ion},\mathrm{e}/\mathrm{p}=18/16=1.125; \mathrm{for} {\mathrm{Cl}}^{-}\mathrm{ion},\mathrm{e}/\mathrm{p}=18/17=1.058$$\mathrm{For} {\mathrm{K}}^{+}\mathrm{ion},\mathrm{e}/\mathrm{p}=18/19=0.947; \mathrm{for} {\mathrm{Ca}}^{+2} \mathrm{ions},\mathrm{e}/\mathrm{p}=18/20=0.900$

Hence, the decreasing order of radii of ions is

${\mathrm{S}}^{-2}>{\mathrm{Cl}}^{-}>{\mathrm{K}}^{+}>{\mathrm{Ca}}^{+2}$