Q.Which pairs of ions are isoelectronic?

Ionic radius is the distance from an ion's nucleus out to which it exerts influence over the surrounding electron cloud. Pauling's method finds the individual cation and anion radii of a uni-univalent ionic crystal from its measured inter-ionic distance $d = r_{C^+} + r_{A^-}$, using the assumption that for ions sharing a noble-gas electron configuration, radius is inversely proportional to effective nuclear charge: $r \propto 1/Z_{eff}$. Dividing the cation and anion proportionalities gives their radius ratio purely from Slater $Z_{eff}$ values, which combined with the measured $d$ solves for both radii (worked example: Na+ and F- in NaF, $d = 231$ pm, gives $r_{Na^+} = 95.9$ pm and $r_{F^-} = 135.1$ pm). …