Ionic radius is :

$\text{ Ionic radius }\propto \frac{1}{{\mathrm{Z}}_{\text{eff }}}$

$Z_{\text{eff }}=\text{ effective nuclear charge }$

${\mathrm{Z}}_{\text{eff }}\text{ is calculated as follows: }$

${\mathrm{Z}}_{\text{eff }}=Z-\text{ screening constant }\left(\sigma \right)$

This value of screening constant is based upon the number of electrons in valence shell as well as in penultimate shells.