Classification of Elements Part 1

For iso-electronic system, radius is inversely proportional to the Z‌eff. ‌. The effective nuclear charge (Z‌eff ‌) is net positive charge, experienced by an electron in a multi-electron atom.

Na+‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌Mg2+‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌Al3+ ╰───────────▾──────────────╯ Z=11‌‌‌‌‌‌‌‌‌Z=12‌‌‌‌‌‌‌‌‌‌‌‌‌Z=13

(10‌electrons in each)

(Z = atomic number)
We know that, more is the positive charge on an ion, smaller will be its ionic radii. Whereas more is the negative charge on an ion more will be its ionic radii.
So, order of ionic size
Na+>Mg2+>Al3+
Ionic radii ofNa+(1.02˙A)>Mg2+(0.72˙A)>Al3+(0.54˙A)