The radii of the iron cations are less than that of an iron atom, and Fe3+ is smaller than Fe2+.

The trend of radii is: Fe > Fe²⁺ > Fe³⁺. There is an increase in the effective nuclear charge in this series. As electrons are removed, the repulsion between the electrons decreases. The larger the effective nuclear charge, the more the electrons are pulled towards the nucleus and the smaller the atom or ion becomes.