One important function of atomic and ionic radius is in regu

One important function of atomic and ionic radius is in regulating the uptake of oxygen by haemoglobin, for the change in ionic radius that accompanies the conversion of Fe(II) to Fe(III) when O2 attaches triggers a conformational change in the protein. Which do you expect to be larger: Fe2+ or Fe3+?

Solution

The ionic radius of Fe2+ is greater than the radius of Fe3+. This is because, for a given atom/ion, the radius decreases with increase in the positive charge on the ion. When the Fe2+ is going to Fe3+, it loses one electron and the effective nuclear charge on the outer electrons is greater in Fe3+. As a result, the electrons are drawn in towards the nucleus more stronlgy in Fe3+ as compared to Fe2+. Therefore, the ionic radius decreases in Fe3+