Th e central iron ion of cytochrome c changes between the +2 and +3 oxidation states as the protein shuttles electrons between complex III and complex IV of the respiratory chain (Section 5.10). Which do you expect to be larger: Fe2+ or Fe3+ ? Why?

Fe2+ atom has a larger size than Fe3+.

Explanation:

The electron transport chain is important for the production of the ATP molecule. The oxygen molecule acts as the final electron acceptor in the electron transport chain.

The size of the cation is always small as compared with the parental atoms. Two electrons are removed from Fe2+ and three electrons are removed from  Fe3+. This removal of electron increases the attraction force between the electron of the outermost shell and the nucleus and decreases the repulsion force between the electrons and thus decreases in size.

gold i'm pretty sure i might be wrong