Fe2+ has the maximum number of unpaired d-electrons. Zn(30)=[Ar]3d10,4s2 Zn2+=[Ar]3d10: No unpaired electron Fe(26)=[Ar]3d6,4s2 Fe2+=[Ar]3d6: Four unpaired electrons. Ni(28)=[Ar]3d8,4s2 Ni3+=[Ar]3d7: Three unpaired electrons. Cu(29)=[Ar]3d10,4s1 Cu+=[Ar]3d10: No unpaired electrons.