f-block elements called inner- transition elements as they have n-2 orbital incomplete. electronic configuration is 5f in complete and 7s filled. There is a steady decrease in the atomic and ionic radii M3+as the atomic number increases. This contraction in size is quite regular which is called actinoid contraction.) Lanthanoids mainly exhibit +3 oxidation state. In addition, they show lower oxidation state of +2 and higher oxidation state of +4. Actinoids also exhibit +3 oxidation state. In addition, they show a wide range of higher oxidation states from +4 to +7. This is due to comparable energies of 5f, 6d and 7s orbitals in actinoids.