The change in ionic radii is Lanthanide series is very small, their chemical properties are small. Thus, the separation of Lanthanide is difficult due to consequence of Lanthanide contraction. The ionic radii of same positive charge of to decreases. So, the polarizing power towards hydroxyl group increases. Therefore the covalent character increases. Thus, is more basic than . and have the almost same radius as the electrons in the elements after are added into the deep seated orbitals which do not take part in the chemical reaction.