What is Lanthanoid contraction ?Mention the cause for it .

Explanation

Lanthanoid contraction : The overall decrease in atomic and ionic radii from lanthanum to leutetium is observed in the lanthanoids. This phenomenon is called lanthanoid contraction it is due to the fact that with every additional proton in the nucleus, the corresponding electron goes into a 4f nucleus as effectively as the more localized inner shell. Hence, the attraction of the nucleus for the outermost electrons increases steadily with the atomic number.
Consequences of Lanthanoid Contraction: The important conquences of lanthanoid contraction are as follows:
(i) Basic character of oxides and hydrpxides:
Due to the lanthanoid contraction the covalent nature of La-OH bond increases and thus, the basic character of oxides and hydroxides decreases from La `(OH)_(3)` to Lu `(OH)_(3)`.
(iii) Similarity in the size of elements of second and third transition series: Because of lanthanoid contraction, elements which follow the third transition series are considerably smaller than would otherwise be expected. The normal size increases from `Sc rarr Y rarr La` and disappears after lanthanides. Thus paris of elements such as `Zr//Hf`, `Hb//Ta` and `Mo//w` are almost identical in size. Due to almost similar size, such pairs have very similar properties which makes their separation difficult.
(iii) Separation of ianthanoids : Due to lanthanoid contraction, there is a difference in some properties of lanthanoid like solubility, degree of hydration and complex formation. These difference enable the separation of lanthanoids by ion exchange method.