What is lanthanoid contraction ? What are the consequences of lanthanoid contraction?

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 additionai proton in the nucleus, the corresponding electron goes into a 4f-subshell. This is too diffused to screen the nucleus as effectively as the more localised inner shell. Hence, the attraction of the nucleus for the outermost electrons increases steadily with the atomic number.
Consequences of Lanthanoid Contraction : The important consequences of lanthanoid contraction are as follows :
i) Basic character of oxides and hydroxides : 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)`.
ii) 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, pairs of elements such as Zr/Hf, Nb(fa 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 lanthauoids : 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.