Lanthanoid contraction : The overall decrease in atomic and ionic radii from lanthanum to leutetium is observed in the lanthanoids. This phenomenon is called lantqanoid contraction. It is due to the fact that with every additional 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.
Consequence of Lanthanoid Contraction: The important consequences of lanthanoid contraction are as follows:
i) Basic character of oxides and hydroxides : pue to the lanthanoid contraction, the covalent nature of La-OH bond increases and thus, the basic chatracter 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/Ta and Mo/w are almost identical in size.
Due to almost simillar size, such pairs have very similar properties which makes their separation difficult.
iii) Separation of lathanoids : 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.
