Define Lanthanoid contraction. Explain its effects.

Explanation

Lanthanoid contraction and its effects :
The gradual decrease in atomic and ionic radii of lanthanoids with the increase in atomic number is called lanthanoid contraction.
Effects :
(1) Decrease in basicity :
(i) In lanthanoids due to lanthanoid contraction, as the atomic number increases, the size of the lanthanoid atoms and their tripositive ions decreases, i.e. from `La^(3)` + to `Lu^(3+)`.
(ii) (ii) As size of the cation decreases, according to Fajan's rule, the polarizability increases and thus the covalent character of the M-OH bond increases, and ionic character decreases.
(iii) Therefore the basic nature of the hydroxides decreases.
(iv) Basicity and ionic character decrease in the order `La(OH)_(3) gt Ce(OH)_(3) gt Lu(OH)_(3)`.
(2) Ionic radii of post-lanthanoids :
(i) Elements following the lanthanoids in the 6th period (third transition series, i.e. 5d-series) are known as post-lanthanoids.
(ii) Due to lanthanoid contraction the atomic radii (size) of elements which follow lanthanum in the 6th period (3rd transition series -hf, Ta, W,Re) - are similar to the elements of the 5th period 4d-series Zr,Nb,Mo Tc).
(iii) Due to similarity in their size, post-lanthanoid elements (5d-series) have closely similar properties to the elements of the 2nd transition series (4d -series) which lie immediately above them.
(iv) Pairs of elements namely `Zr-Hf(Gr -4)` ,Nb-Ta (Gr-5), Mo-W (Gr-6),Tc-Re (Gr-7) are called chemical twins since they possess almost identical sizes and similar properties.