What is meant by lanthanoids contraction ? What effect does it have on the chemistry of element which follow lanthanoids ?

Explanation

Refer to HSC Paper, March 2018, Answer 8.
Decrease in basicity : (i) In lanthanoids due to lanthanoid contraction, as the atomic number increase, the size of the lanthanoid atoms and their tripositive ions decreases, i.e., from `La^(3+)` to `Lu^(3+)`
(ii) As size of the polarizability increases and thus the convalent character of the M-OH bond increases, and ionic character decreases. Therefore the basic nature of the hycroxides decreases.
(iii) BAsicity and ionic character decrease in the order La `(OH)_(3)ltCe(OH)_(3)lt . . . . Lu(OH)_(3)`
Ionic radii of post -lanthanoids :
(i) due to lanthanoid contraction the atomic radii (size) of elements with 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).
(ii) 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.
(iii) 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 indentical sizes and similar properties.