Question

Define lanthanoid contraction. Explain its effects.

Answer 1

Lanthanoid contraction: The gradual decrease in atomic and ionic radii of lanthanoids with the increase in atomic number is called lanthanoid contraction. It has a definite effect on the properties of lanthanoids.
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 $L{a}^{3+}$ to $L{u}^{3+}$.
(ii) As size of the cation decreases, according to Fajan's rule, the polarizability increases and thus the convalent character of the M-OH bond increases, and ionic character decreases. Therefore the basic nature of the hydroxides decreases.
(iii) Basicity and ionic character decrease in the order $La(OH{)}_3>Ce(OH{)}_3>Lu(OH{)}_3$.
Ionic radii of post-lanthanoids:
(i) Due to lanthanoid contraction the atomic radii(size) of elements which follow lanthanum in the $6$th period ($3$rd transition series Hf, Ta, W, Re)are similar to the elements of the $5$th period($4$d-series Zr, Nb, Mo, Tc).
(ii) Due to similarity in their size, post-lanthanoid elements($5$d-series) have closely similar properties to the elements of the $2$nd transition series($4$d-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.