explain anthanoids and lanthanoids contraction
Answers
Answer:
Lanthanoid contraction, also called lanthanide contraction, in chemistry, the steady decrease in the size of the atoms and ions of the rare earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71)
Answer:
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 La
3+
to Lu
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 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
(iv) Pairs of elements namely Zr-Hf(Gr4), 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.
Answered By neha
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