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Define Lanthanoid contraction. Explain its effects. |
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Answer» Solution :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 theorder`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 TRANSITIONSERIES -hf, Ta, W,Re) - are similarto theelementsof the 5thperiod 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 the2nd transition series(4d -series) whichlie immediately abovethem. (iv) Pairs of elements namely `Zr-Hf(Gr -4)` ,Nb-Ta (Gr-5),Mo-W (Gr-6),Tc-Re (Gr-7) are calledchemicaltwins since they possessalmostidenticalsizesand similarproperties. |
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