question_answer 17)

What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

Answer:

In going from La3+ to Lu3+ in Lanthanoid series, the size of ion decreases from 1.06 Å to 0.85 Å. This total decrease of 0.21 Å in the Lanthanoid series is known as lanthanoid contraction. Cause. The lanthanoid contraction arises due to imperfect (poor) shielding of one 4f electron by another present in the same subshell. As a result, nuclear charge felt by outer most electron increases so force of attraction also increases. This results in the contraction of 4f subshell. Consequences or effects of Lanthanoid contraction. (i) Similarity in Properties. Due to Lanthanoid contraction, the size of elements which follow lanthanoids (Hf—Hg) are almost similar to the size of the elements of previous row (Zr—Cd). Thus, Zr—Hf, Nb–Ta and Mb—Ta pairs have similar sizes, similar properties, similar chemistry and hence these are difficult to separate. (ii) Basicity difference. Due to lanthanoid contraction, in going from La3+ to Lu3+ the size decreases. Thus, covalent character of bond between M+3 and OH– increases and tendency to release OH– decreases. Hence basic character of hydroxides also decreases i.e. why La(OH)3 is most basic while Lu(OH)3 is least basic.