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Explain the effects of lanthanoid contraction.

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Effects of lanthanoid contraction:

i. Decrease in basicity:

a. The size of Ln3+ cation decreases with increase in the atomic number, due to lanthanoid contraction. This decreases the ionic character of M-OH bond and gradually increases the covalent character of M-OH bond.

b. Therefore the basic strength of the corresponding hydroxides decreases from La(OH)3 to Lu(OH)3. Thus, La(OH)3 is most basic and Lu(OH)3 is least basic.

Note: As per Fajan’s principle, “With increase in the size of cation, the tendency of the hydroxide to dissociate, increases. This increases the strength of base.”

ii. Ionic radii of post lanthanoids:

a. There is a regular increase in size from Sc to Y to La. But after the lanthanoids, the increase in radii from second to third transition series almost vanishes. 

b. Pairs of elements such as Zr–Hf (group 4), Nb–Ta (group 5), Mo–W (group 6) and Tc-Re (group 7) possess almost same size. These pair of elements are called ‘chemical twins’. The properties of these elements are also similar. So due to lanthanoid contraction, elements of second and third series resemble each other.

Note: 

a. The elements which follow the lanthanoids in the third transition series are known as post-lanthanoids.

b. The ionic radii of 1st, 2nd and 3rd transition series elements are listed in the following table.

Ionic radii of 1st, 2nd and 3rd transition series elements:

Group → Series ↓ 4 5 6 7
1st transition series Ti (132 pm) V (122 pm) Cr (106 pm) Mn (94 pm)
2nd transition series Zr (145 pm) Nb (134 pm) Mo (129 pm) Tc (114 pm)
3rd transition series Hf (144 pm) Ta (134 pm) W (130 pm) Re (114 pm)

iii. Similarity among lanthanoids: Lanthanoids show very small change in radii so their chemical properties are quite similar. Thus it is very difficult to separate the elements in pure state.

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