Question

Explain effects of lanthanoid contraction.

Answer 1

1. Effects on the basic strength of hydroxides: As the size of the lanthanoid ions decreases from La³⁺ to Lu³⁺, the covalent character of the hydroxides increases. Thus, the basic strength decreases. Hence, La(OH)₃ is most basic, while Lu(OH)₃ is the least basic.
2. Ionic radii of post lanthanoids: The elements which follow the lanthanoids in the third transition series are known as post lanthanoids. As a result of lanthanoid contraction, the atomic radii (size) of the elements which follow lanthanum (Hf, Ta, W etc.) are similar to those of the elements of the previous period. There is normal increase in size from Sc to Y to La. This trend disappears after the lanthanoids, and pairs of elements Zr–Hf (group 4), Nb–Ta (group 5), Mo–W (group 6) and Tc–Re (group 7) have almost identical sizes. These atoms possess similar number of valence electrons and similar properties. These pairs of elements are called ‘chemical twins’. The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series.
3. Difficulty in separation of lanthanoids:- Because the changes in ionic radii (size of the ions) in the lanthanoids are very small, their chemical properties are similar. This makes the separation of the lanthanoids in the pure state difficult. However, lanthanoid contraction results in slight difference in the size of the lanthanoids which results in the differences in properties such as solubility, complex formation, hydration, basic character of their hydroxides etc. Because of these differences, the lanthanoids can be separated by the ion exchange method.

Answer 2

I. Decrease in Basicity:

1. 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.”
2. The size of the Ln³⁺ cation decreases with increase in the atomic number, due
   to lanthanoid contraction.
3. This decreases the ionic character of M−OH bond and gradually increases the covalent character of M−OH bond.
4. Therefore, the basic strength of the corresponding hydroxides decreases
   from La(OH)₃ to Lu(OH)₃.
5. Thus, La(OH)₃ is most basic and Lu(OH)₃ is least basic.

II. Ionic radii of post-lanthanoids:

1. The elements which follow the lanthanoids in the third transition series are known as post-lanthanoids.
2. There is a regular increase in size from Sc to Y to La.
3. But after the lanthanoids, the increase in radii from second to third transition series almost vanishes.
4. 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 the second and third series resemble each other.