On the basis of lanthanoid contraction, explain the following:
(i) Nature of bonding in `La_(2)O_(3)` and `Lu_(2)O_(3)`.
(ii) Trends in the stability of oxo salts of lanthanoids from La to Lu.
(iii) Stability of the complexes of lanthanoids.
(iv) Radii of 4d and 5d block elements.
(v) Trends in acidic character of lanthanoids oxides.

Explanation

(a) `La_(2)O_(3)` is expected to be ionic in nature because the element La is present in group 3 (d-block). The element Lu is the last member of lanthanoid family. Since the atomic size decreases along period, its compound `Lu_(2)O_(3)` is expected to be covalent in nature.
(ii) Sice the atomic size decreases from La to Lu in the family of lanthanoids, the oxo salts tar expecte to a more and more covalent character. The stability of these salts is expected to decrease .
(iii) With the decrease in the atomic size or atomic radius along lanthanoids, the bonding beteeen the metal atom ( or ion ) and hte ligands is expected to become stronger. The stability of the complexes is therefore, expected to increase.
(iv) Atomic radius of the element belonging to 4d and 5d series but presenet in the same group are expected to the similar due to lanthanoid contraction.
(v) The oxides of the lanthanoid family `M_(2)O_(3)` dissolve in water to form `M(OH)_(3)`. Due to gradual decrease in the size of `M^(3+)` ion, there is an increase in covalent character of M-OH bond. The cleavage of O-H bond becomes easy and the acidic character of the lanthanoid oxides is expected to increase from left to right.