Question

Compare the chemistry of actinoids with that of lanthanoids with special reference to

(i) electronic configuration
(ii) Oxidation state
(iii) Atomic and ionic radii
(iv) Chemical reactivity

Answer 1

(i) The general electronic configuration of lanthanoids is [Xe]⁵⁴ 4f^1-14 5d^0-1 6s² where as that of actinoids in [Rn]⁸⁶ 5f^1-14 6d^0-1 7s². Thus lanthanoids belong to 4f series whereas actinoids belong to 5f series.

(ii) Lanthanoids show limited oxidation states (+2, +3, +4) out of which +3 is most common. This is because of the large energy gap between 4f and 5d subshells. On the other hand, actinoids show a large number of oxidation states because of a small energy gap between 5f, 6d and 7s subshells.

(iii) Both show a decrease in the size of their atoms or ions in the +3 oxidation state. In lanthanoids, this decrease is called lanthanoid contraction, whereas in actinoids it is called actinoid contraction. However, the contraction is greater from element to element in actinoids due to poor shielding of 5f electrons than that by 4f electrons in lanthanoids.

(iv) Chemical reactivity of lanthanoids

The chemical behaviour of actinoids:

1. They react with boiling water to give a mixture of hydride and oxide.
2. They combine with most of the non-metals at high temperatures.
3. All these metals are attacked by hydro chloric acid, but the effect of nitric acid is very small due to the formation of a protective oxide layer on their surface.
4. Alkalies have no action on actinoids.