Question

The chemistry of the actinoid elements is not so smooth as that of the Lanthanoids. Justify this statement by giving some examples from the oxidation state of these elements.

Answer 1

The oxidation states of lanthanoids are +2, +3, and +4. Of these, +3 is the most common state. The limited number of oxidation states is due to the large energy differences among the 4f, 5d, and 6s suborbitals. In contrast, actinoids exhibit multiple oxidation states, including +2, +3, +4, +5, +6, and +7, although their most common state is +3. This is because there is a small energy difference between the 5f, 6d, and 7s sub-orbitals.

Answer 2

1. The actinoids are radioactive elements, with the older members exhibiting relatively long half-lives, while the latest members have half-lives ranging from one day to three minutes, with lawrencium (Z = 103) having the shortest. The latter members could only be prepared in nanogram quantities. These facts complicate their study.
2. The greater range of oxidation states is due to the comparable energies of the 5f, 6d, and 7s levels. The actinoids usually exhibit a +3 oxidation state. The elements in the first half of the series frequently have higher oxidation states.
3. For example, the maximum oxidation state increases from +4 in Th to +5, +6 and +7, respectively, in Pa, U and Np, but decreases in succeeding elements.
4. The actinoids, such as lanthanoids, exhibit a greater number of compounds in the +3 oxidation state compared to the +4 state. However, +3 and +4 ions exhibit a tendency to undergo hydrolysis. The distribution of oxidation states among the actinoids is markedly irregular and varies significantly between the earlier and later elements. It is inadequate to evaluate their chemistry solely on the basis of oxidation states.