d and f Block Elements
Oxidation state
The important oxidation states exhibited by actinides are compiled below in the tabular form. Some of them are stable but most of these oxidation states are unstable. It may be seen from these oxidation states that the +2 state is shown by Th and Am only in the few compounds like ThBr2, ThI2, ThS, etc. The +3 oxidation state is exhibited by all the elements and it becomes more and more stable as the atomic number increases. The +4 oxidation state is shown by the elements from Th to Bk, the +5 oxidation state by Th to Am, the +6 state by the elements from U to Am and the +7 state is exhibited by only two elements, viz, Np and Pu. Np in the +7 state acts as an oxidising agent.
The principal cations given by actinide elements are M3+, M4+ and oxo-cations such as MO2 + (oxidation state of M = + 5) and MO22+ (oxidation state of M = +6). The examples of oxo-cations are UO2 +, PuO2 +, UO22+ and PuO22+ which are stable in acid and aqueous solutions. Most of the M3+ ions are more or less stable in aqueous solution. Np3+ and Pu3+ ions in solution are oxidized to Np4+ and Pu4+ by air. The latter ions are further oxidized slowly to UO22+ and PuO22+ by air. Various oxidation states of the actinides are listed below:
| Element | Oxidation states | Element | Oxidation states |
| Th | +2, +3, +4 | Bk | +3, +4 |
| Pa | +3, +4, +5 | Cf | +3 |
| U | +3, +4, +5, +6 | Es | +3 |
| Np | +3, +4, +5, +6, +7 | Fm | +3 |
| Pu | +3, +4, +5, +6, +7 | Md | +3 |
| Am | +2, +3, +4, +5, +6 | No | +3 |
| Cm | +3, +4 | Lr | +3 |
Composition of the oxidation states of lanthanides with those of actinides indicates that the +3 oxidation state is most common for both the series of elements. The lighter elements up to Am show variable oxidation states, the maximum being for Np, Pu and Am, but the heavier elements show constant oxidation state of +3.This oxidation state becomes increasingly stable with increasing atomic number in the actinide series. The increasing stability of +3 oxidation state is illustrated by the increasing difficulty of oxidation above the +3 oxidation state. Actinides show a greater multiplicity of oxidation states. For the first half of the actinide series (i.e. lower actinides) the energy required for the conversion 5f→6d is less than 4f→5d, the lower actinides should show higher oxidation state such as +4, +5, +6 and +7. Also, in the second half of the actinide series (i.e. higher actinides), the energy required for the conversion 5f→6d is more than 4f→5d, and the higher actinides should show much lower oxidation states such as +2.The tripositive oxidation state occurs widely in each series. The two groups of elements are not entirely comparable in this respect.
