Actinide elements redox behaviour

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

From table 3, it can be seen that for the actinides in the first half of the transition series a multiplicity of valence states are possible, whilst those in the second half have a more restricted number of valence states available and have more in common with the lanthanide elements. This multiplicity of oxidation states can lead to some extremely complicated solution chemistry, but, fortunately, all of the actinides have one oxidation state which is dominant under fairly well-defined solution conditions. As a result, actinide redox behaviour is understood to a reasonable extent under physiological conditions although there are exceptions as will be discussed. It is, however, worthwhile discussing, briefly, general actinide reduction-oxidation behaviour because valence states which dominate in the environment and which impinge on biochemical/biologi-cal systems may not dominate under physiological conditions. 

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