Actinide elements lanthanide-like behaviour

The exact position of the crossover depends however, on the type of compound that is formed from an actinide. It appears usually for the radioactive heavier actinides. In order to study it, therefore, compoimds of Pu, Am, Cm have to be investigated. Compounds of elements preceding them in the actinide series present properties due to the itineracy of the 5 f electrons, which are somewhat similar to the d-transition elements compounds (especially 3d). Heavier actinides are lanthanide-like although their properties may depart from a true lanthanide behaviour unfortunately, their rarity and the difficulty of their handling is such that very few photoemission results are available for them. 

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... 

---