The chemical specificities of actinides

With an atomic number Z ranging from 90 to 103, actinide (An) elements have very heavy nuclei and a large number of electrons. To compare their behavior to that of the lanthanides (Ln) would be an over-simplification, even if sometimes useful. It is hard to summarize the chemical properties of the An, given the large number of oxidation states. The An(III) and An(IV) ions are hydrated with a first, well-structured layer of water molecules (8 to 10), while the An(V) and An(VI) take the form of actinyl ions (except Pa) with 5 water molecules in the equatorial plane. The An(VII) are oxo-cations in an acid environment and oxo-anions in a basic environment. This multitude of oxidation states is made even more complex by disproportionation reactions and the kinetic charge transfer constraints. (A.C.)