Actinium

The actinide series of elements encompasses all the 15 chemical elements that have properties attributable to the presence of low-lying 7p, 6d, and 5f orbitals such that their tripositive ions have electronic configurations 7p⁰6d⁰5fⁿ, where n = 0,1,2,…,14. According to this definition, actinium, element 89, is the first member of the actinide series of elements, although it has no 5f electrons in its metallic, gaseous, or ionic forms. As such, its position in group 3 (in current IUPAC terminology) or group 3B (commonly used in some American textbooks) of the periodic table is analogous to that of its homolog, lanthanum, in the lanthanide series. This definition, which includes actinium as the first of the actinides (Seaborg, 1994), parallels the accepted inclusion of lanthanum as the first member of the lanthanide series (Moeller, 1963).

The chemistry of actinium closely follows that of lanthanum. There are no qualitative differences between them; the only quantitative differences are those attributable to the difference in their ionic radii (1.12 Å for Ac³⁺ and 1.032 Å for La³⁺ in six-fold coordination) (Shannon, 1976 and Chapter 15, section 7.5, of this book). Because of this similarity, lanthanum is a nearly ideal surrogate for actinium in the development of preparative or analytical procedures. As a carrier for trace amounts of actinium, lanthanum suffers from only one disadvantage: Once mixed, the two elements behave like any pair of adjacent rare earths and can be separated only by ion-exchange chromatography, solvent extraction, or fractional crystallization.