Actinides: d-or ƒ—Transition Metals?

Abstract

The chemical and physical properties of elements depend on their electron configuration and, in particular, on the number and the nature of the bonds formed by the outer ("valence") electrons. Crystal structure and lattice parameters, as well as thermodynamic and electronic properties reflect the nature and strength of these chemical bonds. For instance, the crystal radii of elements are a function of their atomic number and oxidation number, hence of their electron configuration in the solid state. Thermodynamics give access to the strength of the bond: melting temperatures, vapour pressures and enthalpies of dissolution are measures of the tendency of the elemental crystal to pass into the liquid, gaseous or solution phase. Specific heat and electrical resistivity as well as magnetic susceptibility furnish additional information on the electrons, their energy and their participation in bonding. Finally, spectroscopy (optical, photoemission) can provide information on the mixing of different quantum characters in the bond (hybridization). Actinide elements (Z = 89 -103) include the heaviest natural and most of the synthetic transuranium elements. They form a series of transition elements, characterized by the filling of an inner -the 5f-electron shell. The elements from Ac (Z = 89) to Es (Z = 99) are available in quantities sufficient for solid state studies. Elemental actinides are metallic. The methods of metal preparation and characterization have been improved to yield samples of known purity and crystal structure, sometimes in the form of single crystals. Recent measurements of structural, thermodynamic and electronic properties have emphasized elements in the beginning and in the centre of the actinide series. In the following, methods for preparation, purification and characterization of actinide. metals are reviewed. Properties are presented, the theoretical interpretation of which underlines the special nature of the actinides in comparison with d or 4f (lanthanide) transition metals. 0-8412-0568-X/80/47-131-183$05.00/0