The history of the rare earths has entered its third century; trans uranium elements are now a half century old. Both the lanthanide and actinide ele ments, 30 elements altogether, are f elements, meaninj that their metallic 2 1 1 electronic configurations are typically 6s 5d 4f" and 7s 6d 5f" respectively. To an elementary approximation as summarized in the 'average inorganic chemistry textbook, these configurations cause their chemistry to be described by the trivalent state accompanied by less interesting effects such as the lanthanide contraction. However, the discovery of divalent and...