rhenium (rēˈnēəm) [key], metallic chemical element; symbol Re; at. no. 75; at. wt. 186.207; m.p. about 3,180°C; b.p. about 5,625°C; sp. gr. 21.02 at 20°C; valence - 1, +2, +3, +4, +5, +6, or +7. Rhenium is a very dense, high-melting, silver-white metal. Of the elements, only carbon and tungsten have higher melting points and only iridium, osmium, and platinum are more dense. The chemical properties of rhenium are like those of technetium, the element above it in Group 7 of the periodic table. A number of rhenium compounds are known, among them halides, oxides, and sulfides. The heptavalent oxide, Re2O7, on dissolving in water forms perrhenic acid, HReO4, from which many other compounds are prepared. Rhenium is not found uncombined in nature. It is widely distributed in the earth's crust in platinum and molybdenum ores and in many minerals, but is not abundant. In the United States rhenium is obtained commercially as a byproduct of the roasting of copper sulfide ores from Arizona and Utah. Rhenium metal is obtained as a powder by reduction of its compounds with hydrogen. The powder is compacted, sintered, annealed, and formed into wire, foil, rods, or strips. Rhenium is used in alloys with tungsten; it gives improved ductility and high-temperature strength. These alloys are used for electrical contacts, electronic filaments, and thermocouples and in photographic flash lamps. Rhenium forms a superconductive alloy with molybdenum. Rhenium is used as a catalyst for hydrogenation and petroleum cracking. Based on his periodic law, Mendeleev predicted the existence of rhenium, which he called dvi-manganese. The accuracy of prediction of the properties of the element led to its discovery in 1925 by Walter Nodack, Ida Tacke, and Otto Berg in platinum ores and the mineral columbite.