RHENIUM

(Re), chemical element, very rare metal of Group VIIb of the periodic table, one of the densest elements. Predicted by the Russian chemist Dmitry Ivanovich Mendeleyev (1869) as chemically related to manganese, rhenium was discovered (1925) by the German chemists Ida and Walter Noddack and Otto Carl Berg. The metal and its alloys have found limited application as fountain pen points, high-temperature thermocouples (with platinum), catalysts, electrical contact points, and instrument-bearing points and in electrical components. Rhenium is widely distributed in a variety of minerals but usually in concentrations averaging about 0.001 parts per million. It occurs up to about 20 parts per million in molybdenite and to a lesser extent in sulfidic copper ores. The recovery of rhenium is aided by the concentration of its volatile heptoxide (Re2O7) in the flue dust during the smelting of molybdenite ore or from its concentration with the platinum metals in the anode sludge during electrolytic copper refining. The black metal powder is prepared by the hydrogen reduction of ammonium perrhenate (NH4ReO4). The powder may be compressed and sintered into bars in hydrogen at elevated temperatures. Cold-working and annealing permit the fabrication of wire or foil. Rhenium metal is silvery-white and extremely hard; it resists wear and corrosion very well and has one of the highest melting points of the elements. The metal powder slowly oxidizes in air above 150 C (300 F) and rapidly at higher temperatures to form the yellow tetroxide. The metal is not soluble in hydrochloric acid and dissolves only slowly in other acids. There is evidence for the existence of rhenium in each of the oxidation states from 0 to +7; the most common states are +3, +4, +5, and especially +7. Perrhenic acid (HReO4) and its anhydride, the heptoxide, and the perrhenates are common stable rhenium(VII) compounds. Natural rhenium is a mixture of the stable isotope rhenium-185 (37.07 percent) and the radioactive rhenium-187 (62.93 percent, 7 1010-year half-life). atomic number 75 atomic weight 186.2 melting point 3,180 C (5,756 F) boiling point 5,627 C (10,161 F) specific gravity 20.5 (20 C) valence 1, 2, 3, 4, 5, 6, 7 electronic config. 2-8-18-32-13-2 or (Xe)4f 145d56s2