Rhenium: Last Naturally Occurring Stable Precious Metal DiscoveredIn the periodic table of elements, rhenium is found as a third-row transition metal in group 7. Known to be one of the rarest precious metals in the Earth's crust, rhenium has an average concentration of 1 part per billion. It is obtained mainly as a by-product of the refinement of two other chemical elements - copper and molybdenum. Rhenium was discovered as a trace element in the mineral columbite and in platinum ores. Three German chemists - Otto Berg and the couple Walter Noddack and Ida Tacke - made the discovery in 1925. This find made rhenium the last identified naturally occurring precious metal with stable isotopes. Actually, naturally occurring rhenium is composed of 2 stable isotopes and 26 unstable ones. Following is a list of some of the properties of rhenium: General: • Chemical Symbol: Re • Atomic Number: 75 • Category (as an element): Transition Metal • Group/ Period/ Block (in the Periodic Table): 7/ 6/ d • Atomic Weight: 186.207 g.mol-1 • Electron Configuration: [Xe] 4f14 5d5 6s2 Physical: • Density (near room temperature): 21.02 g.cm-3 • Liquid Density (at melting point): 18.9 g.cm-3 • Melting Point: 3186°C, 5767°F, 3459°K • Boiling Point: 5596°C, 10105°F, 5869°K • Heat of Fusion: 60.43 kJ.mol-1 • Heat of Vaporization: 704 kJ.mol-1 Atomic: • Oxidation States: 7, 6, 5, 4, 3, 2, 1, 0, -1 • Electronegativity: 1.9 (Pauling scale) • Atomic Radius: 137 picometre • Covalent Radius: 151±7 picometre • Ionization Energies: 760 kJ.mol-1 (first), 1260 kJ.mol-1 (second), 2510 kJ.mol-1 (third) Rhenium is silvery-white in appearance. It is the third element (after tungsten and carbon) with the highest melting point and the fourth densest (after platinum, iridium, and osmium). Commercially, rhenium is traded in powder form. Its principal application is in the making of certain parts of jet engines. Here, the metal is added to high-temperature nickel-based superalloys. Other uses of rhenium are as follows: 1. As catalysts in making lead-free, high-octane gasoline. 2. As filaments in making ion gauges, mass spectrographs, and photoflash lamps. 3. As electrical contact materials, due to its high resistance to arc corrosion and wear. 4. As catalysts for hydrogenation of fine chemicals, because of its high resistance to chemical poisoning from phosphorus, sulfur, and nitrogen. 5. As treatment for liver cancer, because of its radioactive isotopes. Rhenium resources are identified in several countries, which include Chile, Peru, Armenia, Mexico, Russia, Canada, Kazakhstan, and Uzbekistan. The estimated total resource from these eight countries is 6,000 tonnes (6 million kilograms). The United States alone, on the other hand, has an estimated total resource of 5,000 tonnes (5 million kilograms). These are identified in the states of Arizona, Miami, and Utah. But in spite of these significant resources, the U.S. continues to import a big part of its total consumption of the precious metal from some of the countries mentioned above. Since rhenium and its compounds are used in very small amounts, very little is known about their toxicity. So far, only a few rhenium compounds have been tested for toxicity, and these include rhenium trichloride and potassium perrhenate. The price of rhenium is about 250 U.S. dollars per troy ounce (about 8,300 U.S. dollars per kilogram).
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