molybdenum (məlĭbˈdənəm) [key] [Gr., = leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.96; m.p. about 2,617°C; b.p. about 4,612°C; sp. gr. 10.22 at 20°C; valence +2, +3, +4, +5, or +6. Molybdenum is a hard, malleable, ductile, high-melting, silver-white metal with a body-centered cubic crystalline structure. It is below chromium in Group 6 of the periodic table. Molybdenum resists corrosion at ordinary temperatures. In forming compounds, as in oxides, sulfides, and halides, it exhibits variable valence. In its most important compounds, however, it has an oxidation state of +6, as in the trioxide, which forms a series of compounds known as the molybdates. Molybdenum does not occur uncombined in nature. Its chief ore is molybdenite (molybdenum disulfide, MoS2). It also occurs in wulfenite (a lead molybdate) and powellite (a calcium molybdate-tungstate). It is widely but sparingly distributed throughout the world; it is found in the United States, Canada, Europe, Australia, Chile, Russia, and China. Large amounts of molybdenite are mined at Climax, Colo. Molybdenum ore is also obtained as a byproduct of copper mining. The ores are usually concentrated by the flotation process before being refined. The actual refining process depends on the ultimate use. The molybdenite may be purified for use in lubricants. Almost all molybdenum ore is converted by roasting to molybdic oxide, MoO3. The oxide may be added directly to steel or may be converted to ferromolybdenum by a thermal process; this alloy is used to add molybdenum to other iron and steel alloys. The oxide may be further purified by sublimation, or converting directly from the solid to vapor state, and then reduced to molybdenum powder by reaction with carbon, aluminum, or hydrogen. The oxide may be dissolved in ammonium hydroxide; the solution is filtered and evaporated to yield ammonium molybdate, (NH4)2Mo2O7. In alloy, steel molybdenum acts as a hardening agent and also improves the properties of the alloy at high temperatures; such alloys are used in making high-speed cutting tools, aircraft parts, and forged automobile parts. The pure metal in the form of thin sheets or wire is used in X-ray tubes, electronic tubes, and electric furnaces because it can withstand high temperatures. It was used in early incandescent light bulbs. Because it retains its strength and structure at very high temperatures, it has found use in certain critical rocket and missile parts. Useful compounds of molybdenum include molybdenum disulfide, used as a lubricant; ammonium molybdate, used in chemical analysis for phosphates; and lead molybdate, used as a pigment in ceramic glazes. Molybdenum was recognized as a distinct element in 1778 by K. W. Scheele; its ore had earlier been confused with lead ore, hence its name. The element was isolated by P. J. Hjelm in 1782.
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