MOLYBDENUM

(Mo), chemical element, silver-gray refractory metal of Group VIb of the periodic table, used to impart superior strength to steel and other alloys at high temperature. A brief treatment of molybdenum follows. For the mining, recovery, and applications of molybdenum, see Industries, Extraction and Processing: Molybdenum. The Swedish chemist Carl Wilhelm Scheele had demonstrated (c. 1778) that the mineral molybdaina (now molybdenite), for a long time thought to be a lead ore or graphite, certainly contains sulfur and possibly a previously unknown metal. At Scheele's suggestion, Peter Jacob Hjelm, another Swedish chemist, successfully isolated the metal (1782) and named it molybdenum, from the Greek molybdos, lead. Molybdenum is not found free in nature. A relatively rare element, it is about as abundant as tungsten, which it resembles. Most commercial production is from ores containing the mineral molybdenite. The concentrated mineral is usually roasted in an excess of air to yield molybdenum trioxide (MoO3), also called technical molybdic oxide. Subsequent treatment depends on the ultimate use of molybdenum. Molybdenum may be added to steel in the furnace in the form of either technical oxide or ferromolybdenum. Ferromolybdenum (containing at least 60 percent molybdenum) is produced by igniting a mixture of technical oxide and iron oxide. Molybdenum metal is produced in the form of a powder by hydrogen reduction of chemically pure molybdic oxide or ammonium molybdate, (NH4)2MoO4. The powder is converted to massive metal by the powder-metallurgy process or by the arc-casting process. Molybdenum-base alloys and the metal itself have useful strength at temperatures above which most other metals and alloys are molten. The major use of molybdenum, however, is as an alloying agent in the production of ferrous and nonferrous alloys, to which it uniquely contributes hot strength and corrosion resistance. It is one of the most effective elements for increasing hardenability of iron and steel, and it also contributes to the toughness of quenched and tempered steels. The high corrosion resistance needed in the stainless steels used for processing pharmaceuticals and in the chromium steels for automotive trim is uniquely enhanced by small additions of molybdenum. Metallic molybdenum has been used for such electric and electronic parts as filament supports, anodes, and grids. Rod or wire is used for heating elements in electric furnaces operating up to 1,700 C (3,092 F). Coatings of molybdenum adhere firmly to steel, iron, aluminum, and other metals and show excellent resistance to wear. Natural molybdenum is a mixture of seven stable isotopes: molybdenum-92 (15.84 percent), molybdenum-94 (9.04 percent), molybdenum-95 (15.72 percent), molybdenum-96 (16.53 percent), molybdenum-97 (9.46 percent), molybdenum-98 (23.78 percent), and molybdenum-100 (9.13 percent). Molybdenum exhibits valences of 2 to 6 and is considered zerovalent in the carbonyl Mo(CO)6. Molybdenum(VI) appears in the trioxide, the most important compound, from which most of its other compounds are prepared, and in the molybdates (containing the anion MoO42-), used to produce pigments and dyes. Molybdenum disulfide (MoS2), which resembles graphite, is used as a solid lubricant or as an additive to greases and oils. Molybdenum is an essential trace element in plants; in legumes as a catalyst it assists bacteria in fixing nitrogen. Molybdenum trioxide and sodium molybdate (Na2MoO4) have been used as micronutrients. atomic number 42 atomic weight 95.94 melting point 2,610 C (4,730 F) boiling point 5,560 C (10,040 F) specific gravity 10.2 (20 C) valence 0, 2, 3, 4, 5, 6 electronic config. 2-8-18-13-1 or (Kr)4d55s1