molybdenum

molybdenum
/meuh lib"deuh neuhm/, n. Chem.
a silver-white metallic element, used as an alloy with iron in making hard, high-speed cutting tools. Symbol: Mo; at. wt.: 95.94; at. no.: 42; sp. gr.: 10.2.
[1810-20; < NL, alter. of earlier molybdena < L molybdaena < Gk molýbdaina galena, equiv. to mólybd(os) lead + -aina suffix of appurtenance]

* * *

Metallic chemical element, one of the transition elements, chemical symbol Mo, atomic number 42.

It is a silvery gray, relatively rare metal with a high melting point (4,730 °F [2 ,610 °C]) that does not occur uncombined in nature. Since molybdenum and its alloys have useful strength at temperatures that melt most other metals and alloys, they are used in high-temperature steels. Applications include reaction vessels; aircraft, missile, and automobile parts; and electrodes, heating elements, and filament supports. Some molybdenum compounds (in which it has various valences) are used as pigments and catalysts. Molybdenum disulfide is a solid lubricant, used alone or added to greases and oils.

* * *

 chemical element, silver-gray refractory metal of Group 6 (VIb) of the periodic table, used to impart superior strength to steel and other alloys at high temperature.

      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. For molybdenum the chief ore is molybdenite—molybdenum disulfide, MoS2—but molybdates such as lead molybdate, PbMoO4 (wulfenite), and MgMoO4 are also found. 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, which, after purification, can be reduced with hydrogen to the metal. 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. (For the mining, recovery, and applications of molybdenum, see molybdenum processing.)

      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, e.g., in jet engines, combustion liners, and afterburner parts. 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.

      Molybdenum is rather resistant to attack by acids, except for mixtures of concentrated nitric and hydrofluoric acids, and it can be attacked rapidly by alkaline oxidizing melts, such as fused mixtures of potassium nitrate and sodium hydroxide or sodium peroxide; aqueous alkalies, however, are without effect. It is inert to oxygen at normal temperature but combines with it readily at red heat, to give the trioxides, and is attacked by fluorine at room temperature, to give the hexafluorides.

      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 oxidation states of +2 to +6 and is considered to display the zero oxidation state in the carbonyl Mo(CO)6. Molybdenum(+6) 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 forms hard, refractory, and chemically inert interstitial compounds with boron, carbon, nitrogen, and silicon upon direct reaction with those elements at high temperatures.

      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)
oxidation states
0, +2, +3, +4, +5, +6
electronic config.
[Kr]4d55s1

* * *


Universalium. 2010.

Игры ⚽ Поможем написать курсовую

Look at other dictionaries:

  • molybdenum — Symbol: Mo Atomic number: 42 Atomic weight: 95.94 Silvery white, hard metallic transition element. It is chemically unreactive and is not affected by most acids. It oxidizes at high temperatures. There are seven natural isotopes, and four… …   Elements of periodic system

  • Molybdenum — Mol yb*de num, n. [NL.: cf. F. molybd[ e]ne. See {Molybdena}.] (Chem.) A rare element of the chromium group, occurring in nature in the minerals molybdenite and wulfenite, and when reduced obtained as a hard, silver white, difficulty fusible… …   The Collaborative International Dictionary of English

  • molybdenum — (n.) metallic element, 1816, from molybdena, used generally for lead like minerals, from Gk. molybdos lead, also black graphite, related to L. plumbum lead (see PLUMB (Cf. plumb) (n.)), and like it probably borrowed from a lost Mediterranean… …   Etymology dictionary

  • molybdenum — [mə lib′də nəm] n. [ModL: so named (1781) by K. W. Scheele (see SCHEELITE) after its isolation by P. J. Hjelm (1746 1813), Swed chemist < molybdaena, molybdenite, term used because of resemblance to lead ore < L molybdaena, lead, galena… …   English World dictionary

  • Molybdenum — niobium ← molybdenum → technetium Cr ↑ Mo ↓ W …   Wikipedia

  • molybdenum — noun Etymology: New Latin, from molybdena, a lead ore, molybdenite, molybdenum, from Latin molybdaena galena, from Greek molybdaina, from molybdos lead Date: 1814 a metallic element that resembles chromium and tungsten in many properties, is used …   New Collegiate Dictionary

  • molybdenum — A silvery white metallic element, atomic no. 42, atomic wt. 95.94; a bioelement found in a number of proteins ( e.g., xanthine oxidase). See m. target tube. [G. molybdaina, a piece of lead; a …   Medical dictionary

  • molybdenum-99 — A reactor produced radioisotope of molybdenum with a half life of 2.7476 days, used in radionuclide generators for the production of technetium 99m …   Medical dictionary

  • molybdenum — мед. молибден (molybdenum)  Микроэлемент.   Способствует метаболизму жиров и углеводов.   Жизненно важен для усвоения железа.   Участвует в производстве мочевой кислоты. см. тж микроэлементы …   Универсальный дополнительный практический толковый словарь И. Мостицкого

  • molybdenum — molibdenas statusas T sritis chemija apibrėžtis Cheminis elementas. simbolis( iai) Mo atitikmenys: lot. molybdaenum angl. molybdenum rus. молибден …   Chemijos terminų aiškinamasis žodynas

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”