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molybdenum

  (mə-lĭb'də-nəm) pronunciation
n. (Symbol Mo)

A hard, silvery-white metallic element used to toughen alloy steels and soften tungsten alloy. An essential trace element in plant nutrition, it is used in fertilizers, dyes, enamels, and reagents. Atomic number 42; atomic weight 95.94; melting point 2,617°C; boiling point 4,612°C; specific gravity 10.22 (at 20°C); valence 2, 3, 4, 5, 6.

[New Latin, from earlier molybdena, lead ore, from Latin molybdaena, galena, from Greek molubdaina, from molubdos, lead.]


 
 

A chemical element, Mo, atomic number 42, and atomic weight 95.94, in the periodic table in the triad of transition elements that includes chromium (atomic number 24) and tungsten (atomic number 74). Research has revealed it to be one of the most versatile chemical elements, finding applications not only in metallurgy but also in paints, pigments, and dyes; ceramics; electroplating; industrial catalysts; industrial lubricants; and organometallic chemistry. Molybdenum is an essential trace element in soils and in agricultural fertilizers. Molybdenum atoms have been found to perform key functions in enzymes (oxidases and reductases), with particular interest being directed toward its role in nitrogenase, which is employed by bacteria in legumes to convert inert nitrogen (N2) of the air into biologically useful ammonia (NH3). See also Periodic table.

Molybdenum is widely distributed in the Earth's crust at a concentration of 1.5 parts per million by weight in the lithosphere and about 10 parts per billion in the sea. It is found in at least 13 minerals, mainly as a sulfide [molybdenite (MoS2)] or in the form of molybdates [for example, wulfenite (PbMoO4) and magnesium molybdate (MgMoO4)].

Although molybdenum is closer to chromium in atomic weight and atomic number, its chemical behavior is usually very similar to that of tungsten, which has nearly the same atomic radius. (This is due to the so-called lanthanide contraction in which atomic radii decrease for elements 57 to 71 found in the period between molybdenum and tungsten.) See also Chromium; Lanthanide contraction; Tungsten.

Molybdenum atoms contain six valence electrons (4d55s1), which are employed with great versatility in forming compounds and complexes in which electronic configurations vary from d0 (no d electrons in oxidation state + 6) to d8 (8 d electrons in oxidation state −2). The +6 state is preferred, but all states from −2 to +6 are known. States usually exhibit a variety of coordination numbers (4 to 9), and include polynuclear complexes and metal-metal bonds in metallic clusters with two to six metal atoms in their metallic cores. Molybdenum forms a very large number of compounds with oxygen. Low-valent molybdenum [for example, Mo(CO)6 and Mo2, Mo3, and Mo6 clusters] has a very rich organometallic chemistry, including clusters that are being studied as models for molybdenum metal surfaces that catalyze organic reactions employed in industrial syntheses and oil refining. The ability of molybdenum atoms to vary oxidation state, coordination number, and coordination geometry and to form metal-metal bonds in clusters accounts in part for the large number of industrial catalysts and biological enzymes in which Mo atoms are found at the active site for catalysis. See also Chemical bonding; Coordination chemistry; Electron configuration.

Molybdenum is a high-melting silver-gray metal, strong even at high temperatures, hard, and resistant to corrosion (see table). It also exhibits high conductivity, a high modulus of elasticity, high thermal conductivity, and a low coefficient of expansion. Its major use is in alloy steels, for example, as tool steels (≤10% molybdenum), stainless steel, and armor plate. Up to 3% molybdenum is added to cast iron to increase strength. Up to 30% molybdenum may be added to iron-, cobalt-, and nickel-based alloys designed for severe heat- and corrosion-resistant applications. It may be used in filaments for light bulbs, and it has many applications in electronic circuitry. See also Alloy.

Physical properties of molybdenum metal

Property

Value

Density

10.22 g/cm3 (5.911 oz/in.3)

Heat of vaporization

491 kJ/mol

Heat of fusion

28 kJ/mol

Specific heat

0.267 J/g°C

Thermal conductivity

1.246 J/s/cm2/cm°C (200°C)

0.923 J/s/cm2/cm°C (2200°C)

Electrical conductivity

34% International Copper Standard

Electrical resistivity

5.2 microhm-cm, 20°C

78.2 microhm-cm, 2525°C

Magnetic susceptibility

0.93 × 10−6 emu, 25°C

1.11 × 10−6 emu, 1825°C

Mean linear expansion coefficient

6.65 × 10−6/°C, 20–1600°C

Modulus of elasticity

0.324 N/m2

Lattice parameter

0.314767 nm (body-centered cube)

Molybdenum trioxide, molybdates, sulfo-molybdates, and metallic molybdenum are found in thousands of industrial catalysts used in oil refining, ammonia synthesis, and industrial syntheses of organic chemicals. Monomeric molybdenum(IV) in aqueous solution is a powerful catalyst for the reduction of inert oxo-anions such as perchlorate (ClO4) or nitrate (NO3) as well as other oxidized nonmetals such as azide ion (N3) and dinitrogen (N2). The trinuclear cation MO3O44+ is inert, unreactive, and noncatalytic. See also Catalysis; Homogeneous catalysis.

The molybdenum enzymes comprise two major categories. The first category contains the single, highly important enzyme nitrogenase, which is responsible for biological nitrogen fixation. The second category contains all other known molybdenum enzymes, which are crucial for the metabolism of bacteria, plants, and animals, including humans.


 
Food and Nutrition: molybdenum

A dietary essential mineral, required for a number of enzymes, including xanthine, aldehyde, and pyridoxal oxidases, where it forms the functional part of the coenzyme molybdopterin. Deficiency is unknown; US/Canadian RDA is 45 μ/day.

 
Food and Fitness: molybdenum

An essential mineral used in iron metabolism, and needed for normal growth and development. Because deficiency is not known, there is no Recommended Daily Amount (RDA) but the UK and USA estimated safe and adequate daily intake is 0.15-0.5 mg. Cereal crops and meat are good sources. High intakes may disturb copper metabolism and cause gout-like symptoms.

 
Dental Dictionary: molybdenum

n
Mo

A grayish metallic element with an atomic number of 42 and an atomic weight of 95.94. Molybdenum is poisonous if ingested in large quantities.

 

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.

For more information on molybdenum, visit Britannica.com.

 

A trace element that is an essential component of many enzymes. Deficiency is rare.

 
(məlĭb'dənəm) [Gr.,=leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.94; 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.


 

A hard, silvery-white, metallic element, atomic number 42, atomic weight 95.94, symbol Mo. It is an essential trace element, being a component of the enzymes xanthine oxidase, aldehyde oxidase and nitrate reductase.
Molybdenum poisoning causes a secondary hypocuprosis, and clinical signs including chronic diarrhea, illthrift and depigmentation of hair. It occurs most commonly on pastures growing on soils naturally rich in the element, but can be caused by excessive pasture supplementation in an attempt to stimulate the growth of Rhizobium spp., the nitrogen-fixing bacteria of legume roots.


 
Wikipedia: molybdenum
42 niobiummolybdenumtechnetium
Cr

Mo

W
Mo-TableImage.png
General
Name, Symbol, Number molybdenum, Mo, 42
Chemical series transition metals
Group, Period, Block 6, 5, d
Appearance gray metallic
Mo,42.jpg
Standard atomic weight 95.94(2)  g·mol−1
Electron configuration [Kr] 4d5 5s2
Electrons per shell 2, 8, 18, 13, 1
Physical properties
Phase solid
Density (near r.t.) 10.28  g·cm−3
Liquid density at m.p. 9.33  g·cm−3
Melting point 2896 K
(2623 °C, 4753 °F)
Boiling point 4912 K
(4639 °C, 8382 °F)
Heat of fusion 37.48  kJ·mol−1
Heat of vaporization 617  kJ·mol−1
Heat capacity (25 °C) 24.06  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2742 2994 3312 3707 4212 4879
Atomic properties
Crystal structure cubic body centered
Oxidation states 2, 3, 4, 5, 6
(strongly acidic oxide)
Electronegativity 2.16 (scale Pauling)
Ionization energies
(more)
1st:  684.3  kJ·mol−1
2nd:  1560  kJ·mol−1
3rd:  2618  kJ·mol−1
Atomic radius 145pm
Atomic radius (calc.) 190  pm
Covalent radius 145  pm
Miscellaneous
Magnetic ordering no data
Electrical resistivity (20 °C) 53.4 n Ω·m
Thermal conductivity (300 K) 138  W·m−1·K−1
Thermal expansion (25 °C) 4.8  µm·m−1·K−1
Speed of sound (thin rod) (r.t.) 5400  m·s−1
Young's modulus 329  GPa
Shear modulus 20  GPa
Bulk modulus 230  GPa
Poisson ratio 0.31
Mohs hardness 5.5
Vickers hardness 1530  MPa
Brinell hardness 1500  MPa
CAS registry number 7439-98-7
Selected isotopes
Main article: Isotopes of molybdenum
iso NA half-life DM DE (MeV) DP
92Mo 14.84% Mo is stable with 50 neutrons
93Mo syn 4×103 y ε - 93Nb
94Mo 9.25% Mo is stable with 52 neutrons
95Mo 15.92% Mo is stable with 53 neutrons
96Mo 16.68% Mo is stable with 54 neutrons
97Mo 9.55% Mo is stable with 55 neutrons
98Mo 24.13% Mo is stable with 56 neutrons
99Mo syn 65.94 h β- 0.436, 1.214 99Tc
γ 0.74, 0.36,
0.14
-
100Mo 9.63% 7.8×1018 y β-β- 3.04 100Ru
References

Molybdenum (IPA: /məˈlɪbdənəm/, from the Greek meaning "lead-like"), is a chemical element with the symbol Mo and atomic number 42. It has the sixth highest melting point of any element, and for this reason it is often used in high-strength steel alloys. Molybdenum is found in trace amounts in plants and animals, although excess molybdenum can be toxic in some animals. Molybdenum was discovered in 1778 by Carl Wilhelm Scheele and first isolated in 1781 by Peter Jacob Hjelm.

Characteristics

Molybdenum is a transition metal with an electronegativity of 1.8 on the Pauling scale and an atomic mass of 95.9 g/mole.[1] It does not react with oxygen or water at room temperature. At elevated temperatures, molybdenum trioxide is formed in the reaction 2Mo + 3O2 → 2MoO3.[2]

In its pure metal form, molybdenum is silvery white and very hard, though it is somewhat more ductile than tungsten. It has a melting point of 2623°C, and only tantalum, osmium, rhenium and tungsten have higher melting points.[3] Molybdenum burns only at temperatures above 600°C.[4] It also has the lowest heating expansion of any commercially used metal.[5]

Molybdenum has a value of approximately $65,000 per tonne as of 4 May 2007. It maintained a price at or near $10,000 per tonne from 1997 through 2002, and reached a high of $103,000 per tonne in June 2005.[6]

Applications

The ability of molybdenum to withstand extreme temperatures without significantly expanding or softening make it useful in applications that involve intense heat, including aircraft parts, electrical contacts, industrial motors, and filaments.[7][5] Molybdenum is also used in alloys for its high corrosion resistance and weldability.[4][8] Most high-strength steel alloys are .25% to 8% molybdenum.[3] Despite being used in such small portions, more than 43×106kg of molybdenum is used as an alloying agent each year in stainless steels, tool steels, cast irons, and high temperature superalloys.[4]

Because of its lower density and more stable price, molybdenum is implemented in the place of tungsten.[4] Molybdenum can be implemented both as an alloying agent and as a flame-resistant coating for other metals. Although its melting point is 2623 °C, molybdenum rapidly oxidizes at temperatures above 760 °C, making it better-suited for use in vacuum environments.[7]

Molybdenum disulphide (MoS2) is used as a lubricant and an agent. It forms strong films on metallic surfaces, and is highly resistant to both extreme temperatures and high pressure. Sodium molybdate is a bright orange pigment used with ceramics and plastics.[5] Molybdenum trioxide (MoO3) is used as an adhesive between enamels and metals.[9] Molybdenum powder is used as a fertilizer for some plants, such as cauliflower.[4]

History

Molybdenite (from the Greek Μόλυβδος molybdos, meaning lead),[3] the principal ore from which molybdenum is now extracted, was previously known as molybdena. Molybdena was confused with and often implemented as though it were graphite. Even when the two ores were distinguishable, molybdena was thought to be a lead ore.[5] In 1754, Bengt Qvist examined the mineral and determined that it did not contain lead.[10]

It was not until 1778 that Swedish chemist Carl Wilhelm Scheele realized molybdena was neither graphite nor lead.[9][11] He and other chemists then correctly assumed that it was the ore of a distinct new element, named molybdenum for the mineral in which it was discovered. Peter Jacob Hjelm successfully isolated molybdenum using carbon and linseed oil in 1781.[5][12] For a long time there was no industrial use for molybdenum. The French Schneider Electrics company produced the first steel molybdenum alloy armor plates in 1894. Until World War I most other armor factories also used molybdenum alloys. In World War I, some British tanks were protected by 75 mm manganese plating, but this proved to be ineffective. The manganese plates were then replaced with 25 mm molybdenum plating. These allowed for higher speed, greater manueverability and, despite being thinner, better protection.[5] The high demand of molybdenum in World War I and World War II and the step decrease after the wars had a great influence on prices and production of molybdenum.

Occurrence

Molybdenum output in 2005
Enlarge
Molybdenum output in 2005

The world's largest producers of molybdenum materials are the United States, Canada, Chile, Russia, and China.[13][5]

Though molybdenum is found in such minerals as wulfenite (PbMoO4) and powellite (CaMoO4), the main commercial source of molybdenum is molybdenite (MoS2). Molybdenum is mined as a principal ore, and is also recovered as a byproduct of copper and tungsten mining.[3] The large mining areas in Colorado (Climax) and in British Columbia yield molybdenite while the Chuquicamata mines in northern Chile produce molybdenum as a byproduct of copper mining. The Knaben mine in southern Norway was opened in 1885, making it the first molybdenum mine. It remained open until 1973.

Molybdenum is the 42nd most abundant element in the universe, and the 25th most abundant element in Earth's oceans, with an average of 10.8 mt/km³.[4] The Russian Luna 24 mission discovered a single molybdenum-bearing grain (1 × 0.6 µm) in a pyroxene fragment taken from Mare Crisium on the Moon.[14]

A side product of molybdenum mining is rhenium. As it is always present in small varying quantities in molybdenite the only commercial source for rhenium are molybdenum mines.

Biological role

The most important use of the molybdenum atom in mammals and other animals is in enzymes.[specify] In a 70 kg human body, there is approximately 9.3 mg molybdenum, comprising .00001% of the total body mass.[15] It occurs in higher concentrations in the liver and kidneys, and in lower concentrations in the vertebrae.[4] Molybdenum is also present within human tooth enamel and may help prevent the decaying thereof.[16] Pork, lamb, and beef liver each have approximately 1.5 parts molybdenum per million. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, and cereal grain.[5]

The average daily intake of molybdenum is .3 mg. Daily intake above .4 mg can be toxic.[5] Molybdenum deficiency, caused by less than .05 mg/day,[5] can cause stunted growth, reduced appetite, and impaired reproduction.[17] Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues.[4]

Though molybdenum forms compounds with various organic molecules, including carbohydrates and amino acids, it is transported throughout the body as MoO42-.[18] Molybdenum is present in approximately 20 enzymes, including aldehyde oxidase, sulfite oxidase, xanthine oxidase.[5] In some animals, the oxidation of xanthine to uric acid, a process of purine catabolism, is catalyzed by xanthine oxidase, a molybdenum-containing enzyme. The activity of xanthine oxidase is directly proportional to the amount of molybdenum in the body. However, an extremely high concentration of molybdenum reverses the trend, and can act as an inhibitor in both purine catabolism and other processes. Molybdenum concentrations also affect protein synthesis, metabolism, and growth.[18]

Copper-molybdenum antagonism

High amounts of molybdenum can interfere with the body's uptake of copper, both by preventing plasma proteins from binding the copper and by increasing the amount of copper that is excreted in urine. Ruminants that consume high amounts of molybdenum develop symptoms including diarrhea, stunted growth, anaemia, and achromotrichia. These symptoms can be alleviated by the administration of more copper into the system, both in dietary form and by injection.[19] The condition can be aggravated by excess sulfur.[4]

Isotopes

There are 35 known isotopes of molybdenum ranging in atomic mass from 83 to 117, as well as four metastable nuclear isomers. Seven isotopes occur naturally, with atomic masses of 92, 94, 95, 96, 97, 98, and 100. Of these naturally occurring isotopes, five are stable, with atomic masses from 94 to 98. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.[20]

Molybdenum-92 and molybdenum-100 are the only naturally occurring isotopes which are not stable. Molybdenum-100 has a half-life of approximately 1×1019 y and undergoes double beta decay into ruthenium-100. Molybdenum-98 is the most common isotope, comprising 24.14% of all molybdenum. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately .15 μs.[20]

Precautions

Molybdenum dusts and fumes, as can be generated by mining or metalworking, are not toxic. There are no long-term effects associated with exposure to molybdenum; however, prolonged exposure can cause irritation to the eyes and skin. The direct inhalation or ingestion of molybdenum should also be avoided.[21] OSHA regulations specify the maximum permissible molybdenum exposure in an 8-hour day to be 5 mg/m³. Chronic exposure to 60 to 600 mg Mo/m³ can cause symptoms including fatigue, headaches, and joint pains.[22]

See also

  • Molybdenum compounds.

References

  1. ^ Properties of Molybdenum. Integral Scientist Periodic Table. Qivx, Inc. (2003). Retrieved on 2007-06-10.
  2. ^ Winter, Mark. Chemistry. Molybdenum. The University of Sheffield. Retrieved on 2007-06-10.
  3. ^ a b c d
  4. ^ a b c d e f g h i Considine, Glenn D., ed. (2005), "Molybdenum", Van Nostrand's Encyclopedia of Chemistry, New York: Wylie-Interscience, pp. 1038-1040, 0-471-61525-0
  5. ^ a b c d e f g h i j k
  6. ^ Dynamic Prices and Charts for Molybdenum. InfoMine Inc. (2007). Retrieved on 2007-05-07.
  7. ^ a b
  8. ^ Molybdenum Statistics and Information. U.S. Geological Survey (2007-05-10). Retrieved on 2007-05-10.
  9. ^ a b
  10. ^ Van der Krogt, Peter (2006-01-10). Molybdenum. Elementymology & Elements Multidict. Retrieved on 2007-05-20.
  11. ^ C. W. K. Scheele (1779). "Versuche mit Wasserbley;Molybdaena". svenska vetensk. Academ. Handlingar 40: 238. 
  12. ^ P. J. Hjelm (1788). "Versuche mit Molybdäna, und Reduction der selben Erde". svenska vetensk. Academ. Handlingar 49: 268. 
  13. ^ Lide, David R., ed. (2006), CRC Handbook of Chemistry and Physics, vol. 4, Chemical Rubber Publishing Company, pp. 22-23, 0-8493-0487-3
  14. ^ American Mineralogist, Volume 87, pages 181-184, 2002. Retrieved on 2007-04-09.
  15. ^ Lide, David R., ed. (2006), CRC Handbook of Chemistry and Physics, vol. 7, Chemical Rubber Publishing Company, pp. 18, 0-8493-0487-3
  16. ^ Ismail, Mumtaz. Dental Problems and Diet. Health and Nutrition. Bawarchi. Retrieved on 2007-05-19.
  17. ^ Guide to Healthy Minerals. Cherokee Naturals. Retrieved on 2007-05-17.
  18. ^ a b Mitchell, Phillip C. H. (2003). Overview of Environment Database. International Molybdenum Association. Retrieved on 2007-05-05.
  19. ^ Suttle, N. F. (December 1974). "Recent studies of the copper-molybdenum antagonism". Proceedings of the Nutrition Society 33 (3): 299-305. CABI Publishing. DOI:10.1079/PNS19740053. Retrieved on 2007-05-11. 
  20. ^ a b Lide, David R., ed. (2006), CRC Handbook of Chemistry and Physics, vol. 11, CRC, pp. 87-88, 0-8493-0487-3
  21. ^ Material Safety Data Sheet - Molybdenum. The REMBAR Company, Inc. (2000-09-19). Retrieved on 2007-05-13.
  22. ^ NIOSH Documentation for ILDHs Molybdenum. National Institute for Occupational Safety and Health (1996-08-16). Retrieved on 2007-05-31.

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