Molybdenum

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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
Atomic mass 95.94(2) g/mol
Electron configuration [Kr] 4d5 5s1
Electrons per shell 2, 8, 18, 13, 1
Physical properties
Phase solid
Density (near r.t.) 10.28 g/cm³
Liquid density at m.p. 9.33 g/cm³
Melting point 2896 K
(2623 °C, 4753 °F)
Boiling point 4912 K
(4639 °C, 8382 °F)
Heat of fusion 37.48 kJ/mol
Heat of vaporization 617 kJ/mol
Heat capacity (25 °C) 24.06 J/(mol·K)
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 (Pauling scale)
Ionization energies
(more)
1st: 684.3 kJ/mol
2nd: 1560 kJ/mol
3rd: 2618 kJ/mol
Atomic radius 145 pm
Atomic radius (calc.) 190 pm
Covalent radius 145 pm
Miscellaneous
Magnetic ordering no data
Electrical resistivity (20 °C) 53.4 nΩ·m
Thermal conductivity (300 K) 138 W/(m·K)
Thermal expansion (25 °C) 4.8 µm/(m·K)
Speed of sound (thin rod) (r.t.) 5400 m/s
Speed of sound (thin rod) (r.t.) 329 m/s
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
Notable 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-  ? 100Ru

Molybdenum (chemical symbol Mo, atomic number 42) is a silvery white, soft metal. It has one of the highest melting points of all pure elements. It is used mainly in alloys, especially to make high-strength and high-temperature steels. It is also a catalyst in the petroleum industry. Molybdenum disulfide is a good lubricant, and molybdenum pigments are used in paints, inks, plastics, and rubber compounds. Molybdenum in trace amounts has been found to have a role in the biology of all classes of organisms. If ingested in excess, however, molybdenum dust and its water-soluble compounds can be toxic.

Contents

Occurrence

The element molybdenum (from the Greek molybdos, meaning "lead-like") is not found free in nature. The main commercial source of molybdenum is the mineral molybdenite (MoS2), but it is also found in minerals such as wulfenite (PbMoO4) and powellite (CaMoO4).

Molybdenum is obtained by mining molybdenite directly and is also recovered as a byproduct of copper mining. Molybdenum is present in ores from 0.01 percent to about 0.5 percent. About half of the world's molybdenum is mined in the United States.

The Russian Luna 24 mission discovered a single grain (1 × 0.6 micrometer) of pure molybdenum in a pyroxene fragment taken from Mare Crisium on the Moon.

History

Until the late eighteenth century, the compounds of molybdenum were confused with those of other elements, such as carbon or lead. In 1778, Carl Wilhelm Scheele was able to determine that molybdenum was separate from graphite and lead, and he isolated the oxide of the metal from molybdenite. In 1782, Hjelm isolated an impure extract of the metal by reducing the oxide with carbon. Molybdenum was little used and remained in the laboratory until the late nineteenth century. Subsequently, a French company (Schneider and Co.) tried molybdenum as an alloying agent in steel armor plating and noted its usefulness as a hardener of steel. Molybdenum use soared during World War I, when the increased demand for tungsten made that element scarce and high-strength steels were at a premium.

Notable characteristics

Molybdenum is a transition metal that lies in period five of the periodic table, between niobium and technetium. In addition, it is located in group six (former group 6B), between chromium and tungsten.

Pure molybdenum has a melting point of 2623°C, which is among the highest melting points of all elements. The pure metal has a tendency to flake apart during machining, but it is useful as an additive that hardens steel.

Isotopes

Molybdenum has six stable isotopes and many radioisotopes, most of which have very short half-lives. Mo-99 is used to create Tc-99 for the nuclear isotope industry.

Compounds

  • Ammonium tetrathiomolybdate ((NH4)2MoS4): This bright red ammonium salt is an important reagent in the chemistry of molybdenum and has been used as a building block in bioinorganic chemistry. The thiometallate anion (MoS4-2) has the distinctive property of undergoing oxidation at the sulfur centers, concomitant with reduction of the metal from Mo(VI) to Mo(IV).
  • Molybdic acid: It refers to hydrated forms of molybdenum trioxide. The simplest form is the monohydrate, H2MoO4, but the dihydrate is also found. The salts of molybdic acid are called molybdates. As molybdenum can have various oxidation states, it can form a wide variety of salts.
  • Sodium molybdate (Na2MoO4): It is often found as the dihydrate (Na2MoO4 . 2H2O) and is useful as a source of molybdenum. It is also used in biochemistry and medicinal chemistry to track various colorless organic chemicals that, in the presence of the salt, can be stained blue. The blue color, also called molybdenum blue, is a complex of molybdates (Mo(VI), Mo(V)).
  • Molybdenum disulfide or molybdenum sulfide or molybdenum(IV) sulfide (MoS2): This black, crystalline sulfide occurs as the mineral molybdenite. Its structure, appearance, and feel are similar to graphite. It consists of a sandwich of layers of molybdenum atoms between layers of sulfur atoms. The weak interactions between the sheets gives MoS2 a lubricating effect. Finely powdered MoS2 is a common dry lubricant. It is also often mixed into various oils and greases, which allows the lubricated parts to keep running for a while beyond the almost complete loss of oil. It is used in aircraft engines, constant-velocity joints and universal joints in automobiles, and as a lubricating additive to special plastics, notably nylon and teflon. Synthetic MoS2 is a catalyst for desulfurization in petroleum refineries.[1]

Applications

  • Over two-thirds of all molybdenum is used in alloys, especially to make high-strength and high-temperature steels. Such alloys are used for oil pipelines, aircraft and missile parts, and filaments. Special alloys (such as the Hastelloys) are notably heat-resistant and corrosion-resistant.
  • Molybdenum also finds use as a catalyst in the petroleum industry, especially in catalysts for removing organic sulfurs from petroleum products.
  • It is used to form the anode in some X-ray tubes, particularly in mammography applications.
  • It is found in some electronics applications as the conductive metal layers in thin-film transistors (TFTs).
  • Molybdenum disulfide is a good lubricant, especially at high temperatures.
  • The isotope Mo-99 is used in the nuclear isotope industry.
  • Molybdenum pigments, ranging in color from red-yellow to a bright red-orange, are used in paints, inks, plastics, and rubber compounds.

Biological role

Molybdenum has been found to have a role in the biology of all classes of organisms. It is found in two groups of enzymes—the nitrogenases and the molybdopterins.

The nitrogenases are found in bacteria (that may dwell in plants) and are involved in the pathways of nitrogen fixation. The molybdenum atom is present in a cluster that includes iron and sulfur atoms. The name molybdopterin is misleading, as this group of enzymes includes tungsten-containing enzymes, and the word "molybdopterin" does not actually refer to the metal atom. This group may also be referred to as "mononuclear molybdenum enzymes," as the metal atom is not present in a cluster. These enzymes are involved in various processes that are part of the global sulfur, nitrogen, and carbon cycles.

There is a requirement for trace amounts of molybdenum in plants, and soils can be barren on account of molybdenum deficiencies. Plants and animals generally have molybdenum present in amounts of a few parts per million. In animals, molybdenum is a cofactor of the enzyme xanthine oxidase, which is involved in certain metabolic pathways (purine degradation and formation of uric acid). In some animals, adding a small amount of dietary molybdenum enhances growth.

Precautions

Molybdenum dusts and some molybdenum compounds, such as molybdenum trioxide and water-soluble molybdates, may have slight toxicities if inhaled or ingested orally. Laboratory tests suggest, however, that molybdenum is of relatively low toxicity, compared to many heavy metals. Acute toxicity in humans is unlikely, because the dose required would be exceptionally high. There is the potential for molybdenum exposure in mining and refining operations, as well as the chemical industry, but to date, no instance of harm from this exposure has been reported. Also, water-insoluble molybdenum compounds, such as the lubricant molybdenum disulfide, are considered nontoxic.

In ruminants, molybdenum toxicity occurs if the animals are allowed to graze on soil that is rich in molybdenum but deficient in copper. The molybdenum causes excretion of copper reserves from the animal, leading to copper deficiency. In young calves, the molybdenum toxicity is manifested as "teart" or shooting diarrhea, where the dung is watery, full of air bubbles and with a fetid odor. In pigs and sheep, molybdenum toxicity combined with copper deficiency can lead to a condition called sway back or paralysis of hind quarters. In black-coated animals, the toxicity of this metal is characterized by depigmentation of the skin surrounding the eyes, often referred to as "spectacled eyes."

Regulations by the U.S. Occupational Safety and Health Administration (OSHA) specify maximum molybdenum exposure in an eight-hour day (40-hour week) to be 15 milligrams (mg) per cubic meter. The National Institute for Occupational Safety and Health (NIOSH) recommends exposure limit of 5,000 mg per cubic meter.

Notes

  1. Hydrodesulfurization

See also

References

  • Chang, Raymond. Chemistry (ninth ed.) New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
  • Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry (4th ed.), New York: Wiley, 1980. ISBN 0-471-02775-8
  • Greenwood, N.N.; and A. Earnshaw. Chemistry of the Elements (2nd Edition). Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654. Online version available at Knovel. Retrieved August 24, 2007.
  • Molybdenum Los Alamos National Laboratory. Retrieved August 24, 2007.
  • Topsøe, H., B. S. Clausen, and F. E. Massoth. Hydrotreating Catalysis, Science and Technology. Berlin: Springer-Verlag, 1996.

External links

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