molybdenum

42 niobium ← molybdenum → technetium Cr ↑ Mo ↓ W Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	molybdenum, Mo, 42
Element category	transition metals
Group, Period, Block	6, 5, d
Appearance	gray metallic
Standard atomic weight	95.94(2)  g·mol−1
Electron configuration	[Kr] 4d5 5s1
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
Specific 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	6, 5, 4, 3, 2, 1[1] (strongly acidic oxide)
Electronegativity	2.16 (Pauling scale)
Ionization energies (more)	1st:  684.3  kJ·mol−1
	2nd:  1560  kJ·mol−1
	3rd:  2618  kJ·mol−1
Atomic radius	145  pm
Atomic radius (calc.)	190  pm
Covalent radius	145  pm
Miscellaneous
Magnetic ordering	paramagnetic[2]
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	126  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
Most-stable isotopes
Main article: Isotopes of molybdenum iso NA half-life DM DE (MeV) DP 92Mo 14.84% 92Mo is stable with 50 neutrons 93Mo syn 4×103 y ε - 93Nb 94Mo 9.25% 94Mo is stable with 52 neutrons 95Mo 15.92% 95Mo is stable with 53 neutrons 96Mo 16.68% 96Mo is stable with 54 neutrons 97Mo 9.55% 97Mo is stable with 55 neutrons 98Mo 24.13% 98Mo 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 (pronounced /məˈlɪbdənəm/, from the Greek word for the metal "lead"), is a Group 6 chemical element with the symbol Mo and atomic number 42. The free element, which is a silvery metal, has the sixth-highest melting point of any element. It readily forms hard, stable carbides, and for this reason it is often used in high-strength steel alloys. Molybdenum does not occur as the free metal in nature, but rather in a variety of oxidation states in minerals. Industrially molybdenum compounds are used in high-pressure and temperature resistant greases between metals, as pigments, and catalysts.

Molybdenum minerals have long been known, but the element was "discovered" (in the sense of differentiating it as a new entity from minerals salts of other metals) in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm.

Most of molybdenum's compounds have poor water-solubility, but the molybdate ion (MoO₄^2-) is soluble, and will form if molybdenum-containing minerals are in contact with free oxygen and water. Recent theories suggest that the release of free oxygen by early life was important in removing molybdenum from minerals into a soluble form in the early oceans, where it was available to be used as a catalyst by single-celled organisms. This sequence may have been important in the history of life, because molybdenum-containing enzymes then became the most important catalysts used by some bacteria to break the bond in atmospheric molecular nitrogen, allowing biological nitrogen fixation. This, in turn allowed biologically driven nitrogen-fertilization of the oceans, and thus the development of more complex organisms. Aside from bacterial enzymes involved with nitrogen fixation, about 20 different molybdenum-containing enzymes are known today in animals. Molybdenum is a required element for life in these higher organisms, though not in all bacteria.

Characteristics

Physical

Molybdenum is a transition metal with an electronegativity of 1.8 on the Pauling scale and an atomic mass of 95.94 g/mole.^[3] It does not react with oxygen or water at room temperature. At elevated temperatures, molybdenum trioxide is formed in the reaction 2 Mo + 3 O₂ → 2MoO₃.^[4]

In its pure metal form, molybdenum is silvery white with a Mohs hardness of 5.5, though it is somewhat more ductile than tungsten. It has a melting point of 2,623 °C (4,753 °F); of the naturally occurring elements, only tantalum, osmium, rhenium, tungsten, and carbon have higher melting points.^[5] Molybdenum burns only at temperatures above 600 °C (1,112 °F).^[6] It has the lowest heating expansion of any commercially used metal.^[7]

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.^[8]

Occurrence

Molybdenum output in 2005

The world's largest producers of molybdenum materials are the United States, Canada, Chile, Russia, and China.^[7][9]

Molybdenite on quartz

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.^[5] Large mines in Colorado (such as the now inactive Climax mine)^[10] and in British Columbia yield molybdenite, while many porphyry copper deposits such as the Chuquicamata mine 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.^[6] 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.^[11]

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 is molybdenum mines.

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, only molybdenum-92 and molybdenum-100 are unstable.^[12] All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.^[13]

Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of approximately 1×10¹⁹ y and undergoes double beta decay into ruthenium-100. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately .15 μs.^[12][13]

As also noted below, the most common isotopic molybdenum application involves molybdenum-99, which is a fission product. It is used as a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer which is used in various imaging applications in medicine.^[14]

Compounds and chemistry

Molybdenum has several common oxidation states, +2, +3, +4, +5 and +6. The chemistry and the compounds show more similarity to those of tungsten than that of chromium. An example is the instability of molybdenum(III) and tungsten(III) compounds compared to the stability of the chromium(III) compounds. The highest oxidation state is common in the molybdenum(VI) oxide MoO₃ while the normal sulfur compound is molybdenum disulfide MoS₂.

Keggin structure of the phosphomolybdate anion (P[Mo₁₂O₄₀]^3-), an example of a polyoxometalate

Molybdenum(VI) oxide is soluble in alkaline water, forming molybdates ([MoO₄]^2-). Molybdates are weaker oxidants than chromates, but they show a similar tendency to form complex oxyanions by condensation at lower pH values, such as [Mo₇O₂₄]^6- and [Mo₈O₂₆]^4-. Polymolybdates can incorporate other ions into their structure, forming polyoxometalates.^[16] The dark blue phosphorus-containing heteropolymolybdate P[Mo₁₂O₄₀]^3- is used for the spectroscopic detection of phosphorus. Molybdenum has a broad range of oxidation states, several of which are demonstrated by the various compounds of molybdenum and chlorine:

• Molybdenum(II) chloride MoCl₂ (yellow solid)
• Molybdenum(III) chloride MoCl₃ (dark red solid)
• Molybdenum(V) chloride MoCl₅ (dark green solid)
• Molybdenum(VI) chloride MoCl₆ (brown solid)

The structure of the MoCl₂ is composed of Mo₆Cl₈⁴⁺ clusters with four chloride ions to compensate the charge.

Like chromium and some other transition metals molybdenum is able to form quadruple bonds. Mo₂(CH₃COO)₄ is an example for a quadruple bond. This compound can be transformed into the chlorine compound Mo₂Cl₈^4-.

The oxidation state 0 is possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)₆.

History

Molybdenite (from the Ancient Greek Μόλυβδος molybdos, meaning lead),^[5] 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.^[7] In 1754, Bengt Qvist examined the mineral and determined that it did not contain lead.^[17]

It was not until 1778 that Swedish chemist Carl Wilhelm Scheele realized molybdena was neither graphite nor lead.^[18][19] 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.^[7][20] 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 (3.0 in) manganese plating, but this proved to be ineffective. The manganese plates were then replaced with 25 mm (0.98 in) molybdenum plating. These allowed for higher speed, greater maneuverability, and, despite being thinner, better protection.^[7] The high demand for molybdenum in World War I and World War II and the steep decrease after the wars had a great influence on prices and production of molybdenum.

Production

The molybdenite is first heated to a temperature of 700 °C (1,292 °F) and the sulfide is oxidized into molybdenum(VI) oxide by air:

2MoS₂ + 7O₂ → 2MoO₃ + 4SO₂

The oxidized ore is then either heated to 1,100 °C (2,010 °F) to sublimate the oxide, or leached with ammonia which reacts with the molybdenum(VI) oxide to form water-soluble molybdates:

MoO₃ + 2 NH₄OH → (NH₄)₂(MoO₄) + H₂O

Copper, an impurity in molybdenite, is less soluble in ammonia. To completely remove it from the solution, it is precipitated with hydrogen sulfide.

Pure molybdenum is produced by reduction of the oxide with hydrogen, while the molybdenum for steel production is reduced by the aluminothermic reaction with addition of iron to produce ferromolybdenum. A common form of ferromolybdenum contains 60% molybdenum.^[7][21]

Applications

The ability of molybdenum to withstand extreme temperatures without significantly expanding or softening makes it useful in applications that involve intense heat, including the manufacture of aircraft parts, electrical contacts, industrial motors, and filaments.^[7][22] Molybdenum is also used in alloys for its high corrosion resistance and weldability.^[6][23] Molybdenum contributes corrosion resistance to type 316 stainless steel by 'gettering' residual carbon, preventing the formation of chromium carbide at grain boundaries. Most high-strength steel alloys are .25% to 8% molybdenum.^[5] Despite being used in such small portions, more than 43 million kg of molybdenum is used as an alloying agent each year in stainless steels, tool steels, cast irons, and high-temperature superalloys.^[6]

Because of its lower density and more stable price, molybdenum is implemented in the place of tungsten.^[6] An example is the 'M' series of high-speed steels such as M2, M4, and M42 as substitution for the 'T' series of HSS. Molybdenum can be implemented both as an alloying agent and as a flame-resistant coating for other metals. Although its melting point is 2,623 °C (4,753 °F), molybdenum rapidly oxidizes at temperatures above 760 °C (1,400 °F), making it better-suited for use in vacuum environments.^[22]

Molybdenum based alloys have only limited applications. Due to the corrosion resistance against molten zinc, molybdenum and the molybdenum tungsten alloy (70%/30%) are used for piping, stirrers and pump impellers which come into contact with molten zinc.^[24]

Molybdenum-99 is used as a parent radioisotope to the daughter radioisotope Technetium-99m, which is used in many medical procedures.

Molybdenum disulfide (MoS₂) is used as a solid lubricant and an extreme pressure (EP) antiwear agent. It forms strong films on metallic surfaces, and is highly resistant to both extreme temperatures and high pressure, and for this reason, it is a common additive to extreme pressure application greases; in case of a catastrophic failure, the thin layer of molybdenum prevents metal-on-metal contact.

Molybdenum trioxide (MoO₃) is used as an adhesive between enamels and metals.^[18] Molybdenum powder is used as a fertilizer for some plants, such as cauliflower.^[6]

Lead molybdate (Wulfenite) co-precipitated with lead chromate and lead sulfate is a bright-orange pigment used with ceramics and plastics.^[25]

Also used in NO, NO₂, NO_x analyzers in power plants for pollution controls. At 350 °C (662 °F) the element acts as a catalyst for NO₂/NO_x to form only NO molecules for consistent readings by infrared light.

Biological role

The most important use of the molybdenum atom in living organisms is as a metal hetero-atom at the active site in certain enzymes. In nitrogen fixation in certain bacteria, the nitrogenase enzyme which is involved in the terminal step of reducing molecular nitrogen, usually contains molybdenum in the active site (though replacement of Mo with iron or vanadium is known). The structure of the catalytic center of the enzyme is similar to that in iron-sulfur proteins, it incorporates a Fe₄S₃ and MoFe₃S₃ cluster.^[26]

In March 2008, researchers reported that they had found strong evidence for the hypothesis that a scarcity of molybdenum in the Earth's early oceans was a limiting factor in the further evolution of eukaryotic life (which includes all plants and animals) as eukaryotes cannot fix nitrogen and must acquire it from prokaryotic bacteria.^[27][28][29] The scarcity of molybdenum resulted from the relative lack of oxygen in the early ocean. Oxygen dissolved in seawater is the primary mechanism for dissolving molybdenum from minerals on the sea bottom.

Molybdenum containing cofactor molybdopterin

Though molybdenum forms compounds with various organic molecules, including carbohydrates and amino acids, it is transported throughout the human body as MoO₄^2-.^[30] Molybdenum is present in approximately 20 enzymes in animals, including aldehyde oxidase, sulfite oxidase and xanthine oxidase.^[7] 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.^[30] These enzymes in plants and animals catalyze the reaction of oxygen in small molecules, as part of the regulation of nitrogen-, sulfur- and carbon cycles.^[31]

In a 70 kg (150 lb) human body, there is approximately 9.3 mg of molybdenum, comprising .00001% of the total body mass.^[32] It occurs in higher concentrations in the liver and kidneys, and in lower concentrations in the vertebrae.^[6] Molybdenum is also present within human tooth enamel and may help prevent the decaying thereof.^[33] Pork, lamb, and beef liver each have approximately 1.5 parts per million of molybdenum. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, and cereal grain.^[7]

The average daily intake of molybdenum varies between 120 µg and 240 µg but strongly depending on the molybdenum content of the food.^[34] Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state. Studies on rats show a median lethal dose (LD₅₀) as low as 180 mg/kg for some Mo compounds.^[35] Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/kg of molybdenum can cause diarrhea, growth retardation, sterility, low birth weight, and gout, as well as affecting the lungs, kidneys, and liver. Molybdenum deficiency is not usually seen in healthy people.^[36] Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues.^[6]

Dietary deficiency in molybdenum from low soil concentration has been associated with increased rates of esophageal cancer in a geographical band from northern China to Iran.^[37][38] Compared to the United States, which has a greater supply of molybdenum in the soil, those living in these areas have a 16 times greater risk for esophageal squamous cell carcinoma.^{[citation needed]}

Copper-molybdenum antagonism

High levels of molybdenum can interfere with the body's uptake of copper, producing copper deficiency. Molybdenum prevents plasma proteins from binding to copper, and it also increases the amount of copper that is excreted in urine. Ruminants that consume high amounts of molybdenum develop symptoms including diarrhea, stunted growth, anemia, and achromotrichia (loss of hair pigment). These symptoms can be alleviated by the administration of more copper into the system, both in dietary form and by injection.^[39] The condition can be aggravated by excess sulfur.^[6]

Precautions

Molybdenum dusts and fumes, as can be generated by mining or metalworking, can be toxic, especially if ingested (including dust trapped in the sinuses and later swallowed).^[35] Low levels of prolonged exposure can cause irritation to the eyes and skin. The direct inhalation or ingestion of molybdenum and its oxides should also be avoided.^[40][41] 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.^[42]

References

External links

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