|Name, Symbol, Number||molybdenum, Mo, 42|
|Chemical series||transition metals|
|Group, Period, Block||6, 5, d|
|Atomic mass||95.94(2) g/mol|
|Electron configuration||[Kr] 4d5 5s1|
|Electrons per shell||2, 8, 18, 13, 1|
|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)|
|Crystal structure||cubic body centered|
|Oxidation states||2, 3, 4, 5, 6
(strongly acidic oxide)
|Electronegativity||2.16 (Pauling scale)|
|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|
|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|
|Vickers hardness||1530 MPa|
|Brinell hardness||1500 MPa|
|CAS registry number||7439-98-7|
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.
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.
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.
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.
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.
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.
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