Molybdenum

Atomic Number:	42	Melting Point:	2623 ºC
Atomic Symbol:	Mo	Boiling Point:	4639 ºC
Atomic Weight:	95.94 amu	Density:	21090 kg/m³
Atomic Radius:	136.2 pm	Oxidation States:	6
Covalent Radius:	128 pm	Electron Configuration:	[Kr]5s¹4d⁵
van der Waals Radius:	175 pm	State of Matter:	solid

History

(Gr. molybdos: lead) Before Scheele recognized molybdenite as a distinct ore of a new element in 1778, it was confused with graphite and lead ore. The metal was prepared in impure form in 1782 by Hjelm. Molybdenum does not occur natively, but is obtained principally from molybdenite. Wulfenite, and Powellite are also minor commercial ores.

Properties

The metal is silvery white, very hard, but is softer and more ductile than tungsten. It has a high elastic modulus, and only tungsten and tantalum, of the more readily available metals, have higher melting points. It is a valuable alloying agent, as it contributes to the harden ability and toughness of quenched and tempered steels. It also improves the strength of steel at high temperatures.

Sources

Molybdenum is also recovered as a by-product of copper and tungsten mining operations. The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. Molybdenum is present in ores from 0.01% to about 0.5%. About half of the world's molybdenum is mined in the United States.

Uses

Over 2/3 of all molybdenum is used in alloys. It is used in certain nickel-based alloys, such as the "Hastelloys(R)" which are heat-resistant and corrosion-resistant to chemical solutions. Molybdenum oxidizes at elevated temperatures. The metal has found recent application as electrodes for electrically heated glass furnaces and forehearths. The metal is also used in nuclear energy applications and for missile and aircraft parts. Molybdenum is valuable as a catalyst in the refining of petroleum. It has found applications as a filament material in electronic and electrical applications.

Molybdenum is an essential trace element in plant nutrition; some lands are barren for lack of this element in the soil. Almost all ultra-high strength steels with minimum yield points up to 300,000 psi (lb/in.2) contain molybdenum in amounts from 0.25 to 8%. Biologically, molybdenum as a trace element is necessary for nitrogen fixation and other metabolic processes.

Molybdenum oranges are pigments, ranging from red-yellow to a bright red orange, used in paints, inks, plastics, and rubber compounds. Molybdenum is also used in some electronic applications, as the conductive metal layers in thin-film transistors (TFTs).

Molybdenum sulfide is useful as a lubricant, especially at high temperatures where oils would decompose.

Hazards

Molybdenum dusts and molybdenum compounds, such as molybdenum trioxide and water soluble molybdates, may have slight toxicities if inhaled or ingested orally. Laboratory tests suggest, compared to many heavy metals, that molybdenum is of relatively low toxicity. Acute toxicity in humans is unlikely because the dose required would be exceptionally large. 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. Though water-soluble molybdenum compounds can have a slight toxicity, those that are insoluble, such as the lubricant molybdenum disulfide, are considered to be nontoxic.

However, environmental chains of events can end in serious molybdenum-related health consequences. In 1996 in Sweden an increase in acid rain near Uppsala caused a depletion in the natural foods of moose in nearby rural areas. This caused the moose to venture into the fields of oat farmers who had been heavily liming their soil to compensate for the effect of the acid. The lime caused changes to the levels of cadmium and other trace metals in the soil, causing the oat crops to uptake trace molybdenum in large quantities. Ingestion of the oats by hundreds of moose brought on a severe disturbance in the ratio of molybdenum to copper in their livers, which caused emaciation, hair discoloration, ulcers, diarrhea, convulsions, blindness, osteoporosis and finally heart failure.