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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 (MoO42-) 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. 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.

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. Molybdenum burns only at temperatures above 600 °C (1,112 °F). It has the lowest heating expansion of any commercially used metal.

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.

Occurrence
Molybdenum output in 2005The world's largest producers of molybdenum materials are the United States, Canada, Chile, Russia, and China.

Molybdenite on quartzThough 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. Large mines in Colorado (such as the now inactive Climax mine) 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. 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.

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
Main article: Isotopes of molybdenum
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. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.

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.

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
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. Molybdenum is also used in alloys for its high corrosion resistance and weldability. 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. 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.

Because of its lower density and more stable price, molybdenum is implemented in the place of tungsten. 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.

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.

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. Molybdenum powder is used as a fertilizer for some plants, such as cauliflower.

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

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