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Molybdenum

Molybdenum

Gist

Molybdenum (Mo) is a chemical element, a silvery-white, hard transition metal with the atomic number 42. It's known for its high melting point and is a crucial alloying agent, particularly in steel, enhancing strength and stability at high temperatures. Molybdenum is also an essential trace element for both plants and animals, including humans, playing a vital role in various enzymatic functions.

Molybdenum is primarily used as an alloying agent in steel and cast iron, enhancing their strength, hardness, and resistance to corrosion and wear. It also finds applications in chemical processes as catalysts and pigments, and in plant nutrition as a fertilizer. Additionally, it's used in some medical treatments and industrial applications like heating elements and sputtering targets.

Summary

Molybdenum is a chemical element; it has symbol Mo (from Neo-Latin molybdaenum) and atomic number 42. The name derived from Ancient Greek mólybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered (in the sense of differentiating it as a new entity from the mineral salts of other metals) in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm.

Molybdenum does not occur naturally as a free metal on Earth; in its minerals, it is found only in oxidized states. The free element, a silvery metal with a grey cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of the world production of the element (about 80%) is used in steel alloys, including high-strength alloys and superalloys.

Most molybdenum compounds have low solubility in water. Heating molybdenum-bearing minerals under oxygen and water affords molybdate ion, which forms quite soluble salts. Industrially, molybdenum compounds (about 14% of world production of the element) are used as pigments and catalysts.

Molybdenum-bearing enzymes are by far the most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation. At least 50 molybdenum enzymes are now known in bacteria, plants, and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. Most nitrogenases contain an iron–molybdenum cofactor FeMoco, which is believed to contain either Mo(III) or Mo(IV). By contrast Mo(VI) and Mo(IV) are complexed with molybdopterin in all other molybdenum-bearing enzymes. Molybdenum is an essential element for all higher eukaryote organisms, including humans. A species of sponge, Theonella conica, is known for hyperaccumulation of molybdenum.

Details

Molybdenum (Mo) is a chemical element, silver-gray refractory metal of Group 6 (VIb) of the periodic table, used to impart superior strength to steel and other alloys at high temperature.

The Swedish chemist Carl Wilhelm Scheele had demonstrated (c. 1778) that the mineral molybdaina (now molybdenite), for a long time thought to be a lead ore or graphite, certainly contains sulfur and possibly a previously unknown metal. At Scheele’s suggestion, Peter Jacob Hjelm, another Swedish chemist, successfully isolated the metal (1782) and named it molybdenum, from the Greek molybdos, “lead.”

Molybdenum is not found free in nature. A relatively rare element, it is about as abundant as tungsten, which it resembles. For molybdenum the chief ore is molybdenite—molybdenum disulfide, MoS2—but molybdates such as lead molybdate, PbMoO4 (wulfenite), and MgMoO4 are also found. Most commercial production is from ores containing the mineral molybdenite. The concentrated mineral is usually roasted in an excess of air to yield molybdenum trioxide (MoO3), also called technical molybdic oxide, which, after purification, can be reduced with hydrogen to the metal. Subsequent treatment depends on the ultimate use of molybdenum. Molybdenum may be added to steel in the furnace in the form of either technical oxide or ferromolybdenum. Ferromolybdenum (containing at least 60 percent molybdenum) is produced by igniting a mixture of technical oxide and iron oxide. Molybdenum metal is produced in the form of a powder by hydrogen reduction of chemically pure molybdic oxide or ammonium molybdate, (NH4)2MoO4. The powder is converted to massive metal by the powder-metallurgy process or by the arc-casting process.

Molybdenum-base alloys and the metal itself have useful strength at temperatures above which most other metals and alloys are molten. The major use of molybdenum, however, is as an alloying agent in the production of ferrous and nonferrous alloys, to which it uniquely contributes hot strength and corrosion resistance, e.g., in jet engines, combustion liners, and afterburner parts. It is one of the most effective elements for increasing hardenability of iron and steel, and it also contributes to the toughness of quenched and tempered steels. The high corrosion resistance needed in the stainless steels used for processing pharmaceuticals and in the chromium steels for automotive trim is uniquely enhanced by small additions of molybdenum. Metallic molybdenum has been used for such electric and electronic parts as filament supports, anodes, and grids. Rod or wire is used for heating elements in electric furnaces operating up to 1,700 °C (3,092 °F). Coatings of molybdenum adhere firmly to steel, iron, aluminum, and other metals and show excellent resistance to wear.

Molybdenum is rather resistant to attack by acids, except for mixtures of concentrated nitric and hydrofluoric acids, and it can be attacked rapidly by alkaline oxidizing melts, such as fused mixtures of potassium nitrate and sodium hydroxide or sodium peroxide; aqueous alkalies, however, are without effect. It is inert to oxygen at normal temperature but combines with it readily at red heat, to give the trioxides, and is attacked by fluorine at room temperature, to give the hexafluorides.

Natural molybdenum is a mixture of seven stable isotopes: molybdenum-92 (15.84 percent), molybdenum-94 (9.04 percent), molybdenum-95 (15.72 percent), molybdenum-96 (16.53 percent), molybdenum-97 (9.46 percent), molybdenum-98 (23.78 percent), and molybdenum-100 (9.13 percent). Molybdenum exhibits oxidation states of +2 to +6 and is considered to display the zero oxidation state in the carbonyl Mo(CO)6. Molybdenum(+6) appears in the trioxide, the most important compound, from which most of its other compounds are prepared, and in the molybdates (containing the anion), used to produce pigments and dyes. Molybdenum disulfide (MoS2), which resembles graphite, is used as a solid lubricant or as an additive to greases and oils. Molybdenum forms hard, refractory, and chemically inert interstitial compounds with boron, carbon, nitrogen, and silicon upon direct reaction with those elements at high temperatures.

Molybdenum is an essential trace element in plants; in legumes as a catalyst it assists bacteria in fixing nitrogen. Molybdenum trioxide and sodium molybdate (Na2MoO4) have been used as micronutrients.

The largest producers of molybdenum are China, the United States, Chile, Peru, Mexico, and Canada.

Element Properties

atomic number  :  42
atomic weight  : 95.94
melting point  :  2,610 °C (4,730 °F)
boiling point  :  5,560 °C (10,040 °F)
specific gravity  :  10.2 at 20 °C (68 °F)
oxidation states  :  0, +2, +3, +4, +5, +6

Additional Information:

Uses

Molybdenum has a very high melting point so it is produced and sold as a grey powder. Many molybdenum items are formed by compressing the powder at a very high pressure.

Most molybdenum is used to make alloys. It is used in steel alloys to increase strength, hardness, electrical conductivity and resistance to corrosion and wear. These ‘moly steel’ alloys are used in parts of engines. Other alloys are used in heating elements, drills and saw blades.

Molybdenum disulfide is used as a lubricant additive. Other uses for molybdenum include catalysts for the petroleum industry, inks for circuit boards, pigments and electrodes.

Biological role

Although it is toxic in anything other than small quantities, molybdenum is an essential element for animals and plants.

There are about 50 different enzymes used by plants and animals that contain molybdenum. One of these is nitrogenase, found in nitrogen-fixing bacteria that make nitrogen from the air available to plants. Leguminous plants have root nodules that contain these nitrogen-fixing bacteria.

Natural abundance

The main molybdenum ore is molybdenite (molybdenum disulfide). It is processed by roasting to form molybdenum oxide, and then reducing to the metal. The main mining areas are in the USA, China, Chile and Peru. Some molybdenum is obtained as a by-product of tungsten and copper production. World production is around 200,000 tonnes per year.

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