Molybdenum

Chemical element with atomic number 42

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Chemical element, symbol Mo and atomic number 42

Molybdenum is a chemical element; it has symbol Mo (from Neo-Latin molybdaenum) and atomic number 42. The name derived from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores.[10] 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.[11]

Molybdenum does not occur naturally as a free metal on Earth; it is found only in various oxidation states in minerals. 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, but when molybdenum-bearing minerals contact oxygen and water, the resulting molybdate ion MoO2−4 is quite soluble. Industrially, molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications 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. These nitrogenases contain an iron–molybdenum cofactor FeMoco, which is believed to contain either Mo(III) or Mo(IV).[12][13] This is distinct from the fully oxidized Mo(VI) found complexed with molybdopterin in all other molybdenum-bearing enzymes, which perform a variety of crucial functions.[14] The variety of crucial reactions catalyzed by these latter enzymes means that molybdenum is an essential element for all higher eukaryote organisms, including humans.

Characteristics

Physical properties

In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5 and a standard atomic weight of 95.95 g/mol.[15][16] It has a melting point of 2,623 °C (4,753 °F), sixth highest of the naturally occurring elements; only tantalum, osmium, rhenium, tungsten, and carbon have higher melting points.[10] It has one of the lowest coefficients of thermal expansion among commercially used metals.[17]

Chemical properties

Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale. It does not visibly react with oxygen or water at room temperature, but is attacked by halogens and hydrogen peroxide. Weak oxidation of molybdenum starts at 300 °C (572 °F); bulk oxidation occurs at temperatures above 600 °C, resulting in molybdenum trioxide. Like many heavier transition metals, molybdenum shows little inclination to form a cation in aqueous solution, although the Mo3+ cation is known to form under carefully controlled conditions.[18]

Gaseous molybdenum consists of the diatomic species Mo2. That molecule is a singlet, with two unpaired electrons in bonding orbitals, in addition to 5 conventional bonds. The result is a sextuple bond.[19][20]

Isotopes

There are 39 known isotopes of molybdenum, ranging in atomic mass from 81 to 119, as well as 13 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-100 is unstable.[8]

Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 1019 y and undergoes double beta decay into ruthenium-100. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium. Of the synthetic radioisotopes, the most stable is 93Mo, with a half-life of 4,839 years.[9]

The most common isotopic molybdenum application involves molybdenum-99, which is a fission product. It is a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer used in various imaging applications in medicine.[21] In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99.[22]

Compounds

Molybdenum forms chemical compounds in oxidation states −4 and from −2 to +6. Higher oxidation states are more relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are often associated with metal clusters, and very low oxidation states are typically associated with organomolybdenum compounds. The chemistry of molybdenum and tungsten show strong similarities. The relative rarity of molybdenum(III), for example, contrasts with the pervasiveness of the chromium(III) compounds. The highest oxidation state is seen in molybdenum(VI) oxide (MoO3), whereas the normal sulfur compound is molybdenum disulfide MoS2.[23]

From the perspective of commerce, the most important compounds are molybdenum disulfide (MoS2) and molybdenum trioxide (MoO3). The black disulfide is the main mineral. It is roasted in air to give the trioxide:[23]

2

Mo

2

+ 7

O

2

→ 2

MoO

3

+ 4

SO

2

The trioxide, which is volatile at high temperatures, is the precursor to virtually all other Mo compounds as well as alloys. Molybdenum has several oxidation states, the most stable being +4 and +6 (bolded in the table at left).

Molybdenum(VI) oxide is soluble in strong alkaline water, forming molybdates (MoO42−). Molybdates are weaker oxidants than chromates. They tend to form structurally complex oxyanions by condensation at lower pH values, such as [Mo7O24]6− and [Mo8O26]4−. Polymolybdates can incorporate other ions, forming polyoxometalates.[27] The dark-blue phosphorus-containing heteropolymolybdate P[Mo12O40]3− is used for the spectroscopic detection of phosphorus.[28] The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides:[23]

• Molybdenum(II) chloride MoCl2, which exists as the hexamer Mo6Cl12 and the related dianion [Mo6Cl14]2-.
• Molybdenum(III) chloride MoCl3, a dark red solid, which converts to the anion trianionic complex [MoCl6]3-.
• Molybdenum(IV) chloride MoCl4, a black solid, which adopts a polymeric structure.
• Molybdenum(V) chloride MoCl5 dark green solid, which adopts a dimeric structure.
• Molybdenum(VI) chloride MoCl6 is a black solid, which is monomeric and slowly decomposes to MoCl5 and Cl2 at room temperature.[29]

Like chromium and some other transition metals, molybdenum forms quadruple bonds, such as in Mo2(CH3COO)4 and [Mo2Cl8]4−.[23][30] The Lewis acid properties of the butyrate and perfluorobutyrate dimers, Mo2(O2CR)4 and Rh2(O2CR) 4, have been reported.[31]

The oxidation state 0 and lower are possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)6.[23][25]

History

Molybdenite—the principal ore from which molybdenum is now extracted—was previously known as molybdena. Molybdena was confused with and often utilized as though it were graphite. Like graphite, molybdenite can be used to blacken a surface or as a solid lubricant.[32] Even when molybdena was distinguishable from graphite, it was still confused with the common lead ore PbS (now called galena); the name comes from Ancient Greek Μόλυβδος molybdos, meaning lead.[17] (The Greek word itself has been proposed as a loanword from Anatolian Luvian and Lydian languages).[33]

Although (reportedly) molybdenum was deliberately alloyed with steel in one 14th-century Japanese sword (mfd. c. 1330), that art was never employed widely and was later lost.[34][35] In the West in 1754, Bengt Andersson Qvist examined a sample of molybdenite and determined that it did not contain lead and thus was not galena.[36]

By 1778 Swedish chemist Carl Wilhelm Scheele stated firmly that molybdena was (indeed) neither galena nor graphite.[37][38] Instead, Scheele correctly proposed that molybdena was an ore of a distinct new element, named molybdenum for the mineral in which it resided, and from which it might be isolated. Peter Jacob Hjelm successfully isolated molybdenum using carbon and linseed oil in 1781.[17][39]

For the next century, molybdenum had no industrial use. It was relatively scarce, the pure metal was difficult to extract, and the necessary techniques of metallurgy were immature.[40][41][42] Early molybdenum steel alloys showed great promise of increased hardness, but efforts to manufacture the alloys on a large scale were hampered with inconsistent results, a tendency toward brittleness, and recrystallization. In 1906, William D. Coolidge filed a patent for rendering molybdenum ductile, leading to applications as a heating element for high-temperature furnaces and as a support for tungsten-filament light bulbs; oxide formation and degradation require that molybdenum be physically sealed or held in an inert gas.[43] In 1913, Frank E. Elmore developed a froth flotation process to recover molybdenite from ores; flotation remains the primary isolation process.[44]

During World War I, demand for molybdenum spiked; it was used both in armor plating and as a substitute for tungsten in high-speed steels. Some British tanks were protected by 75 mm (3 in) manganese steel plating, but this proved to be ineffective. The manganese steel plates were replaced with much lighter 25 mm (1.0 in) molybdenum steel plates allowing for higher speed, greater maneuverability, and better protection.[17] The Germans also used molybdenum-doped steel for heavy artillery, like in the super-heavy howitzer Big Bertha,[45] because traditional steel melts at the temperatures produced by the propellant of the one ton shell.[46] After the war, demand plummeted until metallurgical advances allowed extensive development of peacetime applications. In World War II, molybdenum again saw strategic importance as a substitute for tungsten in steel alloys.[47]

Occurrence and production