Molybdenum

Molybdenum,  ₄₂Mo

General properties
Name, symbol	molybdenum, Mo
Pronunciation	or or
Appearance	gray metallic
Molybdenum in the periodic table
Cr ↑ Mo ↓ W niobium ← molybdenum → technetium
Atomic number	42
Standard atomic weight	95.95(1)
Element category	transition metal
Group, block	group 6, d-block
Period	period 5
Electron configuration	[Kr] 4d5 5s1
per shell	2, 8, 18, 13, 1
Physical properties
Phase	solid
Melting point	2896 K ​(2623 °C, ​4753 °F)
Boiling point	4912 K ​(4639 °C, ​8382 °F)
Density near r.t.	10.28 g·cm−3
liquid, at m.p.	9.33 g·cm−3
Heat of fusion	37.48 kJ·mol−1
Heat of vaporization	598 kJ·mol−1
Molar heat capacity	24.06 J·mol−1·K−1
vapor P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2742 2994 3312 3707 4212 4879
Atomic properties
Oxidation states	6, 5, 4, 3, 2, 1,[1] −1, −2 ​(a strongly acidic oxide)
Electronegativity	Pauling scale: 2.16
Ionization energies	1st: 684.3 kJ·mol−1 2nd: 1560 kJ·mol−1 3rd: 2618 kJ·mol−1
Atomic radius	empirical: 139 pm
Covalent radius	154±5 pm
Miscellanea
Crystal structure	​body-centered cubic (bcc)
Speed of sound thin rod, at r.t.	5400 m·s−1
Thermal expansion	4.8 µm·m−1·K−1 (at 25 °C)
Thermal conductivity	138 W·m−1·K−1
Thermal diffusivity	54.3 mm2·s−1 (at 300 K)[2]
Electrical resistivity	at 20 °C: 53.4 nΩ·m
Magnetic ordering	paramagnetic[3]
Young's modulus	329 GPa
Shear modulus	126 GPa
Bulk modulus	230 GPa
Poisson ratio	0.31
Mohs hardness	5.5
Vickers hardness	1530 MPa
Brinell hardness	1500 MPa
CAS number	7439-98-7
History
Discovery	Carl Wilhelm Scheele (1778)
First isolation	Peter Jacob Hjelm (1781)
Most stable isotopes
iso NA half-life DM DE (MeV) DP 92Mo 14.84% >1.9×1020 y (β+β+) 1.6491 92Zr 93Mo syn 4×103 y ε – 93Nb 94Mo 9.25% – (SF) <4.485 95Mo 15.92% – (SF) <4.531 96Mo 16.68% – (SF) <5.771 97Mo 9.55% – (SF) <6.226 98Mo 24.13% >1×1014 y (β−β−) 0.1125 98Ru 99Mo syn 65.94 h β− 0.436, 1.214 99mTc γ 0.74, 0.36, 0.14 – 100Mo 9.63% 7.8×1018 y β−β− 3.04 100Ru
Decay modes in parentheses are predicted, but have not yet been observed

Molybdenum is a chemical element with symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores.^[4] 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, but rather in various oxidation states in minerals. The free element, which is a silvery metal with a gray cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of world production of the element (about 80%) is in making many types of steel alloys, including high strength alloys and superalloys.

Most molybdenum compounds have low solubility in water, but the molybdate ion MoO2−
4 is soluble and forms when molybdenum-containing minerals are in contact with oxygen and water. Industrially, molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications, as pigments, and as catalysts.

Molybdenum-containing enzymes are by far the most common catalysts used by some bacteria to break the chemical bond in atmospheric molecular nitrogen, allowing biological nitrogen fixation. At least 50 molybdenum-containing enzymes are now known in bacteria and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. These nitrogenases contain molybdenum in a different form from the other molybdenum-containing enzymes, which all contain fully oxidized molybdenum incorporated into a molybdenum cofactor. Owing to the diverse functions of the various molybdenum cofactor enzymes, molybdenum is a required element for life in all higher eukaryote organisms, though it is not required by all bacteria.

Characteristics

Physical properties

In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5. 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.^[4] Weak oxidation of molybdenum starts at 300 °C (572 °F). It has one of the lowest coefficients of thermal expansion among commercially used metals.^[5] The tensile strength of molybdenum wires increases about 3 times, from about 10 to 30 GPa, when their diameter decreases from ~50–100 nm to 10 nm.^[6]

Isotopes

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-100 is unstable.^[7]

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. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately 150 ns.^[7][8] All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.^[8]

As also noted below, 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.^[9] In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99.^[10]

Compounds and chemistry

Keggin structure of the phosphomolybdate anion (P[Mo₁₂O₄₀]³⁻), an example of a polyoxometalate

Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale and a standard atomic weight of 95.95 g/mol.^[12][13] It does not visibly react with oxygen or water at room temperature, and the bulk oxidation occurs at temperatures above 600 °C, resulting in molybdenum trioxide:

2 Mo + 3 O
2 → 2 MoO
3

The trioxide is volatile and sublimates at high temperatures. This prevents formation of a continuous protective oxide layer, which would stop the bulk oxidation of metal.^[14] Molybdenum has several oxidation states, the most stable being +4 and +6 (bolded in the table). The chemistry and the compounds show more similarity to those of tungsten than that of chromium. An example is the instability of molybdenum(III) and tungsten(III) compounds as compared with the stability of the chromium(III) compounds. The highest oxidation state is common in the molybdenum(VI) oxide (MoO₃), whereas the normal sulfur compound is molybdenum disulfide MoS₂.^[15]

Molybdenum(VI) oxide is soluble in strong alkaline water, forming molybdates (MoO₄²⁻). Molybdates are weaker oxidants than chromates, but they show a similar tendency to form complex oxyanions by condensation at lower pH values, such as [Mo₇O₂₄]⁶⁻ and [Mo₈O₂₆]⁴⁻. Polymolybdates can incorporate other ions into their structure, forming polyoxometalates.^[16] The dark-blue phosphorus-containing heteropolymolybdate P[Mo₁₂O₄₀]³⁻ is used for the spectroscopic detection of phosphorus.^[17] The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides:^[15]

• Molybdenum(II) chloride MoCl₂ (yellow solid)
• Molybdenum(III) chloride MoCl₃ (dark red solid)
• Molybdenum(IV) chloride MoCl₄ (black solid)
• Molybdenum(V) chloride MoCl₅ (dark green solid)
• Molybdenum(VI) chloride MoCl₆ (brown solid)

The structure of the MoCl₂ is composed of Mo₆Cl₈⁴⁺ clusters with four chloride ions to compensate the charge.^[15]

Like chromium and some other transition metals, molybdenum is able to form quadruple bonds, such as in Mo₂(CH₃COO)₄. This compound can be transformed into Mo₂Cl₈⁴⁻, which also has a quadruple bond.^[15]

The oxidation state 0 is possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)₆.^[15]

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.^[18] 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.^[5] (The Greek word itself has been proposed as a loanword from Anatolian Luvian and Lydian languages).^[19]

Although apparent deliberate alloying of molybdenum with steel in one 14th-century Japanese sword (mfd. ca. 1330) has been reported, that art was never employed widely and was later lost.^[20][21] In the West in 1754, Bengt Andersson Qvist examined molybdenite and determined that it did not contain lead, and thus was not the same as galena.^[22]

By 1778 Swedish chemist Carl Wilhelm Scheele stated firmly that molybdena was (indeed) not galena nor graphite.^[23][24] Instead, Scheele went further and 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 by using carbon and linseed oil in 1781.^[5][25]

For about a century after its isolation, molybdenum had no industrial use, owing to its relative scarcity, difficulty extracting the pure metal, and the immaturity of appropriate metallurgical techniques.^[26][27][28] Early molybdenum steel alloys showed great promise in their increased hardness, but efforts to manufacture them on a large scale were hampered by inconsistent results and a tendency toward brittleness and recrystallization. In 1906, William D. Coolidge filed a patent for rendering molybdenum ductile, leading to its use 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.^[29] In 1913, Frank E. Elmore developed a flotation process to recover molybdenite from ores; flotation remains the primary isolation process^[30]

During the first World War, 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 25 mm (1 in) molybdenum steel plating allowing for higher speed, greater maneuverability, and better protection.^[31] The Germans also used molybdenum-doped steel for heavy artillery. This was because traditional steel melted at the heat produced by enough gunpowder to launch a one ton shell.^[32] 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.^m]]_or_
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