Molybdenum

42 niobium ← molybdenum → technetium Cr ↑ Mo ↓ W periodic table
General
Name, Symbol, Number	molybdenum, Mo, 42
Chemical series	transition metals
Group, Period, Block	6, 5, d
Appearance	gray metallic
Atomic mass	95.94(2) g/mol
Electron configuration	[Kr] 4d5 5s1
Electrons per shell	2, 8, 18, 13, 1
Physical properties
Phase	solid
Density (near r.t.)	10.28 g/cm³
Liquid density at m.p.	9.33 g/cm³
Melting point	2896 K (2623 °C, 4753 °F)
Boiling point	4912 K (4639 °C, 8382 °F)
Heat of fusion	37.48 kJ/mol
Heat of vaporization	617 kJ/mol
Heat capacity	(25 °C) 24.06 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 2742 2994 3312 3707 4212 4879
Atomic properties
Crystal structure	cubic body centered
Oxidation states	2, 3, 4, 5, 6 (strongly acidic oxide)
Electronegativity	2.16 (Pauling scale)
Ionization energies (more)	1st: 684.3 kJ/mol
	2nd: 1560 kJ/mol
	3rd: 2618 kJ/mol
Atomic radius	145 pm
Atomic radius (calc.)	190 pm
Covalent radius	145 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 53.4 nΩ·m
Thermal conductivity	(300 K) 138 W/(m·K)
Thermal expansion	(25 °C) 4.8 µm/(m·K)
Speed of sound (thin rod)	(r.t.) 5400 m/s
Speed of sound (thin rod)	(r.t.) 329 m/s
Shear modulus	20 GPa
Bulk modulus	230 GPa
Poisson ratio	0.31
Mohs hardness	5.5
Vickers hardness	1530 MPa
Brinell hardness	1500 MPa
CAS registry number	7439-98-7
Notable isotopes
Main article: [[Isotopes of {{{isotopesof}}}]] iso NA half-life DM DE (MeV) DP 92Mo 14.84% Mo is stable with 50 neutrons 93Mo syn 4×103 y ε - 93Nb 94Mo 9.25% Mo is stable with 52 neutrons 95Mo 15.92% Mo is stable with 53 neutrons 96Mo 16.68% Mo is stable with 54 neutrons 97Mo 9.55% Mo is stable with 55 neutrons 98Mo 24.13% Mo is stable with 56 neutrons 99Mo syn 65.94 h β- 0.436, 1.214 99Tc γ 0.74, 0.36, 0.14 - 100Mo 9.63% 7.8×1018 y β-β- ? 100Ru

Molybdenum (chemical symbol Mo, atomic number 42) is a silvery white, soft metal. It has one of the highest melting points of all pure elements. In small quantities, it is effective at hardening steel. It is important for plant nutrition and is found in certain enzymes, including xanthine oxidase.

Occurrence

The element molybdenum (from the Greek molybdos, meaning "lead-like") is not found free in nature. The main commercial source of molybdenum is the mineral molybdenite (MoS₂), but it is also found in minerals such as wulfenite (PbMoO₄) and powellite (CaMoO₄).

Molybdenum is obtained by mining molybdenite directly and is also recovered as a byproduct of copper mining. 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.

The Russian Luna 24 mission discovered a single grain (1 × 0.6 micrometer) of pure molybdenum in a pyroxene fragment taken from Mare Crisium on the Moon.

History

Until the late eighteenth century, the compounds of molybdenum were confused with those of other elements, such as carbon or lead. In 1778, Carl Wilhelm Scheele was able to determine that molybdenum was separate from graphite and lead, and he isolated the oxide of the metal from molybdenite. In 1782, Hjelm isolated an impure extract of the metal by reducing the oxide with carbon. Molybdenum was little used and remained in the laboratory until the late nineteenth century. Subsequently, a French company (Schneider and Co.) tried molybdenum as an alloying agent in steel armor plating and noted its usefulness as a hardener of steel. Molybdenum use soared during World War I, when the increased demand for tungsten made that element scarce and high-strength steels were at a premium.

Notable characteristics

Molybdenum is a transition metal that lies in period 5 of the periodic table, between niobium and technetium. In addition, it is located in group 6 (former group 6B), between chromium and tungsten.

Pure molybdenum has a melting point of 2623°C, which is among the highest melting points of all elements. The pure metal has a tendency to flake apart during machining, but it is useful as an additive that hardens steel.

Isotopes

Molybdenum has six stable isotopes and many radioisotopes, most of which have very short half-lives. Mo-99 is used to create Tc-99 for the nuclear isotope industry.

Compounds

• Ammonium tetrathiomolybdate ((NH₄)₂MoS₄): This bright red ammonium salt is an important reagent in the chemistry of molybdenum and has been used as a building block in bioinorganic chemistry. The thiometallate anion (MoS₄^-2) has the distinctive property of undergoing oxidation at the sulfur centers, concomitant with reduction of the metal from Mo(VI) to Mo(IV).

• Molybdic acid: It refers to hydrated forms of molybdenum trioxide. The simplest form is the monohydrate, H₂MoO₄, but the dihydrate is also found. The salts of molybdic acid are called molybdates. As molybdenum can have various oxidation states, it can form a wide variety of salts.

• Sodium molybdate (Na₂MoO₄): It is often found as the dihydrate (Na₂MoO₄ . 2H₂O) and is useful as a source of molybdenum. It is also used in biochemistry and medicinal chemistry to track various colorless organic chemicals that, in the presence of the salt, can be stained blue. The blue color, also called molybdenum blue, is a complex of molybdates (Mo(VI), Mo(V)).

• Molybdenum disulfide or molybdenum sulfide or molybdenum(IV) sulfide (MoS₂): This black, crystalline sulfide occurs as the mineral molybdenite. Its structure, appearance, and feel are similar to graphite. It consists of a sandwich of layers of molybdenum atoms between layers of sulfur atoms. The weak interactions between the sheets gives MoS₂ a lubricating effect. Finely powdered MoS₂ is a common dry lubricant. It is also often mixed into various oils and greases, which allows the lubricated parts to keep running for a while beyond the almost complete loss of oil. It is used in aircraft engines, constant-velocity joints and universal joints in automobiles, and as a lubricating additive to special plastics, notably nylon and teflon. Synthetic MoS₂ is a catalyst for desulfurization in petroleum refineries.[1].

Applications

• Over two-thirds of all molybdenum is used in alloys, especially to make high-strength and high-temperature steels. Such alloys are used for oil pipelines, aircraft and missile parts, and filaments. Special alloys (such as the Hastelloys) are notably heat-resistant and corrosion-resistant.
• Molybdenum also finds use as a catalyst in the petroleum industry, especially in catalysts for removing organic sulfurs from petroleum products.
• It is used to form the anode in some X-ray tubes, particularly in mammography applications.
• It is found in some electronics applications as the conductive metal layers in thin-film transistors (TFTs).
• Molybdenum disulfide is a good lubricant, especially at high temperatures.
• The isotope Mo-99 is used in the nuclear isotope industry.
• Molybdenum pigments, ranging in color from red-yellow to a bright red-orange, are used in paints, inks, plastics, and rubber compounds.

Biological role

Molybdenum has been found to have a role in the biology of all classes of organisms. It is found in two groups of enzymes, the nitrogenases and the molybdopterins.

The nitrogenases are found in bacteria, and are involved in the pathways of nitrogen fixation. The bacteria may be found inside plants. The molybdenum atom is present in a cluster (see cluster chemistry), which includes iron and sulfur atoms. The name molybdopterin is misleading as the group of enzymes includes tungsten-containing enzymes, and the word "molybdopterin" does not actually refer to the metal atom. The group may also be referred to as the "mononuclear molybdenum enzymes" as the metal atom is not present in a cluster. This group of enzymes is involved in a variety of processes, as part of the global sulfur, nitrogen and carbon cycles, and generally involve an oxygen atom transfer as part of the process.

There is a trace requirement for molybdenum in plants, and soils can be barren due to molybdenum deficiencies. Plants and animals generally have molybdenum present in amounts of a few parts per million. In animals molybdenum is a cofactor of the enzyme xanthine oxidase which is involved in the pathways of purine degradation and formation of uric acid. In some animals, adding a small amount of dietary molybdenum enhances growth.

Francis Crick suggested that since molybdenum is an essential trace element that plays an important role in many enzymatic reactions, despite being less abundant than the more common elements, such as chromium and nickel, that perhaps this fact is indictative of "Panspermia." Crick theorized that if it could be shown that the elements represented in terrestrial living organisms correlate closely with those that are abundant in some class of star - molybdenum stars, for example, that this would provide evidence of such Directed Panspermia.

Precautions

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 non-toxic.

However, environmental chains of events can end in serious molybdenum-related health consequences. In 1996, an increase in acid rain near Uppsala, Sweden 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.

OSHA regulations specify maximum molybdenum exposure in an 8-hour day (40-hour week) to be 15 milligrams per cubic meter. NIOSH recommends exposure limit of 5000 mg per cubic meter.

Toxicity in animals

In ruminants, the molybdenum toxicity occurs if the animals are let to graze on soil rich in molybdenum, but deficient in copper. The molybdenum causes excretion of copper reserves from the animal and cause copper deficiency. In young calves, the molybdenum toxicity is manifested as "teart" or shooting diarrhoea, where the dung is watery, full of air bubbles and with a fetid odor. In pigs and sheep, molybdenum toxicity combined with copper deficiency can lead to a condition called sway back or paralysis of hind quarters. In black coated animals, the toxicity of this metal is characterized by depigmentation of the skin surrounding the eyes, which is often referred to as "spectacled eyes"

See also

• Chemical element
• Periodic table
• Transition metal

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Molybdenum
• Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis, Science and Technology, Berlin: Springer-Verlag, 1996.

External links

• WebElements.com — Molybdenum
• International Molybdenum Association — environmental data

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