Difference between revisions of "Molybdenum" - New World Encyclopedia

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{{Elementbox_header | number=42 | symbol=Mo | name=molybdenum | left=[[niobium]] | right=[[technetium]] | above=[[chromium|Cr]] | below=[[tungsten|W]] | color1=#ffc0c0 | color2=black }}
 
{{Elementbox_header | number=42 | symbol=Mo | name=molybdenum | left=[[niobium]] | right=[[technetium]] | above=[[chromium|Cr]] | below=[[tungsten|W]] | color1=#ffc0c0 | color2=black }}
 
{{Elementbox_series | [[transition metal]]s }}
 
{{Elementbox_series | [[transition metal]]s }}
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{{Elementbox_brinellhardness_mpa | 1500 }}
 
{{Elementbox_brinellhardness_mpa | 1500 }}
 
{{Elementbox_cas_number | 7439-98-7 }}
 
{{Elementbox_cas_number | 7439-98-7 }}
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{{Elementbox_isotopes_begin | color1=#ffc0c0 | color2=black }}
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{{Elementbox_isotopes_begin | isotopesof=Molybdenum | color1=#ffc0c0 | color2=black }}
 
{{Elementbox_isotopes_stable | mn=92 | sym=Mo | na=14.84% | n=50 }}
 
{{Elementbox_isotopes_stable | mn=92 | sym=Mo | na=14.84% | n=50 }}
 
{{Elementbox_isotopes_decay | mn=93 | sym=Mo
 
{{Elementbox_isotopes_decay | mn=93 | sym=Mo
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{{Elementbox_footer | color1=#ffc0c0 | color2=black }}
 
{{Elementbox_footer | color1=#ffc0c0 | color2=black }}
  
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'''Molybdenum''' ([[International Phonetic Alphabet|IPA]]: {{IPA|/məˈlɪbdənəm/}}, from the [[Greek language|Greek]] meaning "leadlike"), is a [[chemical element]] in the [[periodic table]]. Its symbol is '''Mo''' and its [[atomic number]] 42.
+
'''Molybdenum''' (chemical symbol '''Mo''', [[atomic number]] 42) is a silvery white, soft [[metal]]. It has one of the highest [[melting point]]s of all pure [[element]]s. It is used mainly in [[alloy]]s, especially to make high-strength and high-temperature steels. It is also a [[catalyst]] in the [[petroleum]] industry. Molybdenum disulfide is a good [[lubricant]], and molybdenum pigments are used in [[paint]]s, [[ink]]s, [[plastic]]s, and [[rubber]] compounds. Molybdenum in trace amounts has been found to have a role in the biology of all classes of [[organism]]s. If ingested in excess, however, molybdenum dust and its water-soluble compounds can be toxic.
 +
{{toc}}
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== Occurrence ==
 +
 
 +
The element molybdenum (from the [[Greek language|Greek]] ''molybdos'', meaning "lead-like") is not found free in [[nature]]. The main commercial source of molybdenum is the [[mineral]] [[molybdenite]] (MoS<sub>2</sub>), but it is also found in minerals such as [[wulfenite]] ([[lead|Pb]]Mo[[oxygen|O]]<sub>4</sub>) and [[powellite]] ([[calcium|Ca]]MoO<sub>4</sub>).
 +
 
 +
Molybdenum is obtained by mining molybdenite directly and is also recovered as a byproduct of [[copper]] mining. Molybdenum is present in [[ore]]s from 0.01 percent to about 0.5 percent. About half of the world's molybdenum is mined in the [[United States]].
 +
 
 +
The Russian [[Luna 24]] mission discovered a single grain (1 &times; 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 [[steel]]s were at a premium.
  
 
== Notable characteristics ==
 
== Notable characteristics ==
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Molybdenum is a [[transition metal]]. The pure metal is silvery white in color, fairly soft, and has one of the highest melting points of all pure elements. In small quantities, molybdenum is effective at hardening [[steel]]. Molybdenum is important in plant nutrition, and is found in certain [[enzyme]]s, including [[xanthine oxidase]].
 
  
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The pure metal has a tendency to flake apart during machining.
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Molybdenum is a [[transition metal]] that lies in period five of the [[periodic table]], between [[niobium]] and [[technetium]]. In addition, it is located in group six (former group 6B), between [[chromium]] and [[tungsten]].
 +
 
 +
Pure molybdenum has a melting point of 2623&deg;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 prices have increased from a low of about $2/pound in 2000, to about $25/pound as of June 2006, down from a high of $40/pound in May of 2005[http://investinmetal.com/?page_id=23].
+
Molybdenum has six stable [[isotope]]s and many [[radioisotope]]s, most of which have very short [[half-life|half-lives]]. Mo-99 is used to create [[Tc-99]] for the nuclear isotope industry.
  
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== Applications ==
+
==Compounds==
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Over ⅔ of all molybdenum is used in [[alloy]]s. Molybdenum use soared during [[World War I]], when demand for [[tungsten]] (Wolfram) made tungsten scarce and high-strength [[steel]]s were at a premium. Molybdenum is used to this day in high-strength alloys and in high-temperature steels. Special molybdenum-containing alloys, such as the [[Hastelloy]]s, are notably heat-resistant and [[corrosion]]-resistant. Molybdenum is used in oil pipelines, [[aircraft]] and [[missile]] parts, and in filaments. Molybdenum finds use as a [[catalyst]] in the [[petroleum]] industry, especially in catalysts for removing organic [[sulfur]]s from petroleum products. It is used to form the anode in some [[x-ray tube]]s, particularly in [[mammography]] applications. And is found in some electronic applications as the conductive metal layers in [[thin-film transistor]]s (TFTs). [[Molybdenum disulfide]] is a good [[lubricant]], especially at high temperatures. And Mo-99 is used in the [[atomic nucleus|nuclear]] [[isotope]] industry. Molybdenum pigments range from [[red]]-[[yellow]] to a bright red [[Orange (colour)|orange]] and are used in [[paint]]s, [[ink]]s, [[plastic]]s, and [[rubber]] compounds.
+
 
 +
* ''Ammonium tetrathiomolybdate'' ((NH<sub>4</sub>)<sub>2</sub>MoS<sub>4</sub>): 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 (MoS<sub>4</sub><sup>-2</sup>) 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]], H<sub>2</sub>MoO<sub>4</sub>, but the [[dihydrate]] is also found. The [[salt]]s of molybdic acid are called molybdates. As molybdenum can have various [[oxidation state]]s, it can form a wide variety of salts.
  
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== History ==
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* ''Sodium molybdate'' (Na<sub>2</sub>MoO<sub>4</sub>): It is often found as the dihydrate (Na<sub>2</sub>MoO<sub>4</sub> . 2H<sub>2</sub>O) 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)).
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Molybdenum (from the [[Greek language|Greek]] ''molybdos'' meaning "lead-like") is not found free in nature, and the compounds that can be found were, until the late [[18th century]], confused with compounds 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 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 [[19th century]]. Subsequently, a French company, [[Schneider and Co]], tried molybdenum as an alloying agent in steel armor plate and noted its useful properties.
 
  
−
== Occurrence ==
+
* ''Molybdenum disulfide'' or ''molybdenum sulfide'' or ''molybdenum(IV) sulfide'' (MoS<sub>2</sub>): 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 MoS<sub>2</sub> a lubricating effect. Finely powdered MoS<sub>2</sub> is a common dry lubricant. It is also often mixed into various [[oil]]s and [[grease (lubricant)|grease]]s, which allows the lubricated parts to keep running for a while beyond the almost complete loss of oil. It is used in [[aircraft engine]]s, [[constant-velocity joint]]s and [[universal joint]]s in automobiles, and as a lubricating additive to special [[plastic]]s, notably [[nylon]] and [[teflon]]. Synthetic MoS<sub>2</sub> is a [[catalyst]] for desulfurization in petroleum refineries.<ref> [http://www.che.lsu.edu/COURSES/4205/2000/Mattson/HDS.htm Hydrodesulfurization] </ref>
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Though molybdenum is found in such [[mineral]]s as [[wulfenite]] ([[lead|Pb]]Mo[[oxygen|O]]<sub>4</sub>) or [[powellite]] ([[calcium|Ca]]Mo[[oxygen|O]]<sub>4</sub>), the main commercial source of molybdenum is [[molybdenite]] (Mo[[sulfur|S]]<sub>2</sub>). Molybdenum is mined directly, and is also recovered as a byproduct of [[copper]] mining. Molybdenum is present in [[ore]]s from 0.01% to about 0.5%. About half of the world's molybdenum is mined in the [[United States]], with [[Phelps Dodge Corporation]] being a primary provider. Molybdenum is also sometimes used as a name, mostly for pets of science teachers.
 
  
−
The Russian [[Luna 24]] mission discovered a single grain (1 &times; 0.6 µm) of pure molybdenum in a [[pyroxene]] fragment taken from [[Mare Crisium]] on the [[Moon]].
+
== Applications ==
  
−
''See also [[:category:Molybdenum minerals|molybdenum minerals]].''
+
* Over two-thirds of all molybdenum is used in [[alloy]]s, 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 [[Hastelloy]]s) are notably heat-resistant and [[corrosion]]-resistant.
 +
* Molybdenum also finds use as a [[catalyst]] in the [[petroleum]] industry, especially in catalysts for removing organic [[sulfur]]s from petroleum products.
 +
* It is used to form the anode in some [[X-ray tube]]s, particularly in [[mammography]] applications.
 +
* It is found in some electronics applications as the conductive metal layers in [[thin-film transistor]]s (TFTs).
 +
* Molybdenum disulfide is a good [[lubricant]], especially at high temperatures.
 +
* The isotope Mo-99 is used in the [[atomic nucleus|nuclear]] [[isotope]] industry.
 +
* Molybdenum pigments, ranging in color from [[red]]-[[yellow]] to a bright red-[[Orange (colour)|orange]], are used in [[paint]]s, [[ink]]s, [[plastic]]s, and [[rubber]] compounds.
  
 
== Biological role==
 
== Biological role==
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Molybdenum has been found to have a role in the biology of all classes of organisms. It is found in two groups of [[enzyme]]s, the [[nitrogenase|nitrogenases]] and the [[molybdopterin|molybdopterins]].
 
−
 
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The [[nitrogenase]]s are found in bacteria, and are involved in the pathways of [[nitrogen fixation]]. The bacteria may be found inside [[plant]]s. 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.
 
  
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There is a trace requirement for molybdenum in [[plant]]s, and [[soil]]s can be barren due to molybdenum deficiencies. Plants and animals generally have molybdenum present in amounts of a few parts per million. In [[animal]]s 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.
+
Molybdenum has been found to have a role in the [[biology]] of all classes of [[organism]]s. It is found in two groups of [[enzyme]]s&mdash;the [[nitrogenase|nitrogenases]] and the [[molybdopterin|molybdopterins]].
  
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[[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#Directed Panspermia|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.
+
The [[nitrogenase]]s are found in [[bacteria]] (that may dwell in [[plant]]s) and are involved in the pathways of [[nitrogen fixation]]. The molybdenum [[atom]] is present in a cluster that includes [[iron]] and [[sulfur]] atoms. The name molybdopterin is misleading, as this group of enzymes includes [[tungsten]]-containing enzymes, and the word "molybdopterin" does not actually refer to the metal atom. This group may also be referred to as "mononuclear molybdenum enzymes," as the metal atom is not present in a cluster. These enzymes are involved in various processes that are part of the global sulfur, [[nitrogen]], and [[carbon]] cycles.
  
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== Isotopes ==
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There is a requirement for trace amounts of molybdenum in [[plant]]s, and [[soil]]s can be barren on account of molybdenum deficiencies. Plants and animals generally have molybdenum present in amounts of a few parts per million. In [[animal]]s, molybdenum is a [[cofactor]] of the [[enzyme]] [[xanthine oxidase]], which is involved in certain metabolic pathways ([[purine]] degradation and formation of [[uric acid]]). In some animals, adding a small amount of dietary molybdenum enhances growth.
−
Molybdenum has six [[stable isotope]]s and almost two dozen [[radioisotope]]s, the vast majority of which have [[half-life|half-lives]] measured in seconds. Mo-99 is used in [[sorpation generator]]s to create [[Tc-99]] for the nuclear [[isotope]] industry. The market for Mo-99 products is estimated to be on the order of [[United States dollar|US$]]100 million per year.
 
  
 
== Precautions ==
 
== Precautions ==
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Molybdenum dusts and molybdenum compounds, such as [[molybdenum trioxide]] and water-soluble molybdates, may have slight [[toxic]]ities if inhaled or ingested orally. Laboratory tests suggest, compared to many [[heavy metals]], that molybdenum is of relatively low toxicity. Acute toxicity in [[human]]s 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.
 
  
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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, [[ulcer]]s, [[diarrhea]], [[convulsion]]s, [[blindness]], [[osteoporosis]] and finally [[heart failure]].
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Molybdenum dusts and some molybdenum compounds, such as [[molybdenum trioxide]] and water-soluble molybdates, may have slight toxicities if inhaled or ingested orally. Laboratory tests suggest, however, that molybdenum is of relatively low toxicity, compared to many [[heavy metals]]. Acute toxicity in [[human]]s is unlikely, because the dose required would be exceptionally high. 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. Also, water-insoluble molybdenum compounds, such as the lubricant molybdenum disulfide, are considered nontoxic.
  
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[[Occupational Safety and Health Administration|OSHA]] regulations specify maximum molybdenum exposure in an 8-hour day (40-hour week) to be 15 milligrams per cubic meter. [[National Institute for Occupational Safety and Health|NIOSH]] recommends exposure limit of 5000 mg per cubic meter.
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In [[ruminant]]s, molybdenum toxicity occurs if the animals are allowed to graze on soil that is rich in molybdenum but deficient in [[copper]]. The molybdenum causes excretion of copper reserves from the animal, leading to [[copper deficiency]]. In young calves, the molybdenum toxicity is manifested as "teart" or shooting diarrhea, 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, often referred to as "spectacled eyes."
  
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==Toxicity in animals==
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Regulations by the U.S. [[Occupational Safety and Health Administration]] (OSHA) specify maximum molybdenum exposure in an eight-hour day (40-hour week) to be 15 milligrams (mg) per cubic meter. The [[National Institute for Occupational Safety and Health]] (NIOSH) recommends exposure limit of 5,000 mg per cubic meter.
−
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"
+
==Notes==
 +
<references />
  
 
==See also==
 
==See also==
  
 
* [[Chemical element]]
 
* [[Chemical element]]
 +
* [[Metal]]
 
* [[Periodic table]]
 
* [[Periodic table]]
 
* [[Transition metal]]
 
* [[Transition metal]]
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==References==
 
==References==
  
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*[http://periodic.lanl.gov/elements/42.html Los Alamos National Laboratory &ndash; Molybdenum]
+
* Chang, Raymond. ''Chemistry'' (ninth ed.) New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
 +
* Cotton, F. Albert, and Geoffrey Wilkinson. ''Advanced Inorganic Chemistry'' (4th ed.), New York: Wiley, 1980. ISBN 0-471-02775-8
 +
* Greenwood, N.N.; and A. Earnshaw. Chemistry of the Elements (2nd Edition). Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654. Online version available at [http://www.knovel.com/knovel2/Toc.jsp?BookID=402&VerticalID=0 Knovel]. Retrieved August 24, 2007.
 +
*[http://periodic.lanl.gov/elements/42.html Molybdenum] Los Alamos National Laboratory. Retrieved August 24, 2007.
 +
* Topsøe, H., B. S. Clausen, and F. E. Massoth. ''Hydrotreating Catalysis, Science and Technology''. Berlin: Springer-Verlag, 1996.
  
 
== External links ==
 
== External links ==
−
 
+
All links retrieved November 9, 2022.
−
*[http://www.webelements.com/webelements/elements/text/Mo/key.html WebElements.com] &mdash; Molybdenum
+
*[http://www.webelements.com/webelements/elements/text/Mo/key.html WebElements.com] &ndash; Molybdenum.  
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*[http://www.hse.imoa.info/Default.asp?page=110 International Molybdenum Association] &mdash; environmental data
 
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
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[[Category:Materials science]]
 
[[Category:Materials science]]
  
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{{credit|87539386}}
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{{credit5|Molybdenum|87539386|Ammonium_tetrathiomolybdate|89554445|Molybdic_acid|69645337|Sodium_molybdate|82533436|Molybdenum_disulfide|78506164}}

Latest revision as of 19:51, 9 November 2022


42 niobium ← molybdenum → technetium
Cr
↑
Mo
↓
W
Mo-TableImage.png
periodic table
General
Name, Symbol, Number molybdenum, Mo, 42
Chemical series transition metals
Group, Period, Block 6, 5, d
Appearance gray metallic
Mo,42.jpg
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 Molybdenum
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. It is used mainly in alloys, especially to make high-strength and high-temperature steels. It is also a catalyst in the petroleum industry. Molybdenum disulfide is a good lubricant, and molybdenum pigments are used in paints, inks, plastics, and rubber compounds. Molybdenum in trace amounts has been found to have a role in the biology of all classes of organisms. If ingested in excess, however, molybdenum dust and its water-soluble compounds can be toxic.

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 percent to about 0.5 percent. 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 five of the periodic table, between niobium and technetium. In addition, it is located in group six (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 (that may dwell in plants) and are involved in the pathways of nitrogen fixation. The molybdenum atom is present in a cluster that includes iron and sulfur atoms. The name molybdopterin is misleading, as this group of enzymes includes tungsten-containing enzymes, and the word "molybdopterin" does not actually refer to the metal atom. This group may also be referred to as "mononuclear molybdenum enzymes," as the metal atom is not present in a cluster. These enzymes are involved in various processes that are part of the global sulfur, nitrogen, and carbon cycles.

There is a requirement for trace amounts of molybdenum in plants, and soils can be barren on account of 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 certain metabolic pathways (purine degradation and formation of uric acid). In some animals, adding a small amount of dietary molybdenum enhances growth.

Precautions

Molybdenum dusts and some molybdenum compounds, such as molybdenum trioxide and water-soluble molybdates, may have slight toxicities if inhaled or ingested orally. Laboratory tests suggest, however, that molybdenum is of relatively low toxicity, compared to many heavy metals. Acute toxicity in humans is unlikely, because the dose required would be exceptionally high. 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. Also, water-insoluble molybdenum compounds, such as the lubricant molybdenum disulfide, are considered nontoxic.

In ruminants, molybdenum toxicity occurs if the animals are allowed to graze on soil that is rich in molybdenum but deficient in copper. The molybdenum causes excretion of copper reserves from the animal, leading to copper deficiency. In young calves, the molybdenum toxicity is manifested as "teart" or shooting diarrhea, 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, often referred to as "spectacled eyes."

Regulations by the U.S. Occupational Safety and Health Administration (OSHA) specify maximum molybdenum exposure in an eight-hour day (40-hour week) to be 15 milligrams (mg) per cubic meter. The National Institute for Occupational Safety and Health (NIOSH) recommends exposure limit of 5,000 mg per cubic meter.

Notes

  1. ↑ Hydrodesulfurization

See also

References
ISBN links support NWE through referral fees

  • Chang, Raymond. Chemistry (ninth ed.) New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
  • Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry (4th ed.), New York: Wiley, 1980. ISBN 0-471-02775-8
  • Greenwood, N.N.; and A. Earnshaw. Chemistry of the Elements (2nd Edition). Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654. Online version available at Knovel. Retrieved August 24, 2007.
  • Molybdenum Los Alamos National Laboratory. Retrieved August 24, 2007.
  • Topsøe, H., B. S. Clausen, and F. E. Massoth. Hydrotreating Catalysis, Science and Technology. Berlin: Springer-Verlag, 1996.

External links

All links retrieved November 9, 2022.

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