42
  Mo  
95.940000
Molybdenum

Name: Molybdenum
Symbol: Mo
Atomic Number: 42
AtomicWeight: 95.940000
Family: Transition Metals
CAS RN: 7439-98-7
Description: A hard silvery-white, transition metal
State (25 C): Solid
Oxidation states: +6

Molar Volume: 9.41 cm3/mole
Valence Electrons: 4d55s1

Boiling Point:  4885K, 4612C, 8334F
Melting Point:
2890K, 2617C, 4743F
Electrons Energy Level: 2, 8, 18, 13, 1
Isotopes: 27 + 6 Stable
Heat of Vaporization: 598 kJ/mol
Heat of Fusion: 32 kJ/mol
Density: 10.22 g/cm3 @ 300K
Specific Heat: 0.25J/gK
Atomic Radius: 2.01
Ionic Radius: 0.65
Electronegativity: 2.16 (Pauling); 1.3 (Allrod Rochow)
Vapor Pressure: 3.47 Pa @ 2617C

1s2 2s2p6 3s2p6d10 4s2p6d5 5s1

History

Molybdenum (from the 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 1781 Peter Jacob 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, Schneide and Co., tried molybdenum as an alloying agent in steel armor plate and noted its useful properties.

Characteristics

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 necessary in animal nutrition (including human nutrition) as well.  It is found at the active site of certain enzymes, including xanthine oxidase.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d5
5s1

The pure metal has a tendency to flake apart during machining.

Molybdenum's characteristics are so similar to lead and graphite that for many years before it was "officially discovered", it was used accidentally (in place of lead and graphite).

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.

Occurrence

Though molybdenum is found in such minerals as wulfenite (PbMoO4) or powellite (CaMoO4), the main commercial source of molybdenum is molybdenite (MoS2).  Molybdenum is mined 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, with Phelps Dodge Corporation being a primary provider.

Molybdenite is typically a trace sulphide mineral in oxidised granites, and skarn or hydrothermal related mineralisation associated with granite intrusions.  Molybdenite occurs as an exsolution mineral in the later stages of crystallisation of granite magma occupying interstitial volume between silicate minerals, fracturs and cavities.   Typically this forms a style of mineralisation similar, and often complementary with porphyry copper deposit style mineralisation.

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

Applications

Over two thirds of all molybdenum is used in alloys.  Molybdenum use soared during World War I, when demand for tungsten made the valuable refractory metal scarce and high-strength steels 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 Hastelloys, are notably heat- 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 sulfurs from petroleum products.  It is used to form the anode in some x-ray tubes, particularly in mammography applications.  Molybdenum metal is found in some electronic applications as the conductive layers in thin-film transistors (TFTs).   Molybdenum disulfide is a good dry lubricant, especially at high temperatures. Mo-99 is produced in the nuclear isotope industry and is used as tracer.  Molybdenum pigments range from red-yellow to a bright red-orange and are used in paints, inks, plastics, and rubber materials.

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, 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, 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.

Compounds

Molybdenum Hexafluoride, MoF6 Molybdenum Phosphide, MoP2
Molybdenum Trioxide, MoO3 Molybdic Acid, H2MoO4
Wulfenite, PbMoO4 Powellite, CaMoO4
Molybdenite, Molybdenum (IV) Disulfide, MoS2

Isotopes

Molybdenum has six stable isotopes and almost two dozen radioisotopes, the vast majority of which have half-lives measured in seconds. Mo-99 is used in technetium-99m generators to create the gamma emitting nuclear isotope Tc-99m for the nuclear isotope industry (usually for various nuclear medicine scanning applications). The market for Mo-99 products is estimated to be on the order of $100 million per year.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
83Mo 82.94874 23 ms
84Mo 83.94009 3.8 ms
85Mo 84.93655 3.2 seconds
86Mo 85.93070 19.6 seconds
87Mo 86.92733 14.05 seconds
88Mo 87.921953 8.0 minutes
89Mo 88.919480 2.11 minutes
90Mo 89.913937 5.56 hours
91Mo 90.911750 15.49 minutes
92Mo 91.906811 Stable
93Mo 92.906813 4000 years
94Mo 93.9050883 Stable
95Mo 94.9058421(21) Stable
96Mo 95.9046795(21) Stable
97Mo 96.9060215(21) Stable
98Mo 97.9054082 Stable
99Mo 98.9077119 2.7489 days
100Mo 99.907477 1.00E 19 years
101Mo 100.910347 14.61 minutes
102Mo 101.910297 11.3 minutes
103Mo 102.91321 67.5 seconds
104Mo 103.91376 60 seconds
105Mo 104.91697 35.6 seconds
106Mo 105.918137 8.73 seconds
107Mo 106.92169 3.5 seconds
108Mo 107.92345 1.09 seconds
109Mo 108.92781 0.53 seconds
110Mo 109.92973 0.27 seconds
111Mo 110.93441 ~200 ms
112Mo 111.93684 ~150 ms
113Mo 112.94188 ~100 ms
114Mo 113.94492 ~80 ms
115Mo 114.95029 ~60 ms

Precautions

40px-Skull_and_crossbones.svg.jpg (1420 bytes) 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 diarrhea, where the dung is watery, full of air bubbles, and has 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".

atom.gif (700 bytes)

Molybdenum Data
 

Atomic Structure

  • Atomic Radius: 2.01
  • Atomic Volume: 9.4cm3/mol
  • Covalent Radius: 1.3
  • Cross Section (Thermal Neutron Capture) Barns: 2.6
  • Crystal Structure: Cubic body centered
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d5 5s1
  • Electrons per Energy Level: 2, 8, 18, 13, 1
  • Ionic Radius: 0.65
  • Filling Orbital: 4d5
  • Number of Electrons (with no charge): 42
  • Number of Neutrons (most common/stable nuclide): 54
  • Number of Protons: 42
  • Oxidation States: 6, 5, 4, 3, 2
  • Valence Electrons: 4d5 5s1

Chemical Properties

  • Electrochemical Equivalent: 0.8949 g/amp-hr
  • Electron Work Function: 4.6eV
  • Electronegativity: 2.16 (Pauling); 1.3 (Allrod Rochow)
  • Heat of Fusion: 32 kJ/mol
  • Incompatibilities:
    Strong oxidizers
  • Ionization Potential
    • First: 7.099
    • Second: 16.461
    • Third: 27.16
  • Valence Electron Potential (-eV): 88.6

Physical Properties

  • Atomic Mass Average: 95.94
  • Boiling Point: 4885K, 4612C, 8334F
  • Coefficient of Lineal Thermal Expansion/K-1: 5.43E-6
  • Conductivity
    Electrical: 0.187 106/cm
    Thermal: 1.38 W/cmK
  • Density: 10.22 g/cm3 @ 300K
  • Description:
    Hard silvery-white transition metal.
  • Elastic Modulus:
    • Bulk: 261.2/GPa
    • Rigidity: 125.6/GPa
    • Youngs: 324.8/GPa
  • Enthalpy of Atomization: 653 kJ/mole @ 25C
  • Enthalpy of Fusion: 27.61 kJ/mole
  • Enthalpy of Vaporization: 594.1 kJ/mole
  • Flammablity Class: Combusrible Solid in powder form
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 1500 MN m-2
    • Mohs: 5.5
    • Vickers: 1530 MN m-2
  • Heat of Vaporization: 598 kJ/mol
  • Melting Point: 2890K, 2617C, 4743F
  • Molar Volume: 9.41 cm3/mole
  • Optical Reflectivity: 58%
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.25J/gK
  • Vapor Pressure: 3.47 Pa @ 2617C

Regulatory / Health

  • CAS Number
    • 7439-98-7
  • RTECS: QA4680000
  • OSHA Permissible Exposure Limit (PEL)
    • TWA: 15 mg/m3
  • OSHA PEL Vacated 1989
    • TWA: 10 mg/m3
  • NIOSH Recommended Exposure Limit (REL)
    •  
    • IDLH: 5000 mg/m3
  • Routes of Exposure: Inhalation; Ingestion; Skin and/or eye contact
  • Target Organs: Eyes, respiratory system, liver, kidneys
  • Levels In Humans:
    Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
    • Blood/mg dm-3: 0.001
    • Bone/p.p.m: <0.7
    • Liver/p.p.m: 1.3-5.8
    • Muscle/p.p.m: 0.018
    • Daily Dietary Intake: 0.05-0.35 mg
    • Total Mass In Avg. 70kg human: 5 mg

Who / Where / When / How

  • Discoverer: Carl Wilhelm ScheeleP.J. Hjelm
  • Discovery Location: Uppsala Sweden
  • Discovery Year: 1781
  • Name Origin:
    Greek: molubdos (lead).
  • Abundance:
    • Earth's Crust/p.p.m.: 1.5
    • Seawater/p.p.m.: 0.01
    • Atmosphere/p.p.m.: N/A
    • Sun (Relative to H=1E12): 145
  • Sources:
    Found in the minerals molybdenite (MoS2) and wulfenite (MoO4Pb) also produced as a by-product of copper production. Annual world wide production is around 80,000 tons. Primary mining areas are USA, Australia, Italy, Norway, Bolivia.
  • Uses:
    Used in steel, aircraft, missiles, filaments in electric heaters, lubricants and protective coatings in boiler plates.

Ionization Energy (eV): 7.092 eV
Estimated Crustal Abundance: 1.2 milligrams per kilogram
Estimated Oceanic Abundance:
110-2 milligrams per liter

Transition Metals
Group 3
(IIIB)
4
(IVB)
5
(VB)
6
(VIB)
7
(VIIB)
8
(VIIIB)
9
(VIIIB)
10 (VIIIB) 11
(IB)
12
(IIB)
Period 4 21
Sc
44.95
22
Ti
47.86
23
V
50.94
24
Cr
51.99
25
Mn
54.93
26
Fe
55.84
27
Co
58.93
28
Ni
58.69
29
Cu
63.54
30
Zn
65.39
Period 5 39
Y
88.90
40
Zr
91.22
41
Nb
92.90
42
Mo
95.94
43
Tc
98.00
44
Ru
101.0
45
Rh
102.9
46
Pd
106.4
47
Ag
107.8
48
Cd
112.4
Period 6 57
La
138.9
72
Hf
178.4
73
Ta
180.9
74
W
183.8
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.0
79
Au
196.9
80
Hg
200.5
Period 7 89
Ac
227.0
104
Rf
261.0
105
Db
262.0
106
Sg
266.0
107
Bh
264.0
108
Hs
269.0
109
Mt
268.0
110
Ds
269.0
111
Rg
272.0
112
Uub
277.0