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Molybdenum

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                42            niobium ← molybdenum → technetium
                Cr
                ↑
                Mo
                ↓
                W

                                  Periodic Table - Extended 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] 4d^5 5s^1
                                       Electrons per shell 2, 8, 18, 13, 1
                                                       Physical properties
                                                               Phase solid
                                      Density (near r.t.) 10.28 g·cm^−3
                                    Liquid density at m.p. 9.33 g·cm^−3
                                                     Melting point 2896  K
                                                    (2623 ° C, 4753 ° F)
                                                      Boiling point 4912 K
                                                    (4639 ° C, 8382 ° F)
                                         Heat of fusion 37.48 kJ·mol^−1
                                     Heat of vaporization 617 kJ·mol^−1
                          Heat capacity (25 °C) 24.06 J·mol^−1·K^−1

   CAPTION: 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^−1
                                                    2nd: 1560 kJ·mol^−1
                                                    3rd: 2618 kJ·mol^−1
                                                      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^−1·K^−1
                        Thermal expansion (25 °C) 4.8 µm·m^−1·K^−1
                        Speed of sound (thin rod) ( r.t.) 5400   m·s^−1
                                                   Young's modulus 329 GPa
                                                      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
                                                         Selected isotopes

                CAPTION: Main article: Isotopes of molybdenum

                       iso     NA    half-life    DM    DE ( MeV)     DP
                      ^92Mo  14.84% Mo is stable with 50 neutrons
                      ^93Mo  syn    4×10^3 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%  7.8×10^18 y β^-β^-  ?           ^100Ru

                                                                References

   Molybdenum ( IPA: /məˈlɪbdənəm/, from the Greek meaning "leadlike"), is
   a chemical element in the periodic table. Its symbol is Mo and its
   atomic number 42.

Notable characteristics

   Molybdenum is a transition metal. The pure metal is silvery white in
   colour, 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 enzymes, including xanthine oxidase.

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

   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 .

Applications

   Over ⅔ of all molybdenum is used in alloys. Molybdenum use soared
   during World War I, when demand for tungsten (Wolfram) made tungsten
   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-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 sulfurs from petroleum products. It is
   used to form the anode in some x-ray tubes, particularly in mammography
   applications. And is found in some electronic applications as the
   conductive metal layers in thin-film transistors (TFTs). Molybdenum
   disulfide is a good lubricant, especially at high temperatures. And
   Mo-99 is used in the nuclear isotope industry. Molybdenum pigments
   range from red- yellow to a bright red orange and are used in paints,
   inks, plastics, and rubber compounds.

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 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

   Though molybdenum is found in such minerals as wulfenite (PbMoO[4]) or
   powellite (CaMoO[4]), the main commercial source of molybdenum is
   molybdenite (MoS[2]). 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. Molybdenum is also sometimes used as a name, mostly
   for pets of science teachers.

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

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.

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 sorpation generators to create Tc-99 for the nuclear isotope
   industry. The market for Mo-99 products is estimated to be on the order
   of US$100 million per year.

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 odour. 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"

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