Everything about Molybdenum totally explained
Molybdenum (from the
Greek meaning "
lead-like"), is a
Group 6 chemical element with the symbol
Mo and
atomic number 42. It has the sixth-highest melting point of any element, and for this reason it's often used in high-strength steel alloys. Molybdenum is found in trace amounts in plants and animals, although excess molybdenum can be toxic in some animals. Molybdenum was discovered in 1778 by
Carl Wilhelm Scheele and first isolated in 1781 by
Peter Jacob Hjelm.
Characteristics
Molybdenum is a
transition metal with an
electronegativity of 1.8 on the Pauling scale and an atomic mass of 95.9 g/mole. It doesn't react with oxygen or water at room temperature. At elevated temperatures, molybdenum trioxide is formed in the reaction 2Mo + 3O
2 → 2MoO
3.
In its pure metal form, molybdenum is silvery white with a
Mohs hardness of 5.5, though it's somewhat more
ductile than tungsten. It has a
melting point of 2623°C, and only
tantalum,
osmium,
rhenium, and
tungsten have higher melting points. It also has the lowest heating expansion of any commercially used metal.
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, five are stable, with atomic masses from 94 to 98. All unstable isotopes of molybdenum decay into isotopes of
niobium,
technetium, and
ruthenium.
Molybdenum-92 and molybdenum-100 are the only naturally occurring isotopes that are not stable. Molybdenum-100 has a
half-life of approximately 1×10
19 y and undergoes
double beta decay into
ruthenium-100. Molybdenum-98 is the most common isotope, comprising 24.14% of all molybdenum. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately .15 μs.
Though molybdenum is found in such
minerals as
wulfenite (
PbMo
O4) and
powellite (
CaMo
O4), the main commercial source of molybdenum is
molybdenite (Mo
S2). Molybdenum is mined as a principal ore, and is also recovered as a byproduct of copper and tungsten mining.
A side product of molybdenum mining is
rhenium. As it's always present in small varying quantities in molybdenite, the only commercial source for rhenium is molybdenum mines.
Compounds
Molybdenum has several common
oxidation states, +2 +3 +4 +5 and +6.
The highest oxidation state is common in the
molybdenum(VI) oxide MoO
3 while the normal sulfur compound is
molybdenum disulfide MoS
2. The broad range of oxidation states shows up in the chlorides of molybdenum:
Biological role
The most important use of the molybdenum atom in living organisms is as a metal hetero-atom at the active site in certain enzymes. In
nitrogen fixation in certain bacteria, the
nitrogenase enzyme which is involved in the terminal step of reducing molecular nitrogen, usually contains molybdenum in the active site (though replacement of Mo with iron or vanadium is known).
In March 2008, researchers reported that they'd found strong evidence for the hypothesis that a scarcity of molybdenum in the earth's early oceans was a limiting factor in the further evolution of
eukaryotic life (which includes all plants and animals) as eukaryotes can't fix nitrogen and must acquire it from prokaryotic bacteria.
(External Link
) (External Link) The scarcity of molybdenum resulted from the relative lack of oxygen in the early ocean. Oxygen dissolved in seawater is the primary mechanism for dissolving molybdenum from minerals on the sea bottom.
Though molybdenum forms compounds with various
organic molecules, including
carbohydrates and
amino acids, it's transported throughout the human body as MoO
42-. Molybdenum is present in approximately 20 enzymes in animals, including
aldehyde oxidase,
sulfite oxidase,
xanthine oxidase. It occurs in higher concentrations in the liver and kidneys, and in lower concentrations in the vertebrae. Pork, lamb, and beef liver each have approximately 1.5 parts molybdenum per million. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, and cereal grain. Molybdenum deficiency isn't usually seen in healthy people.
Sodium tungstate is a
competitive inhibitor of molybdenum. Dietary
tungsten reduces the concentration of molybdenum in tissues. The condition can be aggravated by excess
sulfur. Most high-strength steel alloys are .25% to 8% molybdenum.
Molybdenum 99 is used as a parent radioisotope to the radioisotope Technetium 99, which is used in many medical procedures
Molybdenum disulfide (MoS
2) is used as a lubricant and an agent. It forms strong films on metallic surfaces, and is highly resistant to both extreme temperatures and high pressure, and for this reason, it's a common additive to engine motor oil; in case of a catastrophic failure, the thin layer of molybdenum prevents metal-on-metal contact. Lead molybdate co-precipitated with lead chromate and lead sulfate is a bright-orange pigment used with ceramics and plastics.
Molybdenum trioxide (MoO
3) is used as an adhesive between
enamels and metals. the principal ore from which molybdenum is now extracted, was previously known as molybdena. Molybdena was confused with and often implemented as though it were
graphite. Even when the two ores were distinguishable, molybdena was thought to be a
lead ore.
It wasn't until 1778 that
Swedish chemist
Carl Wilhelm Scheele realized molybdena was neither graphite nor lead. He and other chemists then correctly assumed that it was the ore of a distinct new element, named
molybdenum for the mineral in which it was discovered.
Peter Jacob Hjelm successfully isolated molybdenum using
carbon and
linseed oil in 1781.
For a long time there was no industrial use for molybdenum. The French
Schneider Electrics company produced the first steel molybdenum alloy armor plates in 1894. Until
World War I most other armor factories also used molybdenum alloys. In
World War I, some British tanks were protected by 75 mm
manganese plating, but this proved to be ineffective. The manganese plates were then replaced with 25 mm molybdenum plating. These allowed for higher speed, greater maneuverability, and, despite being thinner, better protection.
OSHA regulations specify the maximum permissible molybdenum exposure in an 8-hour day to be 5 mg/m³. Chronic exposure to 60 to 600 mg Mo/m³ can cause symptoms including fatigue, headaches, and joint pains.
Supply and demand
Although current molybdenum production meets demand, refiners, or roasters, are expected to run into a shortfall between 2009 and 2015, depending on demand.
A roaster processes the moly into a fine powder, pellets, or other forms. Total world moly roaster capacity is currently 320 million pounds per year, barely enough to meet demand. There isn't much excess roasting capacity, and no one is actively permitting for the production of any new roasters in the United States. Global roaster capacity also looks limited, and a future roaster shortage is predicted. The data above are based on the assumption that mines will be able to increase output.
Western demand is projected to increase by around 3 percent annually, while China and the CIS demand is projected to increase by around 10 percent annually, increasing overall global demand by around 4.5 percent annually. Increasing demand can be attributed to two main factors. Hydroprocessing catalysts are becoming essential for crude oil. The other contributing factor is the increase in nuclear reactor construction. There are 48 nuclear reactors to be built by 2013, and approximately 100 are to be built by 2020. The International Molybdenum Association (IMOA) says that an average reactor contains about 520,000 feet of stainless steel alloy. Some larger reactors contain over 1 million feet of stainless steel alloy. Unless moly mine production picks up at a rapid pace, shortfalls of the metal are expected to arrive around 2009.
Further Information
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