Molybdenum

Molybdenum

Molybdenum is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known into prehistory, but the element was discovered (in the sense of differentiating it as a new entity from the mineral salts of other metals) in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm.

Molybdenum does not occur naturally as a free metal on Earth, but rather in various oxidation states in minerals. The free element, which is a silvery metal with a gray cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of world production of the element (about 80%) is in making many types of steel alloys, including high strength alloys and superalloys.
Most molybdenum compounds have low solubility in water, but the molybdate ion MoO42− is soluble and forms when molybdenum-containing minerals are in contact with oxygen and water. Industrially, molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications, as pigments and catalysts.

Molybdenum-containing enzymes are by far the most common catalysts used by some bacteria to break the chemical bond in atmospheric molecular nitrogen, allowing biological nitrogen fixation. At least 50 molybdenum-containing enzymes are now known in bacteria and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation, and these nitrogenases contain molybdenum in a different form from the rest. Owing to the diverse functions of the various other types of molybdenum enzymes, molybdenum is a required element for life in all higher organisms (eukaryotes), though not in all bacteria.

Occurrence


Molybdenum is the 54th most abundant element in the Earth's crust and the 25th most abundant element in the oceans, with an average of 10 parts per billion; it is the 42nd most abundant element in the Universe. The Russian Luna 24 mission discovered a molybdenum-bearing grain (1 × 0.6 µm) in a pyroxene fragment taken from Mare Crisium on the Moon. The comparative rarity of molybdenum in the Earth's crust is offset by its concentration in a number of water-insoluble ores, often combined with sulfur, in the same way as copper, with which it is often found. Though molybdenum is found in such minerals as wulfenite (PbMoO4) and powellite (CaMoO4), the main commercial source of molybdenum is molybdenite (MoS2). Molybdenum is mined as a principal ore, and is also recovered as a byproduct of copper and tungsten mining.

Historically, the Knaben mine in southern Norway, opened in 1885, was the first dedicated molybdenum mine. It closed from 1973–2007, but is now reopened. Large mines in Colorado (such as the Henderson mine and the Climax mine)[35] and in British Columbia yield molybdenite as their primary product, while many porphyry copper deposits such as the Bingham Canyon Mine in Utah and the Chuquicamata mine in northern Chile produce molybdenum as a byproduct of copper mining.

The world's production of molybdenum was 250,000 tonnes in 2011, the largest producers being China (94,000 t), United States (64,000 t), Chile (38,000 t), Peru (18,000 t) and Mexico (12,000 t). The total reserves are estimated at 10 million tonnes, and are mostly concentrated in China (4.3 mt), US (2.7 mt) and Chile (1.2 mt). By continent, 93% of world molybdenum production is about evenly split between North America, South America (mainly in Chile), and China. Europe and the rest of Asia (mostly Armenia, Russia, Iran and Mongolia) produce the remainder.

Symbol		Mo
Atomic Number		42
Atomic Weight		95.94
Oxidation States		+6
Electronegativity, Pauling		2.16
State at RT		Solid, Metal
Melting Point, K		2890
Boiling Point, K		4885

Appearance and Characteristics

Harmful effects:

Molybdenum is toxic in all but small quantities.

Characteristics:

• Molybdenum is a silvery-white, high-melting metal.
• It does not react with oxygen or water at room temperature and it also resists corrosion at ordinary temperatures.
• When present in compounds, molybdenum exists mostly in the oxidation state IV and VI.
• Molybdenum is one of the five major refractory metals (metals with very high resistance to heat and wear).
• The Five Refractory Metals – note their close relationship in the periodic table

The other refractory metals are tungsten, tantalum, rhenium and niobium.

• Molybdenum oxide (MoO3) is soluble in alkaline water, forming molybdate salts.

Uses of Molybdenum

• Molybdenum is used in small quantities to harden steel and is used in many alloys.
• Molybdenum’s strength and resistance to expanding or softening at high temperatures is particularly sought after in critical areas where high temperatures are common, such as in nuclear power plants and aircraft engines.
• Molybdenum is used as glass furnace electrodes due to its high melting point.
• It is also used in the petroleum industry, to catalyze the removal of organic sulfur compounds in coal liquification and gas liquification processes.
• Molybdenum is an essential trace element for animals and plants. As with selenium, too much of it is toxic, too little of it is fatal.
• In nitrogen fixing bacteria, molybdenum is a vital component of the nitrogenase enzyme which allows conversion of nitrogen gas in air into nitrates vital for plant growth.
• Molybdenum is also present in 20 or so enzymes needed in animals’ metabolisms.