molybdenum

molybdenum
Number	42
Name	molybdenum
Category	transition metal
Appearance	gray metallic
Atomic weight	95.95
Electron configuration	[Kr] 4d5 5s1
Phase	solid
Melting point	2896 K
Boiling point	4912 K
Density	10.28 g/cm3
Oxidation states	−4, −2, –1, 0, +1, +2, +3, +4, +5, +6
Electronegativity	2.16
Crystal structure	body-centered cubic

molybdenum. A silvery-white metal of the second transition series, molybdenum is distinguished by its exceptionally high melting point and strength at elevated temperatures. It is primarily obtained as a byproduct of copper and tungsten mining, with major reserves found in the United States, China, and Chile. The element is crucial in metallurgy for creating high-strength alloys and finds essential biological functions as a cofactor in several enzymes across all kingdoms of life.

Properties

Molybdenum is a refractory metal, sharing this category with neighbors like tungsten and niobium. Its high melting point, exceeded only by tantalum and tungsten among naturally occurring elements, makes it stable under extreme thermal stress. The metal has a high modulus of elasticity and maintains its strength at temperatures that would cause most steels to creep. Chemically, it is relatively unreactive at room temperature but oxidizes upon heating, forming various oxides with the most stable being molybdenum trioxide. It exhibits a wide range of oxidation states, with the +6 state being the most common in aqueous chemistry and in important compounds like ammonium molybdate. Its chemistry shows similarities to that of chromium in its group and tungsten in its period.

History

The use of molybdenum compounds, though not recognized as a distinct element, dates to ancient times, with molybdenum sulfide (molybdenite) sometimes confused with and used similarly to graphite for writing. In the late 18th century, Carl Wilhelm Scheele distinguished molybdenite from graphite, identifying it as the sulfide of an unknown element. The metal was first isolated in 1781 by Peter Jacob Hjelm, who reduced the oxide with carbon. For over a century, molybdenum had little commercial use until the demands of World War I for armor plate and the development of high-speed tool steels revealed its potent alloying properties. The establishment of the Climax Molybdenum Company in the early 20th century at the massive deposit in Colorado marked the beginning of its large-scale industrial exploitation.

Occurrence and production

Molybdenum is not found native in nature but primarily in the mineral molybdenite. Significant deposits are often associated with granite intrusions and porphyry copper ore bodies. The largest producers globally are China, the United States, Chile, and Peru, with the Henderson Mine and the Climax Mine in Colorado being historically major sources. Production involves flotation of molybdenite from ore, followed by roasting to create molybdenum trioxide, which is then purified. Ferromolybdenum, a common alloying agent, is produced by reducing the oxide with ferrosilicon in a thermite-like reaction. The metal itself is produced by hydrogen reduction of pure molybdenum trioxide or ammonium molybdate.

Applications

The predominant use of molybdenum is as an alloying agent in steel, cast iron, and superalloys, where it enhances strength, hardness, and resistance to wear and corrosion at high temperatures. These alloys are critical in construction, automotive parts, and gas turbine engines. Molybdenum metal is used in high-temperature furnaces for elements and shields due to its strength and high melting point. Molybdenum disulfide is a common dry lubricant. The element is also vital in the petrochemical industry for catalysts used in hydrodesulfurization to remove sulfur from natural gas and refined fuels. Compounds like ammonium molybdate are used in fertilizers and as reagents in analytical chemistry.

Biological role

Molybdenum is an essential trace element for nearly all organisms. It is a critical component of the molybdenum cofactor, which is the active site in several important enzymes. In nitrogen fixation, the enzyme nitrogenase in bacteria like Rhizobium contains molybdenum (though some variants use vanadium). In humans and other eukaryotes, molybdenum enzymes include xanthine oxidase, which is involved in purine metabolism, sulfite oxidase, which is crucial for metabolizing sulfur-containing amino acids, and aldehyde oxidase. A deficiency, though rare, can cause severe neurological problems and developmental delays, while genetic defects in molybdenum cofactor biosynthesis are serious disorders.

Precautions

Molybdenum in metallic and insoluble compound forms is generally of low toxicity. However, dust or fumes from processing, such as from molybdenum trioxide, can be hazardous if inhaled, causing irritation to the eyes and respiratory tract. Soluble molybdenum compounds exhibit greater toxicity, and excessive intake, though uncommon, can interfere with copper metabolism, particularly in ruminant animals like cattle and sheep, leading to a condition similar to copper deficiency. Occupational exposure limits are set by agencies like the Occupational Safety and Health Administration. Proper industrial ventilation and personal protective equipment are standard precautions during machining or handling of fine powders.

Category:Chemical elements Category:Transition metals Category:Dietary minerals