Chromium

Chromium
Name	Chromium
Number	24
Category	transition metal
Appearance	silvery metallic
Standard atomic weight	51.9961(6)
Electron configuration	[Ar] 3d5 4s1
Phase	solid
Melting point	1907 °C
Boiling point	2671 °C
Density	7.19 g/cm3
Oxidation states	−4, −2, −1, 0, +1, +2, +3, +4, +5, +6
Electronegativity	1.66
Atomic radius	128 pm
Crystal structure	body-centered cubic

Chromium. A hard, lustrous transition metal, it is prized for its high corrosion resistance and hardness. The element is primarily extracted from the mineral chromite and is essential in numerous industrial alloys, most notably stainless steel. Its compounds, such as chromium(III) oxide and potassium dichromate, are vital to industries ranging from metallurgy to tanning and pigment production.

Properties

Chromium exhibits a high melting point and exceptional hardness, characteristics typical of many transition metals. In its most stable oxidation state, +3, it forms inert compounds like the green chromium(III) oxide, a key component in refractory materials. The +6 state, found in compounds such as sodium chromate, is a powerful oxidizing agent. The element's electron configuration contributes to its magnetic properties, and it crystallizes in a body-centered cubic structure. Its ability to form a passive, protective oxide layer when exposed to oxygen is fundamental to its use in corrosion-resistant applications.

History

The element was first discovered in 1797 by the French chemist Louis Nicolas Vauquelin while analyzing a red lead mineral from Siberia known as crocoite. He successfully isolated the metal by heating its oxide with charcoal. The name derives from the Ancient Greek word *chrōma*, meaning color, due to the vibrant hues of its compounds. Its industrial significance grew rapidly in the late 19th century, particularly with the development of ferrochromium for steelmaking. The pioneering work of Harry Brearley on stainless steel in Sheffield cemented chromium's role in modern metallurgy.

Production

The primary commercial source is the mineral chromite, mined extensively in nations like South Africa, Kazakhstan, and India. The extraction process typically involves the reduction of chromite in a high-temperature electric arc furnace using coke or silicon to produce ferrochromium. For pure metal, the aluminothermic reaction between chromium(III) oxide and aluminum is employed, a process refined from the Goldschmidt process. Major producers include Glencore and Samancor Chrome, with significant refining operations also located in China and Finland.

Applications

The dominant use is in metallurgy, where it is a crucial alloying element in stainless steel, tool steel, and superalloys used in jet engines. Chrome plating, via electroplating with a solution of chromic acid, provides a decorative and hard-wearing surface on automotive parts and household fixtures. In the chemical industry, potassium dichromate serves as an oxidizing agent in organic synthesis and was historically used in photography and wood preservation. Chromium(III) oxide is a common green pigment in paints, inks, and ceramic glaze, while sodium chromate is used in leather tanning.

Biological role

Trivalent chromium, or chromium(III), is considered an essential trace element in human nutrition, though its precise biochemical role remains under study. It is thought to potentiate the action of insulin and is involved in carbohydrate and lipid metabolism. It is found in foods like broccoli, grape juice, and whole grains. The National Institutes of Health provides dietary reference intakes for it. In contrast, hexavalent chromium, or chromium(VI)], is highly toxic and carcinogenic, with no known biological function. The Environmental Protection Agency strictly regulates its presence in drinking water.

Safety

Occupational exposure to hexavalent chromium compounds, such as those generated in welding or chromate production, poses serious health risks, including lung cancer and dermatitis. This link was famously highlighted in the case against the Pacific Gas and Electric Company in Hinkley, California, as depicted in the film *Erin Brockovich*. Regulatory bodies like the Occupational Safety and Health Administration enforce strict permissible exposure limits. Proper handling, personal protective equipment, and engineering controls like local exhaust ventilation are mandatory in industrial settings. Environmental contamination from industrial waste, such as from chromite ore processing residue, is a significant remediation challenge.

Category:Chemical elements Category:Transition metals