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Cr, a chemical element in Group VI of Mendeleev’s periodic table. Atomic number, 24; atomic weight, 51.996.
A steel-blue metal, chromium occurs in nature in the form of the stable isotopes 50Cr (4.31 percent), 52Cr (87.76 percent), 53Cr (9.55 percent), and 54Cr (2.38 percent). Of the six artificial radioactive chromium isotopes, the most important is 51Cr, with a half-life of 27.8 days, which is used as an isotopic tracer.
History. Chromium was discovered in 1797 by L. N. Vauquelin in the mineral crocoite, which is a natural form of lead chromate, PbCrO4. Its name is derived from the Greek chroma (“color”), because chromium compounds are highly colored. Independently of Vauquelin, the German scientist M. H. Klaproth discovered chromium in crocoite in 1798.
Distribution in nature. The mean content of chromium in the earth’s crust (clarke) is 8.3 × 10–3 percent. Chromium is probably more characteristic of the earth’s mantle, since ultrabasic rocks, which presumably are most similar in composition to the earth’s mantle, are rich in chromium (2 × 10–1 percent). It forms massive and disseminated ores in ultrabasic rocks; the formation of the largest chromium deposits is related to uitrabasic rocks. The chromium content reaches only 2 × 10–2 percent in basic rocks, 2.5 × 10–3 percent in acid rocks, 3.5 × 10–3 percent in sedimentary rocks (sandstones), and 9 × 10–3 percent in shales. Chromium is a rather weak water migrant, and its content in sea-water is 0.00005 mg/liter.
Chromium, on the whole, is a metal of the earth’s plutonic zones. Stony meteorites, whose composition resembles that of the mantle, are also rich in chromium (2.7 × 10–1 percent). More than 20 chromium minerals are known. Only chrome spinellids, which contain up to 54 percent chromium, are of industrial importance. Chromium is also present in several other minerals that often accompany chromium ores but are themselves of little value, for example, uvarovite, volchonskoite, kämmererite, andfuchsite.
A. I. PERELMAN
Physical and chemical properties. Chromium is a solid, heavy, infusible metal. Pure chromium is ductile. Chromium has a body-centered lattice, with a = 2.885 angstroms (Å) at 20°C. Transformation to the face-centered modification, with a = 3.69 Å, is possible at about 1830°C.
The atomic radius of chromium is 1.27 Å, while the ionic radii are 0.83 Å (Cr2+), 0.64 Å (Cr3+), and 0.52 Å (Cr6+). Chromium has a density of 7.19 g/cm3, a melting point of 1890°C, and a boiling point of 2480°C. The specific heat is 0.461 kilojoule/kg·°K, or 0.11 cal/g·°C (at 25°C), the linear coefficient of thermal expansion 8.24 × 10–6 (at 20°C), the coefficient of thermal conductivity is 67 W/m·°K, or 0.16 cal/cm · sec · °C (at 20°C), the specific electrical resistivity is 0.414 μohm · m (at 20°C), and the thermal coefficient of electrical resistivity in the range 20° to 600°C is 3.01 × 10–3. Chromium is an antiferromagnet, with a specific magnetic susceptibility of 3.6 × 10–6. The Brinell hardness of pure chromium is 7–9 meganewtons/m2, or 70–90 kilograms-force/cm2.
The outer electronic configuration of the chromium atom is 3d54s1. In its compounds, chromium usually has an oxidation state of +2, +3, or +6, of which the +3 state is the most stable. A few compounds are known with chromium in the +1, +4, or +5 oxidation state. Chemically, chromium is not very reactive. Under ordinary conditions, it is resistant to oxygen and moisture, although it combines with fluorine, forming CrF3. Above 600°C, it reacts with water vapor to yield Cr2O3, with nitrogen to yield Cr2N and CrN, with carbon to yield Cr23C6, Cr7C3, and Cr3C2, and with sulfur to yield Cr2S3. Upon melting with boron, it forms the boride CrB, and upon melting with silicon, it forms the suicides Cr3Si, Cr2Si3, and CrSi2.
Chromium forms alloys with many metals. Its reaction with oxygen initially proceeds rather vigorously but then becomes sharply inhibited owing to the formation of a metal oxide surface layer. At 1200°C, this layer is decomposed, and oxidation again proceeds rapidly. Chromium burns in oxygen at 2000°C, with the formation of the dark green oxide Cr2O3. In addition to chromic oxide, other compounds of chromium with oxygen are known, for example, CrO and CrO3, which are produced indirectly (for more detailed information seeCHROMIUM OXIDE). Chromium readily reacts with dilute solutions of hydrochloric and sulfuric acids, resulting in the formation of chromium chloride and chromium sulfate, respectively, and the liberation of hydrogen. Aqua regia and nitric acid passivate chromium.
With increasing oxidation state, the acidic and oxidizing properties of chromium increase. Derivatives of Cr2+ are strong reducing agents. The Cr2+ ion forms in the first stage of dissolving chromium in acids or in the reduction of Cr3+ by zinc in acid media. Chromous hydroxide, Cr(OH)2, is converted upon dehydration to Cr2O3. Compounds of Cr3+ are stable in the air and may be either reducing or oxidizing agents. The Cr3+ ion may be reduced in acid media by zinc to Cr2+ or may be oxidized in an alkaline solution by bromine and other oxidizing agents to CrO42–. Chromic hydroxide, Cr(OH)3 (more precisely, Cr2O3 · nH2O), is an amphoteric compound that forms salts with the Cr+ cation or salts of chromous acid, HCrO2, for example, potassium chromite, KCrO2, and sodium chromite, NaCrO2. Compounds of Cr6+ include chromic anhydride, CrO3, and chromic acids and their salts, among which the chromates and dichromates, which are strong oxidizing agents, are the most important. Chromium forms a large number of salts with oxygen-containing acids. Chromium complexes include the especially numerous complexes of Cr3+, in which chromium has a coordination number of 6.
There are also a considerable number of peroxide compounds of chromium.
Production. Depending on its intended use, chromium is produced with varying degrees of purity. Chrome spinellids usually serve as the raw material; they are subjected to concentration and then melted with potash or sodium carbonate in the presence of atmospheric oxygen. The following reaction is applicable to the major component of ores containing Cr3+:
2FeCr2O4 + 4K2CO3 + 3.5O2 = 4K2CrO4 + Fe2O3 + 4CO2
Potassium chromate, K2CrO4, which is formed in this reaction, is leached by hot water and converted to potassium dichromate, K2Cr2O7, by the action of H2SO4. Further action of a concentrated solution of H2SO4 on potassium dichromate yields chromic anhydride, CrO3, while the heating of potassium dichromate with sulfur yields chromic oxide, Cr2O3.
The purest grades of chromium produced industrially are obtained either by the electrolysis of concentrated aqueous solutions of CrO3 or Cr2O3 containing H2SO4 or by the electrolysis of chromic sulfate, Cr2(SO4)3. In this process, chromium is isolated at the aluminum or stainless steel cathode. The complete removal of impurities is achieved by treatment of chromium with high-purity hydrogen at high temperatures (1500°–1700°C).
The production of pure chromium is also possible by the electrolysis of melts of CrF3 or CrCl3 in a mixture with sodium fluoride, potassium fluoride, or calcium fluoride at about 900°C in an argon atmosphere.
In small amounts, chromium is produced by the reduction of Cr2O3 by aluminum or silicon. In aluminothermy, the previously heated charge consisting of Cr2O3, aluminum powder or filings, and an oxidizing agent are placed in a crucible, where the reaction is initiated by ignition of a mixture of Na2O2 and Al until the crucible is filled with chromium and slag. In the silicothermic process, chromium is smelted in arc furnaces. The purity of the chromium produced is measured by the content of impurities in Cr2O3 and in Al or Si, which are used for reduction.
Ferrochrome and ferrochrome silicon are chromium alloys produced in industry on a large scale.
Uses. The use of chromium is based on its heat resistance, hardness, and corrosion resistance. It is primarily used in the production of various chrome steels, such as chromal, chromel, and chromansil. Aluminothermic and silicothermic chromiums are used in the production of nichrome, nimonic, and other nickel alloys and in the production of stellite.
A considerable amount of chromium is used for the production of decorative corrosion-resistant coatings. Chromium powder is widely used in the production of metalloceramic items and materials for welding electrodes. In the form of the Cr3+ ion, chromium is an impurity in rubies, which are used as gems and laser materials. Compounds of chromium are used as mordants in textile dyeing. In the leather industry, some chromium salts are components of tanning solutions. PbCrO4, ZnCrO4, and SrCrO4 are used as paint pigments. Chrome-magnesite refractory materials are made from a mixture of chromite and magnesite.
Chromium compounds, especially compounds of Cr6+, are toxic.
A. B. SUCHKOV
Chromium in organisms. Chromium is a biogenic element and is a component of plant and animal tissues. The average content of chromium in plants is 0.0005 percent, of which 92–95 percent is accumulated in the roots. In animals it varies between 1/10,000 and 1/10,000,000 of 1 percent. In planktons, the accumulation coefficient for chromium is enormous (10,000–26,000), while higher plants do not tolerate chromium concentrations higher than 3 × 10–4 moles/liter. Chromium is present in leaves as a low-molecular-weight complex that is not bonded to subcellular structures. The need of plants for chromium has not been proved. In animals, chromium participates in the metabolism of lipids, proteins (it is a component of the enzyme trypsin), and carbohydrates (it is a structural component of the glucose-stable factor).
The major source of chromium for animals and humans is food. A reduction of the chromium content in food and blood leads to a decrease in the rate of growth, an increase in blood cholesterol, and a reduction in the sensitivity of peripheral tissues to insulin.
M. IA. SSHKOLNIK
Poisoning. Poisoning by chromium is encountered in the production of chromium and chromium compounds, in machine building (electroplating), in metallurgy (alloying additives, alloys, refractory materials), and in the production of leather and paints. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, while Cr(VI) compounds are more toxic than Cr(II) and Cr(III) compounds.
The initial manifestations of chromium poisoning are a sensation of dryness and pain in the nose, a tickling sensation in the throat, difficulty in breathing, and coughing; they may disappear upon cessation of contact with chromium. Upon prolonged contact with chromium compounds, symptoms of chronic poisoning develop, including headache, fatigue, dyspepsia, and weight loss. Disorders of the stomach, liver, and pancreas also occur, as well as bronchitis, bronchial asthma, and diffuse pneumosclerosis. Upon direct contact with the skin, chromium may induce dermatitis or eczema. According to some sources, chromium compounds, primarily Cr(III) compounds, are carcinogenic.
The prevention of chromium poisoning includes periodic medical examination, including examination by an otolaryngologist. Other measures include adequate ventilation in electroplating processes by the installation of side vents in the baths, the use of gloves, and the application of protective salves. Breathing masks and general equipment for dust reduction and removal are used in cases when chromium-containing dust is present.
A. A. KASPAROV
REFERENCESSully, A. H., and E. A. Brandes. Khrom, 2nd ed. Moscow, 1971. (Translated from English.)
Nekrasov, B. V. Osnovy obshchei khimii. Moscow, 1973.
Akhmetov, N. S. Neorganicheskaia khimiia, 2nd ed. Moscow, 1975.
Remy, H. Kurs neorganicheskoi khimii, vols. 1–2. Moscow, 1972–74. (Translated from German.)
Cotton, F., and G. Wilkinson. Sovremennaia neorganicheskaia khimiia, part 3. Moscow, 1969. (Translated from English.)
Grushko, Ia. M. Soedineniia khroma i profilaktika otravlenii imi. Moscow, 1964.
Bowen, H. J. M. Trace Elements in Biochemistry. London-New York, 1966.
ChromiumIntroduced in 2009, two open source software projects are named Chromium: an operating system and a Web browser. Examples are the Chrome OS in Chromebooks and the underlying engine in Chrome, Edge, Brave, Vivaldi and other Web browsers. For more information, visit www.chromium.org.
Chromium Operating System
The Linux-based Chromium OS in Chromebooks is geared for Web applications, and its user interface is essentially the Chromium browser. Various versions have been introduced with names such as Cherry, Zero, Flow and Vanilla Login. In 2012, software that compiles and installs a build for developers on a USB drive was released. See Chromebook and Cloud Ready.
Chromium Web Browser
An advantage to Chromium-based browsers is that they use the same extensions. For example, if Chrome users prefer another browser based on Chromium, they can switch to that browser and download the same extensions they liked in Chrome. Unfortunately, all Chromium browsers do not behave exactly the same, and while most Chrome extensions work, not all do, frustrating users to no end. See browser engine.
Chromium browser source code is available for Windows, Mac and Linux in two modules: the user interface and rendering engine. The engine interacts with the user interface and executes the Web page in a sandbox to prevent illegal system calls. If a Web app crashes, Chromium is able to cancel that operation and keep running. See sandbox.