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{{About|the chemical element|the web browser|Chromium (web browser)}} {{Use dmy dates|date=November 2018}}

factoids

Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.JOURNAL, Brandes, EA, Greenaway, HT, Stone, HEN, 4221048, 1956, Ductility in Chromium, Nature, 178, 4533, 10.1038/178587a0, 587, 1956Natur.178..587B, free, Chromium metal is valued for its high corrosion resistance and hardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. Stainless steel and chrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, and almost 90% of infrared light.WEB, Coblentz, WW, Stair, R, Reflecting power of beryllium, chromium, and several other metals,nvlpubs.nist.gov/nistpubs/jres/2/jresv2n2p343_A2b.pdf, National Institute of Standards and Technology, NIST Publications, 11 October 2018, The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color,χρῶμα, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus because many chromium compounds are intensely colored.Industrial production of chromium proceeds from chromite ore (mostly FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium.In the United States, trivalent chromium (Cr(III)) ion is considered an essential nutrient in humans for insulin, sugar, and lipid metabolism.WEB,ods.od.nih.gov/factsheets/Chromium-HealthProfessional/#h2, Chromium, Office of Dietary Supplements, US National Institutes of Health, 2016, 26 June 2016, However, in 2014, the European Food Safety Authority, acting for the European Union, concluded that there was insufficient evidence for chromium to be recognized as essential.WEB,www.efsa.europa.eu/en/efsajournal/pub/3845, Scientific Opinion on Dietary Reference Values for chromium, 18 September 2014, 20 March 2018, European Food Safety Authority, While chromium metal and Cr(III) ions are considered non-toxic, hexavalent chromium, Cr(VI), is toxic and carcinogenic. According to the European Chemicals Agency (ECHA), chromium trioxide that is used in industrial electroplating processes is a “substance of very high concern” (SVHC).WEB, Substance Information - ECHA,echa.europa.eu/substance-information/-/substanceinfo/100.014.189, 2022-01-17, echa.europa.eu, en-GB, Abandoned chromium production sites often require environmental cleanup.WEB, EPA, Abandoned Mine Site Characterization and Cleanup Handbook,www.epa.gov/sites/production/files/2015-09/documents/2000_08_pdfs_amscch.pdf, United States Environmental Protection Agency, 8 September 2019, August 2000,

Physical properties

Atomic

Chromium is the fourth transition metal found on the periodic table, and has a ground-state electron configuration of Ar] 3d5 4s1. It is the first element in the periodic table whose configuration violates the Aufbau principle. Exceptions to the principle also occur later in the periodic table for elements such as copper, niobium and molybdenum.WEB, The Nature of X-Ray Photoelectron Spectra,www.casaxps.com/help_manual/XPSInformation/IntroductiontoXPS.htm, CasaXPS, Casa Software Ltd., 2005, 10 March 2019, Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less to metallic bonding, and hence the melting and boiling points and the enthalpy of atomisation of chromium are lower than those of the preceding element vanadium. Chromium(VI) is a strong oxidising agent in contrast to the molybdenum(VI) and tungsten(VI) oxides.Greenwood and Earnshaw, pp. 1004–5

Bulk

(File:Chromium.jpg|thumb|left|Sample of pure chromium metal)Chromium is extremely hard, and is the third hardest element behind carbon (diamond) and boron. Its Mohs hardness is 8.5, which means that it can scratch samples of quartz and topaz, but can be scratched by corundum. Chromium is highly resistant to tarnishing, which makes it useful as a metal that preserves its outermost layer from corroding, unlike other metals such as copper, magnesium, and aluminium.Chromium has a melting point of 1907 Â°C (3465 Â°F), which is relatively low compared to the majority of transition metals. However, it still has the second highest melting point out of all the Period 4 elements, being topped by vanadium by 3 Â°C (5 Â°F) at 1910 Â°C (3470 Â°F). The boiling point of 2671 Â°C (4840 Â°F), however, is comparatively lower, having the fourth lowest boiling point out of the Period 4 transition metals alone behind copper, manganese and zinc.The melting/boiling point of transition metals are usually higher compared to the alkali metals, alkaline earth metals, and nonmetals, which is why the range of elements compared to chromium differed between comparisons The electrical resistivity of chromium at 20 Â°C is 125 nanoohm-meters.Chromium has a high specular reflection in comparison to other transition metals. In infrared, at 425 Î¼m, chromium has a maximum reflectance of about 72%, reducing to a minimum of 62% at 750 Î¼m before rising again to 90% at 4000 Î¼m. When chromium is used in stainless steel alloys and polished, the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel. The explanation on why chromium displays such a high turnout of reflected photon waves in general, especially the 90% in infrared, can be attributed to chromium’s magnetic properties.JOURNAL, Lind, Michael Acton, The infrared reflectivity of chromium and chromium-aluminium alloys,lib.dr.iastate.edu/cgi/viewcontent.cgi?article=6216&context=rtd, Iowa State University Digital Repository, Iowa State University, 1972, 4 November 2018, 1972PhDT........54L, Chromium has unique magnetic properties - chromium is the only elemental solid that shows antiferromagnetic ordering at room temperature and below. Above 38 Â°C, its magnetic ordering becomes paramagnetic. The antiferromagnetic properties, which cause the chromium atoms to temporarily ionize and bond with themselves, are present because the body-centric cubic’s magnetic properties are disproportionate to the lattice periodicity. This is due to the magnetic moments at the cube’s corners and the unequal, but antiparallel, cube centers. From here, the frequency-dependent relative permittivity of chromium, deriving from Maxwell’s equations and chromium’s antiferromagnetism, leaves chromium with a high infrared and visible light reflectance.JOURNAL, Bos, Laurence William, Optical properties of chromium-manganese alloys,lib.dr.iastate.edu/cgi/viewcontent.cgi?article=4816&context=rtd, Iowa State University Digital Repository, Iowa State University, 1969, 4 November 2018, 1969PhDT.......118B,

Passivation

Chromium metal left standing in air is passivated - it forms a thin, protective, surface layer of oxide. This layer has a spinel structure a few atomic layers thick; it is very dense and inhibits the diffusion of oxygen into the underlying metal. In contrast, iron forms a more porous oxide through which oxygen can migrate, causing continued rusting.JOURNAL, The oxidation of alloys, Wallwork, GR, 1976, Reports on Progress in Physics, 39, 401–485, 10.1088/0034-4885/39/5/001, 5, 1976RPPh...39..401W, 250853920, Passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acids. Passivation can be removed with a strong reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids.Chromium, unlike iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.BOOK,books.google.com/books?id=CGMrAAAAYAAJ, High-temperature oxidation-resistant coatings: coatings for protection from oxidation of superalloys, refractory metals, and graphite, National Research Council (U.S.). Committee on Coatings, National Academy of Sciences, 1970, 978-0-309-01769-5,

Isotopes

Naturally occurring chromium is composed of four stable isotopes; 50Cr, 52Cr, 53Cr and 54Cr, with 52Cr being the most abundant (83.789% natural abundance). 50Cr is observationally stable, as it is theoretically capable of decaying to 50Ti via double electron capture with a half-life of no less than 1.3{{e|18}} years. Twenty-five radioisotopes have been characterized, ranging from 42Cr to 70Cr; the most stable radioisotope is 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority less than 1 minute. Chromium also has two metastable nuclear isomers.{{NUBASE2020|ref}} The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.{{NUBASE2020|ref}}53Cr is the radiogenic decay product of 53Mn (half-life 3.74 million years).WEB,www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html, Live Chart of Nuclides, International Atomic Energy Agency - Nuclear Data Section, 18 October 2018, Chromium isotopes are typically collocated (and compounded) with manganese isotopes. This circumstance is useful in isotope geology. Manganese-chromium isotope ratios reinforce the evidence from 26Al and 107Pd concerning the early history of the Solar System. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the Solar System.JOURNAL, Geochimica et Cosmochimica Acta, 63, 23–24, 1999, 4111–4117, 10.1016/S0016-7037(99)00312-9, 53Mn-53Cr evolution of the early solar system, Birck, JL, Rotaru, M, Allegre, C, 1999GeCoA..63.4111B, 53Cr has been posited as a proxy for atmospheric oxygen concentration.JOURNAL, Frei, Robert, Gaucher, Claudio, Poulton, Simon W, Canfield, Don E, 4373201, Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes, Nature, 461, 7261, 250–253, 2009, 19741707, 10.1038/nature08266, 2009Natur.461..250F,

Chemistry and compounds

File:Chromium in water pourbiax diagram.png|thumb|left|The (Pourbaix diagram]] for chromium in pure water, perchloric acid, or sodium hydroxidePuigdomenech, Ignasi Hydra/Medusa Chemical Equilibrium Database and Plotting Software {{webarchive |url=https://web.archive.org/web/20130605034847www.kth.se/che/medusa |date=5 June 2013}} (2004) KTH Royal Institute of Technology)Chromium is a member of group 6, of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.WEB, Clark, Jim, Oxidation states (oxidation numbers),www.chemguide.co.uk/inorganic/redox/oxidnstates.html, Chemguide, 3 October 2018, “>

Common oxidation states {|class“wikitable” style@float:right; margin-left:1em”

! colspan=2|Oxidation statesMost common oxidation states of chromium are in bold. The right column lists a representative compound for each oxidation state.{{Greenwood&Earnshaw2nd}}| −4 (d10)

last1=Theopold	title=Chromium: Organometallic Chemistry	encyclopedia=Encyclopedia of Inorganic and Bioinorganic Chemistry	editor-last=Scott	publisher=John Wiley & Sons, Ltd	doi=10.1002/9781119951438.eibc0042	last2=Kucharczyk |first2=Robin R.}}.

	Na	[Cr(CO)	]}}

	Na	[Cr	(CO)	]}}

	bis(benzene)chromium>{{chem	6	6	2}}

	K	[Cr(CN)	NO]}}

+2 (d4)>	Chromium(II) chloride>{{chem	2}}

+3 (d3) >	Chromium(III) chloride>{{chem	3}}

	K	CrF|6}}

	}}

+6 (d0) >	Potassium chromate>{{chem	2	4}}

Chromium(0)

Many Cr(0) complexes are known. Bis(benzene)chromium and chromium hexacarbonyl are highlights in organochromium chemistry.

Chromium(II)

File:Chromium carbide Cr3C2.JPG|thumb|upright|Chromium(II) carbideChromium(II) carbideChromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air. Water-stable chromium(II) chloride {{chem|CrCl|2}} that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is stable at neutral pH. Some other notable chromium(II) compounds include chromium(II) oxide {{chem|CrO}}, and chromium(II) sulfate {{chem|CrSO|4}}. Many chromium(II) carboxylates are known. The red chromium(II) acetate (Cr2(O2CCH3)4) is somewhat famous. It features a Cr-Cr quadruple bond.BOOK, Cotton, FA, F. Albert Cotton, Walton, RA, Multiple Bonds Between Metal Atoms,archive.org/details/multiplebondsbet0000cott, registration, Oxford University Press, Oxford, 1993, 978-0-19-855649-7,

Chromium(III)

(File:Chromium(III)-chloride-purple-anhydrous-sunlight.jpg|thumb|upright|Anhydrous chromium(III) chloride (CrCl3))A large number of chromium(III) compounds are known, such as chromium(III) nitrate, chromium(III) acetate, and chromium(III) oxide.WEB, Chromium(III) compounds,www.npi.gov.au/resource/chromium-iii-compounds, National Pollutant Inventory, Commonwealth of Australia, 8 November 2018, Chromium(III) can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid, but it can also be formed through the reduction of chromium(VI) by cytochrome c7.JOURNAL, Assfalg, M, Banci, L, Bertini, I, Bruschi, M, Michel, C, Giudici-Orticoni, M, Turano, P, NMR structural characterization of the reduction of chromium(VI) to chromium(III) by cytochrome c7, Protein Data Bank, 31 July 2002, 1LM2, 10.2210/pdb1LM2/pdb,www.rcsb.org/structure/1lm2, 8 November 2018, The {{chem|Cr|3+}} ion has a similar radius (63 pm) to {{chem|Al|3+}} (radius 50 pm), and they can replace each other in some compounds, such as in chrome alum and alum.Chromium(III) tends to form octahedral complexes. Commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds are the pale green [CrCl(H2O)5]Cl2 and violet [Cr(H2O)6]Cl3. If anhydrous violetBOOK, George W., Luther, 2016, Hydrate (Solvate) Isomers, Introduction to Transition Metals,books.google.com/books?id=Fz7hCgAAQBAJ&pg=PA244, Inorganic Chemistry for Geochemistry & Environmental Sciences: Fundamentals & Applications, 244, 978-1-118-85137-1, John Wiley & Sons, 2019-08-07, chromium(III) chloride is dissolved in water, the violet solution turns green after some time as the chloride in the inner coordination sphere is replaced by water. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) salts. A tetrahedral coordination of chromium(III) has been reported for the Cr-centered Keggin anion [α-CrW12O40]5–.JOURNAL, Gumerova, Nadiia I., Roller, Alexander, Giester, Gerald, Krzystek, J., Cano, Joan, Rompel, Annette, 2020-02-19, Incorporation of CrIII into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {CrIIIO4} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties, Journal of the American Chemical Society, 142, 7, 3336–3339, 10.1021/jacs.9b12797, 0002-7863, 7052816, 31967803, Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form {{chem|[Cr(OH)|6|]|3-}}. It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), a stable oxide with a crystal structure identical to that of corundum.

Chromium(VI)

Chromium(VI) compounds are oxidants at low or neutral pH. Chromate anions ({{chem|CrO|4|2-}}) and dichromate (Cr2O72−) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH: Chromium(VI) oxyhalides are known also and include chromyl fluoride (CrO2F2) and chromyl chloride ({{chem|CrO|2|Cl|2}}).BOOK, Walter de Gruyter, 1985, 91–100, 1081–1095, 978-3-11-007511-3, Lehrbuch der Anorganischen Chemie, Holleman, Arnold F, Wiber, Egon, Wiberg, Nils, Chromium, de, However, despite several erroneous claims, chromium hexafluoride (as well as all higher hexahalides) remains unknown, as of 2020.JOURNAL, Seppelt, Konrad, 2015-01-28, Molecular Hexafluorides, Chemical Reviews, en, 115, 2, 1296–1306, 10.1021/cr5001783, 25418862, 0009-2665, (File:Chrom(VI)-oxid.jpg|thumb|right|upright|Chromium(VI) oxide)Sodium chromate is produced industrially by the oxidative roasting of chromite ore with sodium carbonate. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possible.Both the chromate and dichromate anions are strong oxidizing reagents at low pH: They are, however, only moderately oxidizing at high pH: File:Chroman sodný.JPG|thumb|upright|Sodium chromateSodium chromateChromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct {{chem|CrO|5|·OR|2}}.Chromic acid has the hypothetical formula {{chem|H|2|CrO|4}}. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red chromium(VI) oxide {{chem|CrO|3}}, the acid anhydride of chromic acid, is sold industrially as “chromic acid”. It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.

Other oxidation states

{{See also|Organochromium chemistry}}Compounds of chromium(V) are rather rare; the oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF5). This red solid has a melting point of 30 Â°C and a boiling point of 117 Â°C. It can be prepared by treating chromium metal with fluorine at 400 Â°C and 200 bar pressure. The peroxochromate(V) is another example of the +5 oxidation state. Potassium peroxochromate (K3[Cr(O2)4]) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown compound is stable at room temperature but decomposes spontaneously at 150–170 Â°C.THESIS, PhD,dokumentix.ub.uni-siegen.de/opus/volltexte/2006/52/, Preparation, Structure and Vibrational Spectroscopy of Tetraperoxo Complexes of CrV+, VV+, NbV+ and TaV+, 2003, Gentiana, Haxhillazi, University of Siegen, Compounds of chromium(IV) are slightly more common than those of chromium(V). The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides ({{chem|CrX|3}}) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water. Organic compounds containing Cr(IV) state such as chromium tetra t-butoxide are also known.JOURNAL, Thaler, Eric G., Rypdal, Kristin, Haaland, Arne, Caulton, Kenneth G., 1989-06-01, Structure and reactivity of chromium(4+) tert-butoxide, Inorganic Chemistry, 28, 12, 2431–2434, 10.1021/ic00311a035, 0020-1669, Most chromium(I) compounds are obtained solely by oxidation of electron-rich, octahedral chromium(0) complexes. Other chromium(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4) pm) has also been described.JOURNAL, Nguyen, T, Sutton, AD, Brynda, M, Fettinger, JC, Long, GJ, Power, PP, 42853922, Synthesis of a stable compound with fivefold bonding between two chromium(I) centers, Science, 310, 5749, 844–847, 2005, 16179432, 10.1126/science.1116789, 2005Sci...310..844N, free, Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.(File:5-fold chromium.png|thumb|Chromium compound determined experimentally to contain a Cr-Cr quintuple bond)

Occurrence

{{category see also|Chromium minerals}}File:Crocoite from Tasmania.jpg|left|thumb|upright|CrocoiteCrocoiteFile:Chromit 1.jpg|thumb|left|upright|ChromiteChromiteChromium is the 21stBOOK, Nature’s Building Blocks: An A–Z Guide to the Elements, Emsley, John, Oxford University Press, 2001, Oxford, England, UK, 978-0-19-850340-8, Chromium, 495–498,archive.org/details/naturesbuildingb0000emsl/page/495, most abundant element in Earth’s crust with an average concentration of 100 ppm. Chromium compounds are found in the environment from the erosion of chromium-containing rocks, and can be redistributed by volcanic eruptions. Typical background concentrations of chromium in environmental media are: atmosphere

Production

File:Chrom 1.jpg|thumb|left|Piece of chromium produced with aluminothermic reactionaluminothermic reaction(File:Chromium - world production trend.svg|upright=1.4|thumb|World production trend of chromium)File:Chromium zone refined and 1cm3 cube.jpg|left|thumb|Chromium, remelted in a horizontal arc zone-refiner, showing large visible crystal grains]]Approximately 28.8 million metric tons (Mt) of marketable chromite ore was produced in 2013, and converted into 7.5 Mt of ferrochromium.File:World Chromium Production 2002.svg|thumb|upright=1.4|Chromium ore output in 2002