Difference between revisions of "Chromium" - New World Encyclopedia

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{{Elementbox_header | number=24 | symbol=Cr | name=chromium | left=[[vanadium]] | right=[[manganese]] | above=- | below=[[molybdenum|Mo]] | color1=#ffc0c0 | color2=black }}
 
{{Elementbox_header | number=24 | symbol=Cr | name=chromium | left=[[vanadium]] | right=[[manganese]] | above=- | below=[[molybdenum|Mo]] | color1=#ffc0c0 | color2=black }}
 
{{Elementbox_series | [[transition metal]]s }}
 
{{Elementbox_series | [[transition metal]]s }}
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{{Elementbox_footer | color1=#ffc0c0 | color2=black }}
  
'''Chromium''' is a [[chemical element]] in the [[periodic table]] that has the symbol '''Cr''' and [[atomic number]] 24.
+
'''Chromium''' (chemical symbol '''Cr''', [[atomic number]] 24) is a hard, shiny, steel-gray metal that takes a high polish and does not tarnish. It is therefore used in [[alloy]]s, such as stainless [[steel]], and in chrome plating. The [[human body]] needs trace amounts of trivalent chromium (chromium(III)) for [[sugar]] [[metabolism]], but hexavalent chromium (chromium(VI)) is very toxic.
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{{toc}}
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Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly [[color]]ed and used in paints and pigments. The red color of rubies derives from the presence of chromium. Some compounds, particularly [[potassium]] and sodium dichromate, are oxidizing agents useful for the oxidation of organic compounds and (with sulfuric acid) for cleaning laboratory glassware. In addition, chromium(VI) oxide is used in manufacturing high-performance audiotapes.
  
== Notable characteristics ==
+
== Discovery and etymology ==
Chromium is a steel-gray, lustrous, hard [[metal]] that takes a high polish, and has a high melting point.  
+
 
 +
In 1761, Johann Gottlob Lehmann found an orange-red mineral in the [[Ural Mountains]] and named it "Siberian red lead." Though misidentified as a [[lead]] compound with [[selenium]] and [[iron]] components, the material was in fact ''lead chromate'', with the chemical formula PbCrO<sub>4</sub>. It is now known as the mineral [[crocoite]].
 +
 
 +
In 1770, [[Peter Simon Pallas]] visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a [[pigment]] in [[paint]]s. The use of Siberian red lead as a paint pigment developed rapidly. In addition, a bright [[yellow]] made from crocoite became a fashionable color.
  
The most common [[oxidation state]]s of chromium are +2, +3, and +6, with +3 being the most stable. +1, +4 and +5 are rare. Chromium compounds of oxidation state 6 are powerful oxidants.
+
In 1797, [[Nicolas-Louis Vauquelin]] received samples of crocoite [[ore]]. By mixing crocoite with [[hydrochloric acid]], he was able to produce [[chromium oxide]], with the chemical formula CrO<sub>3</sub>. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious [[gemstone]]s such as [[ruby]] and [[emerald]].
  
Chromium(0) is unstable in oxygen, immediately producing a thin oxide layer that is impermeable to oxygen and protects the metal below.
+
During the 1800s, chromium was primarily used as a component of paints and in [[tanning]] salts. Now its primary use is for metal alloys, accounting for 85 percent of the use of chromium. The remainder is used in the [[chemical industry]] and [[refractory]] and [[foundry]] industries.
  
== Applications ==
+
Chromium was named after the Greek word "chroma" meaning [[color]], because of the many colorful compounds made from it.
Uses of chromium:
+
 
* In [[metallurgy]], to impart corrosion resistance and a shiny finish:
+
== Occurrence and isolation ==
** as an [[alloy]] constituent, such as in [[stainless steel]] in [[cutlery]]
 
** in [[chrome plating]],
 
** in [[anodising|anodized]] [[aluminium]], literally turning the surface of aluminium into ruby.
 
* As dyes and [[paint]]s.
 
** [[Chromium(III) oxide]] is a metal polish known as green rouge.
 
** Chromium salts color glass an [[emerald]] green.
 
** Chromium is what makes a ruby red, and therefore is used in producing synthetic rubies.
 
* As a [[catalyst]].
 
* [[Chromite]] is used to make molds for the firing of [[brick]]s.
 
* Chromium salts are used in the [[tanning]] of [[leather]].
 
* Potassium dichromate is a chemical [[reagent]], used in cleaning [[laboratory glassware]] and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
 
* Chromium(VI) oxide (CrO<sub>3</sub>) is used to manufacture [[magnetic tape]], where its higher [[coercivity]] than [[iron]] oxide tapes gives better performance.
 
* In well drilling muds as an anti-corrosive.
 
  
== History ==
+
Chromium is mined as [[chromite]] (FeCr<sub>2</sub>O<sub>4</sub>) [[ore]]. Roughly half this ore in the world is produced in [[South Africa]]. In addition, [[Kazakhstan]], [[India]], and [[Turkey]] are substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.
In 1761, Johann Gottlob Lehmann found an orange-red mineral in the [[Ural Mountains]] which he named ''Siberian red lead''. Though misidentified as a [[lead]] compound with [[selenium]] and [[iron]] components, the material was in fact ''lead chromate'' with a formula of PbCrO<sub>4</sub>, now known as the mineral [[crocoite]].
 
  
In 1770, [[Peter Simon Pallas]] visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a [[pigment]] in [[paint]]s. The use of Siberian red lead as a paint pigment developed rapidly. A bright [[yellow]] made from crocoite became a  color in [[fashion]].
+
Deposits of native chromium metal are rare, but they have been discovered. The Udachnaya Mine in [[Russia]] produces samples of the native metal. This mine is a [[kimberlite]] pipe rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond.
  
In 1797, [[Nicolas-Louis Vauquelin]] received samples of crocoite [[ore]]. He was able to produce [[chromium oxide]] with a chemical formula of CrO<sub>3</sub>, by mixing crocoite with [[hydrochloric acid]]. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious [[gemstone]]s, such as [[ruby]], or [[emerald]].
+
To isolate the metal commercially, chromite ore is oxidized by reacting it with molten alkali (sodium hydroxide, NaOH). This produces sodium chromate (Na<sub>2</sub>CrO<sub>4</sub>), which is reduced with carbon to chromium(III) oxide (Cr<sub>2</sub>O<sub>3</sub>). The metal is obtained by heating the oxide in the presence of [[aluminum]] or [[silicon]].
  
During the 1800s chromium was primarily used as a component of paints and in [[tanning]] salts but now the primary use in  is for metal alloys and is responsible for 85% of the use of chromium. The remainder is used in the [[chemical industry]] and [[refractory]] and [[foundry]] industries.
+
About 15 million tons of marketable chromite ore were produced in 2000 and converted into roughly four million tons of ferrochrome (consisting of 70 percent chromium alloyed with iron), with an approximate market value of 2.5 billion U.S. dollars.
  
Chromium was named after the Greek word "chroma" meaning [[color]], because of the many colorful compounds made from it.
+
== Notable characteristics ==
  
== Biological role ==
+
Chromium is a [[transition metal]] in period four of the [[periodic table]], situated between [[vanadium]] and [[manganese]]. It is placed in group six (former group 6B). It melts at a temperature of 1,907&deg;C.
Trivalent chromium (Cr(III), or Cr<sup>3+</sup>) is required in trace amounts for [[sugar]] [[metabolism]] in humans, and its deficiency can cause [[chromium deficiency]]. In contrast, [[hexavalent chromium]] is very toxic.
 
  
== Occurrence ==
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In the presence of oxygen, chromium rapidly produces a thin oxide layer that protects the metal from further reaction with oxygen.
Chromium is mined as [[chromite]] (FeCr<sub>2</sub>O<sub>4</sub>) ore.  Roughly half the chromite ore in the world is produced in [[South Africa]].  [[Kazakhstan]], [[India]] and [[Turkey]] are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.
 
  
Approximately 15 million tons of marketable chromite ore were produced in 2000, and converted into approximately 4 million tons of ferro-chrome with an approximate market value of 2.5 billion [[United States dollar]]s.
+
As a transition element, chromium combines with [[oxygen]] and other elements in several different ratios. Thus it forms compounds in which it has a variety of [[oxidation state]]s. Its common oxidation states are +2, +3, and +6, with +3 being the most stable. In addition, the +1, +4, and +5 states have been observed in rare cases. Chromium compounds of oxidation state +6 are powerful oxidants.
  
Though native chromium deposits are rare, some native chromium metal has been discovered. The Udachnaya Mine in [[Russia]] produces samples of the native metal. This mine is a [[kimberlite]] pipe rich in diamonds, and the reducing environment so provided helped produce both elemental chromium and diamond.
+
=== Isotopes ===
  
''See also [[:category:Chromium minerals|chromium minerals]].''
+
The isotopes of chromium range in [[atomic weight]] from 43 atomic mass units (amu) (<sup>43</sup>Cr) to 67 amu (<sup>67</sup>Cr). Naturally occurring chromium is composed of three stable [[isotope]]s: <sup>52</sup>Cr, <sup>53</sup>Cr, and <sup>54</sup>Cr. Of these, <sup>52</sup>Cr is the most abundant (83.789 percent [[natural abundance]]). In addition, 19 [[radioisotope]]s have been characterized, with the most stable being <sup>50</sup>Cr with a [[half-life]] greater than 1.8x10<sup>17</sup> years. The isotope <sup>51</sup>Cr has a half-life of 27.7 days, and all the other [[radioactive]] isotopes have half-lives under 24 hours; the majority of these have half-lives less than one minute. This element also has two [[meta state]]s.
  
== Isolation ==
+
Chromium isotopic contents in the earth are typically combined with [[manganese]] (Mn) isotopic contents and have found application in [[isotope geology]]. The isotope <sup>53</sup>Cr is produced by the radioactive decay of <sup>53</sup>Mn. Isotope ratios for Mn/Cr reinforce other types of evidence for the early history of the [[solar system]]. Variations in <sup>53</sup>Cr/<sup>52</sup>Cr and Mn/Cr ratios from several meteorites provides supporting evidence for the creation of new atomic nuclei immediately before coalescence of the solar system.
Chromium is obtained commercially by heating the [[ore]] in the presence of [[aluminium]] or silicon.
 
  
 
== Compounds ==
 
== Compounds ==
[[Potassium dichromate]] is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. It is used as a saturated solution in concentrated sulphuric acid for washing the apparatus. For this purpose, however, [[sodium dichromate]] is sometimes used because of its higher solubility (20 g/100 ml and 5 g/100 ml).
 
Chrome green is the green oxide of chromium, Cr<sub>2</sub>O<sub>3</sub>, used in [[enamel paint]]ing, and glass staining.
 
Chrome yellow is a brilliant yellow pigment, PbCrO<sub>4</sub>, used by painters.
 
  
[[Chromic acid]] has the hypothetical structure H<sub>2</sub>CrO<sub>4</sub>. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO<sub>3</sub>, the acid [[anhydride]] of chromic acid, is sold industrially as "chromic acid".
+
'''Chromium(III) oxide'''
 +
 
 +
Chromium(III) oxide (Cr<sub>2</sub>O<sub>3</sub>) also known as chromium sesquioxide or chromia, is one of four oxides of chromium. It is manufactured from the mineral chromite, noted above.
 +
Green in color, it is commonly called chrome green when used as a pigment in enamel painting and glass staining. It can dissolve in acids to give chromium(III) [[salt]]s, and in molten alkali to give [[chromite]]s.
 +
 
 +
'''Potassium dichromate'''
 +
 
 +
Potassium dichromate (K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>) is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. It is used as a saturated solution in concentrated sulphuric acid for washing the apparatus. (Sometimes, however, [[sodium dichromate]] is used for this purpose, based on its higher solubility.) In addition, it can drive the oxidation of organic compounds, as in converting a primary [[alcohol]] to an [[aldehyde]] and then to a [[carboxylic acid]].
 +
 
 +
Potassium dichromate is one of the most common culprits in causing chromium [[dermatitis]]. Chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and forearms, which is chronic and difficult to treat. As with other Cr(VI) products, potassium dichromate is [[carcinogen]]ic and should be handled with gloves and appropriate health and safety protection.
 +
 
 +
'''Chromic acid'''
 +
 
 +
Chromic acid has the hypothetical structure H<sub>2</sub>CrO<sub>4</sub>. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO<sub>3</sub>, the acid [[anhydride]] of chromic acid, is sold industrially as "chromic acid."
  
''See also [[:category:Chromium compounds|chromium compounds]].''
+
'''Lead(II) chromate'''
 +
 
 +
Lead(II) chromate (PbCrO<sub>4</sub>) has a vivid yellow [[color]] and is practically insoluble in [[water]]. For this reason, it was once commonly used as a [[paint]] [[pigment]] under the name "chrome yellow."
  
 
=== Chromium and the quintuple bond ===
 
=== Chromium and the quintuple bond ===
Chromium is notable for its ability to form quintuple [[covalent bond]]s.  Writing in ''[[Science (journal)|Science]]'', [[Tailuan Nguyen]], a graduate student working with [[Philip Power]] of the [[University of California, Davis]] describes the synthesis of a compound of chromium(I) and a [[hydrocarbon]] [[Radical (chemistry)|radical]] which was shown via [[X-ray diffraction]] to contain a quintuple bond of length 183.51(4) pm joining the two central chromium atoms.  This was accomplished through the use of an extremely bulky monodentate ligand which through its sheer size, is able to prevent further coordination.  Chromium currently remains the only element for which quintuple bonds have been observed.
 
  
The compound had the [[Lewis structure]]
+
Chromium is notable for its ability to form quintuple [[covalent bond]]s. Writing in the journal ''Science'', [[Tailuan Nguyen]], a graduate student working with [[Philip Power]] of the [[University of California, Davis]], describes the synthesis of a compound of chromium(I) and a [[hydrocarbon]] [[Radical (chemistry)|radical]].<ref>Tailuan Nguyen, Andrew D. Sutton, Marcin Brynda, James C. Fettinger, Gary J. Long, Philip P. Power, (4 November, 2005). "Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers," ''Science'', Volume 310, Issue 5749, pp. 796-797.</ref> This compound was shown (by [[X-ray diffraction]]) to contain a quintuple bond joining two chromium atoms.
 +
 
 +
The formula for the compound may be written as
 
:<math>
 
:<math>
 
\rm Ar-Cr-Cr-Ar
 
\rm Ar-Cr-Cr-Ar
 
</math>
 
</math>
 +
where <math>\rm Ar</math> represents a specific aromatic group.
  
where <math>\rm Ar</math> is the [[aryl]] group <math>\rm C_6H_3\mbox{-}2,6(C_6H_3\mbox{-}2,6\mbox{-}Pr^i_2)_2</math> (<math>\rm Pr^i</math> is [[isopropyl]])
+
Chromium currently remains the only element for which quintuple bonds have been observed.
  
<ref>Tailuan Nguyen,  Andrew D. Sutton,  Marcin Brynda,  James C. Fettinger,  Gary J. Long,  Philip P. Power, ([[4 November]], [[2005]]). "Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers", ''Science'', Volume 310, Issue 5749, pp. 796-797. </ref>
+
== Applications ==
  
== Isotopes ==
+
Chromium and its compounds have a variety of applications, some of which are noted below.
Naturally occurring chromium is composed of 3 stable [[isotope]]s; <sup>52</sup>Cr, <sup>53</sup>Cr, and <sup>54</sup>Cr with <sup>52</sup>Cr being the most abundant (83.789% [[natural abundance]]). 19 [[radioisotope]]s have been characterized with the most stable being <sup>50</sup>Cr with a [[half-life]] of (more than) 1.8x10<sup>17</sup> years, and <sup>51</sup>Cr with a half-life of 27.7 days.  All of the remaining [[radioactive]] isotopes have half-lifes that are less than 24 hours and the majority of these have half lifes that are less than 1 minute. This element also has 2 [[meta state]]s.
 
  
<sup>53</sup>Cr is the [[radiogenic]] decay product of <sup>53</sup>[[manganese|Mn]]. Chromium [[isotope|isotopic]] contents are typically combined with [[manganese]] isotopic contents and have found application in [[isotope geology]]. [[manganese|Mn]]-Cr isotope ratios reinforce the evidence from <sup>26</sup>[[Aluminium|Al]] and <sup>107</sup>[[Palladium|Pd]] for the early history of the [[solar system]]. Variations in <sup>53</sup>Cr/<sup>52</sup>Cr and Mn/Cr ratios from several meteorites indicate an initial <sup>53</sup>Mn/<sup>55</sup>Mn ratio that suggests Mn-Cr isotope systematics must result from in-situ decay of <sup>53</sup>Mn in differentiated planetary bodies. Hence <sup>53</sup>Cr provides additional evidence for [[nucleosynthesis|nucleosynthetic]] processes immediately before coalescence of the solar system.
+
* In [[metallurgy]], chromium imparts corrosion resistance and a shiny finish. It is therefore a constituent in [[alloy]]s, such as in stainless [[steel]] used for [[cutlery]]. It is also used in [[chrome plating]].
 +
* In [[anodising|anodized]] [[aluminum]], chromium imparts a ruby color to the surface of aluminum.
 +
* Chromium(III) oxide is a metal polish known as green rouge. In addition, it is used as a pigment under the name chrome green, as noted above.
 +
* Chromium salts color glass an [[emerald]] green.
 +
* The presence of chromium makes a ruby red. It is therefore is used in producing synthetic rubies.
 +
* Chromium is a [[catalyst]] for various reactions.
 +
* Chromite is used to make molds for the firing of [[brick]]s.
 +
* Chromium salts are used in the [[tanning]] of [[leather]].
 +
* Potassium dichromate is an oxidizing agent used for oxidizing organic compounds such as alcohols and aldehydes, and for cleaning laboratory glassware (mentioned earlier). It is also used as a mordant (fixing agent) for dyeing fabric, and in [[photography]] and photographic [[screen printing]].
 +
* Chromium(VI) oxide (CrO<sub>3</sub>) is used to manufacture [[magnetic tapes]] (such as audiotapes), which give better performance than [[iron]] oxide tapes.
  
The isotopes of chromium range in [[atomic weight]] from 43 [[atomic mass unit|amu]] (<sup>43</sup>Cr) to 67 amu (<sup>67</sup>Cr). The primary [[decay mode]] before the most abundant stable isotope, <sup>52</sup>Cr, is [[electron capture]] and the primary mode after is [[beta decay]].
+
== Biological role ==
 +
 
 +
Trivalent chromium (Cr(III) or Cr<sup>3+</sup>) is required in trace amounts for [[sugar]] [[metabolism]] in humans, and its deficiency can cause [[chromium deficiency]]. By contrast, [[hexavalent chromium]] (Cr(VI)) is very toxic.
  
 
== Precautions ==
 
== Precautions ==
Chromium metal and chromium(III) compounds are not usually considered health hazards, but [[hexavalent chromium]] (chromium VI) compounds can be [[toxic]] if orally ingested or inhaled. The lethal dose of poisonous chromium (VI) compounds is about one half teaspoon of material. Most chromium (VI) compounds are irritating to eyes, skin and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury, unless properly treated. Chromium(VI) is an established human [[carcinogen]].  Exposure to the poisonous chemical hexavalent chromium in drinking water formed the plot of the motion picture [[Erin Brockovich (film)|Erin Brockovich]].
 
  
[[World Health Organization]] recommended [[maximum allowable concentration]] in drinking water for chromium (VI) is 0.05 [[milligram]]s per [[liter]].  
+
Chromium metal and chromium(III) compounds are not usually considered health hazards, but [[hexavalent chromium]] (chromium VI) compounds can be [[toxic]] if orally ingested or inhaled. Most chromium (VI) compounds are irritating to the [[eye]]s, [[skin]], and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury unless properly treated. In addition, chromium(VI) is an established human [[carcinogen]]. The lethal dose of poisonous chromium (VI) compounds is about one-half teaspoon of material. According to recommendations by the [[World Health Organization]], the maximum allowable concentration of chromium (VI) in drinking water is 0.05 milligrams per liter.
  
As chromium compounds were used in [[dye]]s and [[paint]]s and the [[tanning]] of [[leather]], these compounds are often found in soil and [[groundwater]] at abandoned industrial site, now needing [[environmental cleanup]] and [[remediation]] per the treatment of [[brownfield land]]. [[Primer (paint)|Primer paint]] containing hexavalent chromium is still widely used for [[aerospace]] and [[automobile]] refinishing applications.
+
As chromium compounds have been used in [[dye]]s and [[paint]]s and the [[tanning]] of [[leather]], these compounds are often found in soil and [[groundwater]] at abandoned industrial sites that now need [[environmental cleanup]] and [[remediation]]. [[Primer (paint)|Primer paint]] containing hexavalent chromium is still widely used for [[aerospace]] and [[automobile]] refinishing applications.
  
 
== See also ==
 
== See also ==
Line 140: Line 152:
 
* [[Chromium VI]]
 
* [[Chromium VI]]
  
== References ==
 
 
===Notes===
 
===Notes===
 
<references/>
 
<references/>
  
===General references===
+
== References ==
* [http://periodic.lanl.gov/elements/24.html Los Alamos National Laboratory - Chromium]
+
* [http://periodic.lanl.gov/elements/24.html Los Alamos National Laboratory - Chromium] Retrieved January 14, 2008.
  
 
== External links ==
 
== External links ==
{{Commons|Chromium}}
+
All links retrieved December 10, 2023.
{{wiktionary|chromium}}
+
 
* [http://www.atsdr.cdc.gov/HEC/CSEM/chromium/ Case Studies in Environmental Medicine: Chromium Toxicity]
+
* [https://www.atsdr.cdc.gov/csem/csem.asp?csem=10&po=0 Chromium Toxicity]
* [http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
 
* [http://www.chromium-asoc.com/ International Chromium Development Association]
 
 
* [http://education.jlab.org/itselemental/ele024.html It's Elemental &ndash; The Element Chromium]
 
* [http://education.jlab.org/itselemental/ele024.html It's Elemental &ndash; The Element Chromium]
* [http://www.npi.gov.au/database/substance-info/profiles/24.html National Pollutant Inventory - Chromium (III) compounds fact sheet]
 
* [http://www.merck.com/pubs/mmanual/section1/chapter4/4f.htm The Merck Manual &ndash; Mineral Deficiency and Toxicity]
 
 
* [http://www.webelements.com/webelements/elements/text/Cr/index.html WebElements.com &ndash; Chromium]
 
* [http://www.webelements.com/webelements/elements/text/Cr/index.html WebElements.com &ndash; Chromium]
  
{{ChemicalSources}}
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[[Category:Physical sciences]]
 
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[[Category:Chemistry]]
 
[[Category:Chemical elements]]
 
[[Category:Chemical elements]]
[[Category:Transition metals]]
 
[[Category:Chromium]]
 
<!-- interwiki —>
 
  
[[af:Chroom]]
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{{credit4|Chromium|69969387|Chromium(III)_oxide|60647184|Potassium_dichromate|68190548|Lead(II)_chromate|66951741}}
[[ar:كروم]]
 
[[ca:Crom]]
 
[[cs:Chróm]]
 
[[da:Krom]]
 
[[de:Chrom]]
 
[[et:Kroom]]
 
[[es:Cromo]]
 
[[eo:Kromo]]
 
[[fa:کروم]]
 
[[fr:Chrome]]
 
[[gl:Cromo (elemento)]]
 
[[ko:크로뮴]]
 
[[hr:Krom]]
 
[[io:Kromio]]
 
[[id:Kromium]]
 
[[is:Króm]]
 
[[it:Cromo]]
 
[[he:כרום]]
 
[[ku:Krom]]
 
[[la:Chromium]]
 
[[lv:Hroms]]
 
[[lt:Chromas]]
 
[[hu:Króm]]
 
[[mi:Konukita]]
 
[[nl:Chroom]]
 
[[ja:クロム]]
 
[[no:Krom]]
 
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[[oc:Cròm]]
 
[[ug:خروم]]
 
[[pl:Chrom]]
 
[[pt:Cromo]]
 
[[ru:Хром]]
 
[[sh:Hrom]]
 
[[simple:Chromium]]
 
[[sk:Chróm]]
 
[[sl:Krom]]
 
[[sr:Хром]]
 
[[fi:Kromi]]
 
[[sv:Krom]]
 
[[ta:நீலிறும்பு]]
 
[[th:โครเมียม]]
 
[[tr:Krom]]
 
[[uk:Хром]]
 
[[zh:铬]]
 

Latest revision as of 21:52, 10 December 2023

24 vanadiumchromiummanganese
-

Cr

Mo
Cr-TableImage.png
periodic table
General
Name, Symbol, Number chromium, Cr, 24
Chemical series transition metals
Group, Period, Block 6, 4, d
Appearance silvery metallic
Cr,24.jpg
Atomic mass 51.9961(6) g/mol
Electron configuration [Ar] 3d5 4s1
Electrons per shell 2, 8, 13, 1
Physical properties
Phase solid
Density (near r.t.) 7.15 g/cm³
Liquid density at m.p. 6.3 g/cm³
Melting point 2180 K
(1907 °C, 3465 °F)
Boiling point 2944 K
(2671 °C, 4840 °F)
Heat of fusion 21.0 kJ/mol
Heat of vaporization 339.5 kJ/mol
Heat capacity (25 °C) 23.35 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 1656 1807 1991 2223 2530 2942
Atomic properties
Crystal structure cubic body centered
Oxidation states 6, 4, 3, 2
(strongly acidic oxide)
Electronegativity 1.66 (Pauling scale)
Ionization energies
(more)
1st: 652.9 kJ/mol
2nd: 1590.6 kJ/mol
3rd: 2987 kJ/mol
Atomic radius 140 pm
Atomic radius (calc.) 166 pm
Covalent radius 127 pm
Miscellaneous
Magnetic ordering AFM (rather: SDW)
Electrical resistivity (20 °C) 125 nΩ·m
Thermal conductivity (300 K) 93.9 W/(m·K)
Thermal expansion (25 °C) 4.9 µm/(m·K)
Speed of sound (thin rod) (20 °C) 5940 m/s
Speed of sound (thin rod) (r.t.) 279 m/s
Shear modulus 115 GPa
Bulk modulus 160 GPa
Poisson ratio 0.21
Mohs hardness 8.5
Vickers hardness 1060 MPa
Brinell hardness 1120 MPa
CAS registry number 7440-47-3
Notable isotopes
Main article: Isotopes of chromium
iso NA half-life DM DE (MeV) DP
50Cr syn > 1.8×1017y εε - 50Ti
51Cr syn 27.7025 d ε - 51V
γ 0.320 -
52Cr 83.789% Cr is stable with 28 neutrons
53Cr 9.501% Cr is stable with 29 neutrons
54Cr 2.365% Cr is stable with 30 neutrons

Chromium (chemical symbol Cr, atomic number 24) is a hard, shiny, steel-gray metal that takes a high polish and does not tarnish. It is therefore used in alloys, such as stainless steel, and in chrome plating. The human body needs trace amounts of trivalent chromium (chromium(III)) for sugar metabolism, but hexavalent chromium (chromium(VI)) is very toxic.

Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of rubies derives from the presence of chromium. Some compounds, particularly potassium and sodium dichromate, are oxidizing agents useful for the oxidation of organic compounds and (with sulfuric acid) for cleaning laboratory glassware. In addition, chromium(VI) oxide is used in manufacturing high-performance audiotapes.

Discovery and etymology

In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it "Siberian red lead." Though misidentified as a lead compound with selenium and iron components, the material was in fact lead chromate, with the chemical formula PbCrO4. It is now known as the mineral crocoite.

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. In addition, a bright yellow made from crocoite became a fashionable color.

In 1797, Nicolas-Louis Vauquelin received samples of crocoite ore. By mixing crocoite with hydrochloric acid, he was able to produce chromium oxide, with the chemical formula CrO3. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gemstones such as ruby and emerald.

During the 1800s, chromium was primarily used as a component of paints and in tanning salts. Now its primary use is for metal alloys, accounting for 85 percent of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.

Chromium was named after the Greek word "chroma" meaning color, because of the many colorful compounds made from it.

Occurrence and isolation

Chromium is mined as chromite (FeCr2O4) ore. Roughly half this ore in the world is produced in South Africa. In addition, Kazakhstan, India, and Turkey are substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they have been discovered. The Udachnaya Mine in Russia produces samples of the native metal. This mine is a kimberlite pipe rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond.

To isolate the metal commercially, chromite ore is oxidized by reacting it with molten alkali (sodium hydroxide, NaOH). This produces sodium chromate (Na2CrO4), which is reduced with carbon to chromium(III) oxide (Cr2O3). The metal is obtained by heating the oxide in the presence of aluminum or silicon.

About 15 million tons of marketable chromite ore were produced in 2000 and converted into roughly four million tons of ferrochrome (consisting of 70 percent chromium alloyed with iron), with an approximate market value of 2.5 billion U.S. dollars.

Notable characteristics

Chromium is a transition metal in period four of the periodic table, situated between vanadium and manganese. It is placed in group six (former group 6B). It melts at a temperature of 1,907°C.

In the presence of oxygen, chromium rapidly produces a thin oxide layer that protects the metal from further reaction with oxygen.

As a transition element, chromium combines with oxygen and other elements in several different ratios. Thus it forms compounds in which it has a variety of oxidation states. Its common oxidation states are +2, +3, and +6, with +3 being the most stable. In addition, the +1, +4, and +5 states have been observed in rare cases. Chromium compounds of oxidation state +6 are powerful oxidants.

Isotopes

The isotopes of chromium range in atomic weight from 43 atomic mass units (amu) (43Cr) to 67 amu (67Cr). Naturally occurring chromium is composed of three stable isotopes: 52Cr, 53Cr, and 54Cr. Of these, 52Cr is the most abundant (83.789 percent natural abundance). In addition, 19 radioisotopes have been characterized, with the most stable being 50Cr with a half-life greater than 1.8x1017 years. The isotope 51Cr has a half-life of 27.7 days, and all the other radioactive isotopes have half-lives under 24 hours; the majority of these have half-lives less than one minute. This element also has two meta states.

Chromium isotopic contents in the earth are typically combined with manganese (Mn) isotopic contents and have found application in isotope geology. The isotope 53Cr is produced by the radioactive decay of 53Mn. Isotope ratios for Mn/Cr reinforce other types of evidence for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites provides supporting evidence for the creation of new atomic nuclei immediately before coalescence of the solar system.

Compounds

Chromium(III) oxide

Chromium(III) oxide (Cr2O3) also known as chromium sesquioxide or chromia, is one of four oxides of chromium. It is manufactured from the mineral chromite, noted above. Green in color, it is commonly called chrome green when used as a pigment in enamel painting and glass staining. It can dissolve in acids to give chromium(III) salts, and in molten alkali to give chromites.

Potassium dichromate

Potassium dichromate (K2Cr2O7) is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. It is used as a saturated solution in concentrated sulphuric acid for washing the apparatus. (Sometimes, however, sodium dichromate is used for this purpose, based on its higher solubility.) In addition, it can drive the oxidation of organic compounds, as in converting a primary alcohol to an aldehyde and then to a carboxylic acid.

Potassium dichromate is one of the most common culprits in causing chromium dermatitis. Chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and forearms, which is chronic and difficult to treat. As with other Cr(VI) products, potassium dichromate is carcinogenic and should be handled with gloves and appropriate health and safety protection.

Chromic acid

Chromic acid has the hypothetical structure H2CrO4. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid."

Lead(II) chromate

Lead(II) chromate (PbCrO4) has a vivid yellow color and is practically insoluble in water. For this reason, it was once commonly used as a paint pigment under the name "chrome yellow."

Chromium and the quintuple bond

Chromium is notable for its ability to form quintuple covalent bonds. Writing in the journal Science, Tailuan Nguyen, a graduate student working with Philip Power of the University of California, Davis, describes the synthesis of a compound of chromium(I) and a hydrocarbon radical.[1] This compound was shown (by X-ray diffraction) to contain a quintuple bond joining two chromium atoms.

The formula for the compound may be written as

where represents a specific aromatic group.

Chromium currently remains the only element for which quintuple bonds have been observed.

Applications

Chromium and its compounds have a variety of applications, some of which are noted below.

  • In metallurgy, chromium imparts corrosion resistance and a shiny finish. It is therefore a constituent in alloys, such as in stainless steel used for cutlery. It is also used in chrome plating.
  • In anodized aluminum, chromium imparts a ruby color to the surface of aluminum.
  • Chromium(III) oxide is a metal polish known as green rouge. In addition, it is used as a pigment under the name chrome green, as noted above.
  • Chromium salts color glass an emerald green.
  • The presence of chromium makes a ruby red. It is therefore is used in producing synthetic rubies.
  • Chromium is a catalyst for various reactions.
  • Chromite is used to make molds for the firing of bricks.
  • Chromium salts are used in the tanning of leather.
  • Potassium dichromate is an oxidizing agent used for oxidizing organic compounds such as alcohols and aldehydes, and for cleaning laboratory glassware (mentioned earlier). It is also used as a mordant (fixing agent) for dyeing fabric, and in photography and photographic screen printing.
  • Chromium(VI) oxide (CrO3) is used to manufacture magnetic tapes (such as audiotapes), which give better performance than iron oxide tapes.

Biological role

Trivalent chromium (Cr(III) or Cr3+) is required in trace amounts for sugar metabolism in humans, and its deficiency can cause chromium deficiency. By contrast, hexavalent chromium (Cr(VI)) is very toxic.

Precautions

Chromium metal and chromium(III) compounds are not usually considered health hazards, but hexavalent chromium (chromium VI) compounds can be toxic if orally ingested or inhaled. Most chromium (VI) compounds are irritating to the eyes, skin, and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury unless properly treated. In addition, chromium(VI) is an established human carcinogen. The lethal dose of poisonous chromium (VI) compounds is about one-half teaspoon of material. According to recommendations by the World Health Organization, the maximum allowable concentration of chromium (VI) in drinking water is 0.05 milligrams per liter.

As chromium compounds have been used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites that now need environmental cleanup and remediation. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.

See also

  • Chromium compounds
  • Chromium minerals
  • Chromium VI

Notes

  1. Tailuan Nguyen, Andrew D. Sutton, Marcin Brynda, James C. Fettinger, Gary J. Long, Philip P. Power, (4 November, 2005). "Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers," Science, Volume 310, Issue 5749, pp. 796-797.

References
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External links

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