Difference between revisions of "Chromium" - New World Encyclopedia
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== Notable characteristics == | == Notable characteristics == | ||
− | Chromium is a [[transition metal]] in period 4 of the [[periodic table]], situated between [[vanadium]] and [[manganese]]. It is placed in group 6 (former group 6B). | + | Chromium is a [[transition metal]] in period 4 of the [[periodic table]], situated between [[vanadium]] and [[manganese]]. It is placed in group 6 (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. | In the presence of oxygen, chromium rapidly produces a thin oxide layer that protects the metal from further reaction with oxygen. | ||
− | Chromium has a | + | Chromium 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. |
+ | |||
+ | === Isotopes === | ||
+ | |||
+ | 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 3 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% [[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 1 minute. This element also has 2 [[meta state]]*s. | ||
+ | |||
+ | 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. | ||
+ | |||
+ | == 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. Sometimes, however, [[sodium dichromate]]* is used for this purpose, based on its higher solubility. | ||
+ | |||
+ | Chrome green is the green oxide of chromium, Cr<sub>2</sub>O<sub>3</sub>, used in enamel painting 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 and the quintuple bond === | ||
+ | |||
+ | 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 (of length 183.51(4) picometers) joining two chromium atoms. | ||
+ | |||
+ | The formula for the compound may be written as | ||
+ | :<math> | ||
+ | \rm Ar-Cr-Cr-Ar | ||
+ | </math> | ||
+ | where <math>\rm Ar</math> is a specific aromatic group. | ||
− | Chromium | + | Chromium currently remains the only element for which quintuple bonds have been observed. |
== Applications == | == Applications == | ||
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* Chromium(VI) oxide (CrO<sub>3</sub>) is used to manufacture [[magnetic tape]], where its higher [[coercivity]] than [[iron]] oxide tapes gives better performance. | * 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. | * In well drilling muds as an anti-corrosive. | ||
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== Biological role == | == Biological role == | ||
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− | + | 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. 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 | + | 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 == |
Revision as of 15:53, 18 August 2006
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General | ||||||||||||||||||||||||||||||||||||||||
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Name, Symbol, Number | chromium, Cr, 24 | |||||||||||||||||||||||||||||||||||||||
Chemical series | transition metals | |||||||||||||||||||||||||||||||||||||||
Group, Period, Block | 6, 4, d | |||||||||||||||||||||||||||||||||||||||
Appearance | silvery metallic | |||||||||||||||||||||||||||||||||||||||
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) | |||||||||||||||||||||||||||||||||||||||
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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 | ||||||||||||||||||||||||||||||||||||||||
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Chromium (chemical symbol Cr, atomic number 24) is a steel-gray, shiny, hard metal that takes a high polish.
- a chemical element in the periodic table
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% 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 4 million tons of ferrochrome (consisting of 70% 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 4 of the periodic table, situated between vanadium and manganese. It is placed in group 6 (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.
Chromium 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 3 stable isotopes: 52Cr, 53Cr, and 54Cr. Of these, 52Cr is the most abundant (83.789% 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 1 minute. This element also has 2 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
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. Sometimes, however, sodium dichromate is used for this purpose, based on its higher solubility.
Chrome green is the green oxide of chromium, Cr2O3, used in enamel painting and glass staining.
Chrome yellow is a brilliant yellow pigment, PbCrO4, used by painters.
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."
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 (of length 183.51(4) picometers) joining two chromium atoms.
The formula for the compound may be written as
where is a specific aromatic group.
Chromium currently remains the only element for which quintuple bonds have been observed.
Applications
Uses of chromium:
- In metallurgy, to impart corrosion resistance and a shiny finish:
- As dyes and paints.
- 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 bricks.
- 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 (CrO3) 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.
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
ReferencesISBN links support NWE through referral fees
Notes
- ↑ 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.
General references
External links
- Case Studies in Environmental Medicine: Chromium Toxicity
- IARC Monograph "Chromium and Chromium compounds"
- International Chromium Development Association
- It's Elemental – The Element Chromium
- National Pollutant Inventory - Chromium (III) compounds fact sheet
- The Merck Manual – Mineral Deficiency and Toxicity
- WebElements.com – Chromium
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