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Chromium ( in the Periodic Table that has the symbol '''Cr''' and Atomic Number 24. It is a steel-gray, lustrous, hard Metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable. HISTORY On 26th July 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 PbCrO4, 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 . In 1797, Louis Nicolas Vauquelin received samples of crocoite Ore . He was able to produce Chromium Oxide with a chemical formula of CrO3, 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 . Later that year he successfully isolated chromium atoms. During the 1800s chromium was primarily used as a component of paints and in Tanning salts but now metal alloys account 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 PRODUCTION Chromium is mined as Chromite (FeCr2O4) ore. About two-fifths of the chromite ores and concentrates in the world are produced in South Africa . Kazakhstan , India , Russia 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. Though native chromium deposits are rare, some native chromium metal has been discovered. The Udachnaya Mine in ) Chromium is obtained commercially by heating the Ore in the presence of Aluminium or Silicon . CHEMICAL PROPERTIES Chromium is a member of the Transition Metal s, in group 6. Chromium(0) has an electronic configuration of 4s13d5, due to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states. 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. Chromium is Passivated by oxygen, forming a thin protective oxide surface layer which prevents oxidation of the underlying metal. This makes chromium suitable for a plating material. COMPOUNDS ''See also .'' Potassium Dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any trace organics. It is used as a saturated solution in concentrated sulfuric acid for washing the apparatus. For this purpose, however, Sodium Dichromate is sometimes used because of its higher solubility (5 g/100 ml vs. 20 g/100 ml respectively). Chrome Green is the green oxide of chromium, Cr2O3, used in Enamel Paint ing, 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 Bond s. Writing in '' 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 which was shown via X-ray Diffraction to contain a Quintuple Bond of length 183.51(4) pm (1.835 angstroms) joining the two central chromium atoms.1 This was accomplished through the use of an extremely bulky monodentate ligand which through its sheer size prevents further coordination. Chromium currently remains the only element for which quintuple bonds have been observed. APPLICATIONS Uses of chromium:
BIOLOGICAL ROLE Trivalent chromium (Cr(III), or Cr3+) is required in trace amounts for Sugar Metabolism in humans ( Glucose Tolerance Factor ) and its deficiency may cause a disease called Chromium Deficiency . In contrast, Hexavalent Chromium is very toxic and mutagenic when inhaled as popularized by the film Erin Brockovich . Cr(VI) has not been established as a carcinogen when not inhaled but in solution it is well established as a cause of allergic Contact Dermatitis (ACD).2 ISOTOPES See Also: Isotopes of chromium Naturally occurring chromium is composed of three stable Isotope s; 52Cr, 53Cr, and 54Cr with 52Cr being the most abundant (83.789% Natural Abundance ). Nineteen Radioisotope s have been characterized with the most stable being 50Cr with a Half-life of (more than) 1.8x1017 years, and 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 of these have half-lives that are less than 1 minute. This element also has 2 Meta State s. 53Cr is the Radiogenic decay product of 53 Mn . Chromium Isotopic contents are typically combined with Manganese isotopic contents and have found application in Isotope Geology . Mn -Cr isotope ratios reinforce the evidence from 26 Al and 107 Pd for 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 isotope systematics 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. The isotopes of chromium range in Atomic Weight from 43 U (43Cr) to 67 u (67Cr). The primary Decay Mode before the most abundant stable isotope, 52Cr, is Electron Capture and the primary mode after is Beta Decay . PRECAUTIONS Chromium metal and chromium(III) compounds are not usually considered health hazards; chromium is an essential trace mineral.4 However, 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 . An investigation into hexavalent chromium release into drinking water formed the plot of the motion picture Erin Brockovich . World Health Organization recommended Maximum Allowable Concentration in drinking water for chromium (VI) is 0.05 Milligram s per Liter . Hexavalent chromium is also one of the substances whose use is restricted by the European Restriction Of Hazardous Substances Directive . 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 containing hexavalent chromium is still widely used for Aerospace and Automobile refinishing applications. SEE ALSO REFERENCES EXTERNAL LINKS
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