Misplaced Pages

Revision as of 18:05, 19 September 2006

Template:Elementbox header Template:Elementbox series Template:Elementbox groupperiodblock Template:Elementbox appearance img Template:Elementbox atomicmass gpm Template:Elementbox econfig Template:Elementbox epershell Template:Elementbox section physicalprop Template:Elementbox phase Template:Elementbox density gpcm3nrt Template:Elementbox densityliq gpcm3mp Template:Elementbox meltingpoint Template:Elementbox boilingpoint Template:Elementbox heatfusion kjpmol Template:Elementbox heatvaporiz kjpmol Template:Elementbox heatcapacity jpmolkat25 Template:Elementbox vaporpressure katpa Template:Elementbox section atomicprop Template:Elementbox crystalstruct Template:Elementbox oxistates Template:Elementbox electroneg pauling Template:Elementbox ionizationenergies4 Template:Elementbox atomicradius pm Template:Elementbox atomicradiuscalc pm Template:Elementbox covalentradius pm Template:Elementbox section miscellaneous Template:Elementbox magnetic Template:Elementbox eresist ohmmat20 Template:Elementbox thermalcond wpmkat300k Template:Elementbox thermalexpansion umpmkat25 Template:Elementbox speedofsound rodmpsat20 Template:Elementbox youngsmodulus gpa Template:Elementbox shearmodulus gpa Template:Elementbox bulkmodulus gpa Template:Elementbox poissonratio Template:Elementbox mohshardness Template:Elementbox vickershardness mpa Template:Elementbox brinellhardness mpa Template:Elementbox cas number Template:Elementbox isotopes begin |- ! style="text-align:right;" | Cr | style="text-align:center;" | syn | style="text-align:right;" | > 1.8×10y | εε | style="text-align:right;" | Ti |- ! rowspan="2" style="text-align:right; vertical-align:middle;" | Cr | rowspan="2" style="text-align:center; vertical-align:middle;" | syn | rowspan="2" style="text-align:right; vertical-align:middle;" | 27.7025 d | ε | style="text-align:right;" | V |- | γ | style="text-align:right;" | - Template:Elementbox isotopes stable Template:Elementbox isotopes stable Template:Elementbox isotopes stable Template:Elementbox isotopes end Template:Elementbox footer

Chromium is a chemical element in the periodic table that has the symbol Cr and atomic number 24.

Notable characteristics

Chromium is a steel-gray, lustrous, hard metal that takes a high polish, and has a high melting point.

The most common oxidation states 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 passive with oxygen, i.e. forms a very thin, insoluble, protective oxide which prevents further oxidation of the underlying metal.

this is very boring prep from the gib

History

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 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 paints. 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, Nicolas-Louis 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 gemstones, such as ruby, or emerald.

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.

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

Biological role

Trivalent chromium (Cr(III), or Cr) 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

Chromium is mined as chromite (FeCr₂O₄) 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 dollars.

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.

See also chromium minerals.

Isolation

Chromium is obtained commercially by heating the ore in the presence of aluminium or silicon.

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, 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".

See also chromium compounds.

Chromium and the quintuple bond

Chromium is notable for its ability to form quintuple covalent bonds. 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. 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

${\displaystyle {\rm {Ar-Cr-Cr-Ar}}}$

where ${\displaystyle {\rm {Ar}}}$ is the aryl group ${\displaystyle {\rm {C_{6}H_{3}{\mbox{-}}2,6(C_{6}H_{3}{\mbox{-}}2,6{\mbox{-}}Pr_{2}^{i})_{2}}}}$ (${\displaystyle {\rm {Pr^{i}}}}$ is isopropyl)

Isotopes

Naturally occurring chromium is composed of 3 stable isotopes; Cr, Cr, and Cr with Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being Cr with a half-life of (more than) 1.8x10 years, and 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 states.

Cr is the radiogenic decay product of 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 Al and Pd for the early history of the solar system. Variations in Cr/Cr and Mn/Cr ratios from several meteorites indicate an initial Mn/Mn ratio that suggests Mn-Cr isotope systematics must result from in-situ decay of Mn in differentiated planetary bodies. Hence Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar system.

The isotopes of chromium range in atomic weight from 43 amu (Cr) to 67 amu (Cr). The primary decay mode before the most abundant stable isotope, Cr, is electron capture and the primary mode after is beta decay.

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.

World Health Organization recommended maximum allowable concentration in drinking water for chromium (VI) is 0.05 milligrams per liter.

As chromium compounds were used in dyes and paints 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

• Chromium compounds
• Chromium minerals
• Chromium VI

References

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.

General references

• Los Alamos National Laboratory - Chromium

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

Template:ChemicalSources

• Chemical elements
• Transition metals
• Chromium