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24 vanadium ← chromium → manganese - ↑ Cr ↓ Mo Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	chromium, Cr, 24
Chemical series	transition metals
Group, Period, Block	6, 4, d
Appearance	silvery metallic
Standard atomic weight	51.9961(6)  g·mol−1
Electron configuration	[Ar] 3d5 4s1
Electrons per shell	2, 8, 13, 1
Physical properties
Phase	solid
Density (near r.t.)	7.15  g·cm−3
Liquid density at m.p.	6.3  g·cm−3
Melting point	2180 K (1907 °C, 3465 °F)
Boiling point	2944 K (2671 °C, 4840 °F)
Heat of fusion	21.0  kJ·mol−1
Heat of vaporization	339.5  kJ·mol−1
Heat capacity	(25 °C) 23.35  J·mol−1·K−1
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−1
	2nd:  1590.6  kJ·mol−1
	3rd:  2987  kJ·mol−1
Atomic radius	140  pm
Atomic radius (calc.)	166  pm
Covalent radius	127  pm
Miscellaneous
Magnetic ordering	AFM (rather: SDW)
Electrical resistivity	(20 °C) 125 n Ω·m
Thermal conductivity	(300 K) 93.9  W·m−1·K−1
Thermal expansion	(25 °C) 4.9  µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 5940 m/s
Young's modulus	279  GPa
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
Selected isotopes
Main article: Isotopes of chromium iso NA half-life DM DE (MeV) DP 50Cr 4.345% > 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
References

Chromium (IPA: /ˈkrəʊmiəm/) is a chemical element in the periodic table that has the symbol Cr and atomic number 24.

Contents 1 Notable characteristics 2 Applications 3 History 4 Biological role 5 Occurrence 6 Isolation 7 Compounds 7.1 Chromium and the quintuple bond 8 Isotopes 9 Precautions 10 See also 11 References 12 External links

Chromium 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.

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 passivated by oxygen, forming a thin protective oxide surface layer which prevents oxidation of the underlying metal.

Uses of chromium:

• In metallurgy, to impart corrosion resistance and a shiny finish :
  • as an alloy constituent, such as in stainless steel in cutlery
  • in chrome plating,
  • in anodized aluminium, literally turning the surface of aluminium into ruby.
• 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.
  • also makes a brilliant yellow for painting
• 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(IV) oxide (CrO₂) 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 medicine, as a dietary supplement or slimming aid, usually as chromium (III) chloride or chromium(III) picolinate.
• Chromium hexacarbonyl (Cr(CO)₆) is used as a gasoline additive.
• Chromium boride (CrB) is used as a high-temperature electrical conductor.
• Chromium (III) sulfate (Cr₂(SO₄)₃) is used as a green pigment in paints, in ceramics, varnishes and inks as well as in chrome plating.

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 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, 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 gemstones, such as ruby, or emerald.

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.

Trivalent chromium (Cr(III), or Cr³⁺) is required in trace amounts for sugar metabolism in humans 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 (http://www.atsdr.cdc.gov/tfacts7.html) but in solution it is well established as a cause of allergic contact dermatitis (ACD).

Recently it was shown, that the popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells. In the United States the dietary guidelines for daily chromium uptake were lowered from 50-200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).^[1]

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.

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

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 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 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.^[2] 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.

The compound had the Lewis structure

Ar − Cr − Cr − Ar

where Ar is the aryl group (Prⁱ is isopropyl)

Naturally occurring chromium is composed of three stable isotopes; ⁵²Cr, ⁵³Cr, and ⁵⁴Cr with ⁵²Cr being the most abundant (83.789% natural abundance). Nineteen radioisotopes 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 states.

⁵³Cr is the radiogenic decay product of ⁵³Mn. 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 ²⁶Al and ¹⁰⁷Pd 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.

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. 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 milligrams 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 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.

• Chromium compounds
• Chromium minerals
• Chromium VI

• Los Alamos National Laboratory - Chromium
• WebElements.com – Chromium

1. ^ John B. Vincent (2004). Recent advances in the nutritional biochemistry of trivalent chromium. Proceedings of the Nutrition Society, 63, 41 - 47.
2. ^ 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 Issue 5749, 310:796-797.

• 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

• Links to external chemical sources