Chromium

This article is about the chemical element. For the web browser, see Chromium (web browser).

Chromium, 24Cr
Chromium
Appearancesilvery metallic
Standard atomic weight Ar°(Cr)
Chromium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
–
↑
Cr
↓
Mo
vanadium ← chromium → manganese
Atomic number (Z)24
Groupgroup 6
Periodperiod 4
Block  d-block
Electron configuration[Ar] 3d5 4s1
Electrons per shell2, 8, 13, 1
Physical properties
Phase at STPsolid
Melting point2180 K ​(1907 °C, ​3465 °F)
Boiling point2944 K ​(2671 °C, ​4840 °F)
Density (at 20° C)7.192 g/cm3 [3]
when liquid (at m.p.)6.3 g/cm3
Heat of fusion21.0 kJ/mol
Heat of vaporization347 kJ/mol
Molar heat capacity23.35 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
Oxidation statescommon: +3, +6
−4,[4] −2,[5] −1,[5] 0,[6] +1,[5] +2,[5] +4,[5] +5[5]
ElectronegativityPauling scale: 1.66
Ionization energies
  • 1st: 652.9 kJ/mol
  • 2nd: 1590.6 kJ/mol
  • 3rd: 2987 kJ/mol
  • (more)
Atomic radiusempirical: 128 pm
Covalent radius139±5 pm
Color lines in a spectral range
Spectral lines of chromium
Other properties
Natural occurrenceprimordial
Crystal structure ​body-centered cubic (bcc) (cI2)
Lattice constant
Body-centered cubic crystal structure for chromium
a = 288.49  pm (at 20 °C)[3]
Thermal expansion4.81×10−6/K (at 20 °C)[3]
Thermal conductivity93.9 W/(m⋅K)
Electrical resistivity125 nΩ⋅m (at 20 °C)
Magnetic orderingantiferromagnetic (rather: SDW)[7]
Molar magnetic susceptibility+280.0×10−6 cm3/mol (273 K)[8]
Young's modulus279 GPa
Shear modulus115 GPa
Bulk modulus160 GPa
Speed of sound thin rod5940 m/s (at 20 °C)
Poisson ratio0.21
Mohs hardness8.5
Vickers hardness1060 MPa
Brinell hardness687–6500 MPa
CAS Number7440-47-3
History
Discovery and first isolationLouis Nicolas Vauquelin (1794, 1797)
Isotopes of chromium
Main isotopes[9] Decay
abun­dance half-life (t1/2) mode pro­duct
50Cr 4.34% stable
51Cr synth 27.7025 d ε 51V
γ –
52Cr 83.8% stable
53Cr 9.50% stable
54Cr 2.37% stable
 Category: Chromium
| references

Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.[10]

Chromium is valued for its high corrosion resistance and hardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel.[11] Stainless steel and chrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, and almost 90% of infrared light.[12] The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color,[13] because many chromium compounds are intensely colored.

Industrial production of chromium proceeds from chromite ore (mostly FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium.

Trivalent chromium (Cr(III)) occurs naturally in many foods and is sold as a dietary supplement, although there is insufficient evidence that dietary chromium provides nutritional benefit to people.[14][15] In 2014, the European Food Safety Authority concluded that research on dietary chromium did not justify it to be recognized as an essential nutrient.[16]

While chromium metal and Cr(III) ions are considered non-toxic, chromate and its derivatives, often called "hexavalent chromium", is toxic and carcinogenic. According to the European Chemicals Agency (ECHA), chromium trioxide that is used in industrial electroplating processes is a "substance of very high concern" (SVHC).[17]

  1. ^ "Standard Atomic Weights: Chromium". CIAAW. 1983.
  2. ^ Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (4 May 2022). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
  3. ^ a b c Arblaster, John W. (2018). Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International. ISBN 978-1-62708-155-9.
  4. ^ Cr(–4) is known in Na4Cr(CO)4; see John E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States". Inorganic Chemistry. 45 (8). doi:10.1021/ic052110i.
  5. ^ a b c d e f Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 28. ISBN 978-0-08-037941-8.
  6. ^ Cr(0) is known in Cr(CO)6; see John E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States". Inorganic Chemistry. 45 (8). doi:10.1021/ic052110i.
  7. ^ Fawcett, Eric (1988). "Spin-density-wave antiferromagnetism in chromium". Reviews of Modern Physics. 60: 209. Bibcode:1988RvMP...60..209F. doi:10.1103/RevModPhys.60.209.
  8. ^ Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
  9. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  10. ^ Brandes, EA; Greenaway, HT; Stone, HEN (1956). "Ductility in Chromium". Nature. 178 (4533): 587. Bibcode:1956Natur.178..587B. doi:10.1038/178587a0. S2CID 4221048.
  11. ^ "Chromium (Cr) | AMERICAN ELEMENTS ®". American Elements: The Materials Science Company. Retrieved 18 June 2024.
  12. ^ Coblentz, WW; Stair, R. "Reflecting power of beryllium, chromium, and several other metals" (PDF). National Institute of Standards and Technology. NIST Publications. Archived (PDF) from the original on 27 April 2020. Retrieved 11 October 2018.
  13. ^ χρῶμα Archived 22 April 2021 at the Wayback Machine, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  14. ^ "Chromium". Office of Dietary Supplements, US National Institutes of Health. 2 June 2022. Retrieved 17 October 2024.
  15. ^ "Chromium". Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis. January 2024. Retrieved 17 October 2024.
  16. ^ "Scientific Opinion on Dietary Reference Values for Chromium". European Food Safety Authority. 18 September 2014. Retrieved 17 October 2024.
  17. ^ "Substance Information – ECHA". echa.europa.eu. Archived from the original on 23 November 2021. Retrieved 17 January 2022.