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Chromium

Chemical element, symbol Cr and atomic number 24 / From Wikipedia, the free encyclopedia

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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.[7]

Quick facts: Chromium, Appearance, Standard atomic weight ...
Chromium, 24Cr
Chromium_crystals_and_1cm3_cube.jpg
Chromium
Appearancesilvery metallic
Standard atomic weight Ar°(Cr)
Chromium in the periodic table


Cr

Mo
vanadiumchromiummanganese
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 (near r.t.)7.15 g/cm3
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 states−4, −2, −1, 0, +1, +2, +3, +4, +5, +6 (depending on the oxidation state, an acidic, basic, or amphoteric oxide)
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 = 228.49  pm (at 20 °C)[3]
Speed of sound thin rod5940 m/s (at 20 °C)
Thermal expansion4.9 µm/(m⋅K) (at 25 °C)
Thermal conductivity93.9 W/(m⋅K)
Electrical resistivity125 nΩ⋅m (at 20 °C)
Magnetic orderingantiferromagnetic (rather: SDW)[4]
Molar magnetic susceptibility+280.0×10−6 cm3/mol (273 K)[5]
Young's modulus279 GPa
Shear modulus115 GPa
Bulk modulus160 GPa
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[6] 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
Symbol_category_class.svg Category: Chromium
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Chromium metal 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. 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.[8] The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color,[9] 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.

In the United States, trivalent chromium (Cr(III)) ion is considered an essential nutrient in humans for insulin, sugar, and lipid metabolism.[10] However, in 2014, the European Food Safety Authority, acting for the European Union, concluded that there was insufficient evidence for chromium to be recognized as essential.[11]

While chromium metal and Cr(III) ions are considered non-toxic, hexavalent chromium, Cr(VI), 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).[12]

Abandoned chromium production sites often require environmental cleanup.[13]

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