Chromium

Chromium,  ₂₄Cr

General properties
Appearance	silvery metallic
Standard atomic weight (Ar, standard)	7001519961000000000♠51.9961(6)
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
Group	group 6
Period	period 4
Element category	transition metal
Block	d-block
Electron configuration	[Ar] 3d5 4s1
Electrons per shell	2, 8, 13, 1
Physical properties
Phase at STP	solid
Melting point	2180 K ​(1907 °C, ​3465 °F)
Boiling point	2944 K ​(2671 °C, ​4840 °F)
Density (near r.t.)	7.19 g/cm3
when liquid (at m.p.)	6.3 g/cm3
Heat of fusion	21.0 kJ/mol
Heat of vaporization	347 kJ/mol
Molar heat capacity	23.35 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	6, 5, 4, 3, 2, 1, −1, −2, −4 ​(depending on the oxidation state, an acidic, basic, or amphoteric oxide)
Electronegativity	Pauling scale: 1.66
Ionization energies	1st: 652.9 kJ/mol 2nd: 1590.6 kJ/mol 3rd: 2987 kJ/mol (more)
Atomic radius	empirical: 128 pm
Covalent radius	139±5 pm
Spectral lines
Miscellanea
Crystal structure	​body-centered cubic (bcc)
Speed of sound thin rod	5940 m/s (at 20 °C)
Thermal expansion	4.9 µm/(m·K) (at 25 °C)
Thermal conductivity	93.9 W/(m·K)
Electrical resistivity	125 nΩ·m (at 20 °C)
Magnetic ordering	antiferromagnetic (rather: SDW)
Magnetic susceptibility	+280.0·10−6 cm3/mol (273 K)
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	687–6500 MPa
CAS Number	7440-47-3
History
Discovery and first isolation	Louis Nicolas Vauquelin (1797, 1798)
Main isotopes of chromium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct 50Cr 4.345% stable 51Cr syn 27.7025 d ε 51V γ – 52Cr 83.789% stable 53Cr 9.501% stable 54Cr 2.365% stable
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Chromium is a chemical element with symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard and brittle metal which takes a high polish, resists tarnishing, and has a high melting point. The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, because many chromium compounds are intensely colored.

Ferrochromium alloy is commercially produced from chromite by silicothermic or aluminothermic reactions and chromium metal by roasting and leaching processes followed by reduction with carbon and then aluminium. Chromium metal is of high value 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.

Trivalent chromium (Cr(III)) ion was considered an essential nutrient in trace amounts in humans for insulin, sugar and lipid metabolism. However in 2014, the European Food Safety Authority concluded that lacking evidence of benefits, setting any Adequate Intake is inappropriate.

While chromium metal and Cr(III) ions are not considered toxic, hexavalent chromium (Cr(VI)) is toxic and carcinogenic. Abandoned chromium production sites often require environmental cleanup.