Chromium

Chromium, 24Cr
Chromium
Appearancesilvery metallic
Standard atomic weight Ar°(Cr)
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
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)
Body-centered cubic crystal structure for chromium
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)
Molar magnetic susceptibility+280.0×10−6 cm3/mol (273 K)
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 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 metal

Chromium is a chemical element. Its symbol on the periodic table (a list of all the elements) is Cr. It always has 24 protons, so its atomic number is 24. Its mass number (number of protons and neutrons) is about 54. Most of the times it has 28 neutrons, but different isotopes have more or fewer neutrons. As a metal, it has 24 electrons. Its ions have fewer electrons.

Properties

Chromium is a shiny metal. It is reactive, but it reacts with air to produce a very thin coating of chromium(III) oxide, which prevents it from rusting any more.

Compounds

Chromium occurs in several oxidation states; +2, +3, and +6 are most common. It forms chemical compounds that are colorful.

Trivalent chromium compound

Trivalent chromium

There are two main types of chromium ions, which are found in chromium compounds. One type of chromium ion (Cr3+, trivalent chromium ) is required by our body and by those of other animals. If we do not get enough chromium, certain parts of our body may not work properly. We get most of the chromium we need from our food we eat. Chromium is also put into vitamins to make sure that we get enough. If we get too much of this type of chromium, it can harm our body.

Hexavalent chromium compound

Hexavalent chromium

Another type of chromium (Cr6+ (chromate, hexavalent chromium), in the +6 oxidation state) can cause cancer or make people sick, even though most people do not get exposed to it much. It is found in places that make chromium metal, which is why they need to be cleaned up when they close. Hexavalent chromium can be destroyed by reacting it with reducing agents, turning it into Cr3+ (trivalent chromium). They are strong oxidizing agents.

Chromates

The chromate ion is CrO42-. Chromates are generally yellow.

Dichromates

The dichromate ion is Cr2O72-. Dichromates are red or orange.

Occurrence and preparation

Chromite

Chromium occurs as chromite. Chromite is a mixture of iron(II) oxide and chromium(III) oxide. Its chemical formula is FeCr2O4. It is heated with sodium carbonate to make sodium chromate, iron(III) oxide, and carbon dioxide. The sodium chromate is reacted with sulfuric acid to produce sodium dichromate. The sodium dichromate is reduced by carbon to chromium(III) oxide. The chromium(III) oxide is reacted with aluminum to make chromium metal. The chromates and dichromates are what make chromium production areas toxic.

Uses

Chromium is used in different metal products. Stainless steel is the most common use. It is also used in "chrome plating", where it makes things shiny and stops corrosion. Some chromates were used as pigments, but they are toxic so people do not want to use them anymore.

Safety

Chromium metal is nontoxic. But hexavalent chromium is toxic and carcinogenic, and trivalent chromium is mildly toxic in large quantities.


  1. ↑ "Standard Atomic Weights: Chromium". CIAAW. 1983.
  2. ↑ 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.
  3. ↑ Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
  4. ↑ 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.