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Cobalt,  27Co
General properties
Name, symbol Cobalt, Co
Appearance Lustrous gray metal
Cobalt in the periodic table


Atomic number 27
Standard atomic weight (Ar) 58.933194(4)
Group, block 9; d-block
Period period 4
Electron configuration [Ar] 3d7 4s2
per shell
2, 8, 15, 2
Physical properties
Phase Solid
Melting point 1768 K ​(1495 °C, ​​2723 °F)
Boiling point 3200 K ​(2927 °C, ​5301 °F)
Density near r.t. 8.90 g/cm3
when liquid, at  8.86 g/cm3
Heat of fusion 16.06 kJ/mol
Heat of 377 kJ/mol
Molar heat capacity 24.81 J/(mol·K)
Atomic properties
Oxidation states −3, −1, +1, +2, +3, +4, +5 ​(an amphoteric oxide)
Electronegativity Pauling scale: 1.88
energies 1st: 760.4 kJ/mol
2nd: 1648 kJ/mol
3rd: 3232 kJ/mol
Atomic radius empirical: 125 pm
Covalent radius Low spin: 126±3 pm
High spin: 150±7 pm
Crystal structure ​​hexagonal close-packed (hcp)
Speed of sound thin rod 4720 m/s (at 20 °C)
Thermal expansion 13.0 µm/(m·K) (at 25 °C)
Thermal conductivity 100 W/(m·K)
Electrical resistivity 62.4 Ω·m (at 20 °C)
Magnetic ordering Ferromagnetic
Young's modulus 209 GPa
Shear modulus 75 GPa
Bulk modulus 180 GPa
Poisson ratio 0.31
Mohs hardness 5.0
Vickers hardness 1043 MPa
CAS Registry Number 7440-48-4
Discovery Georg Brandt (1732)
· references

Cobalt is a chemical element with symbol Co and atomic number 27.



Cobalt is a weakly reducing metal, resistant to oxidation, by forming a passivating oxide film. At room temperature, cobalt reacts slowly with mineral acids, and very slowly with moist, but not with dry, air. Hot cobalt reacts with oxygen to give cobalt(II,III) oxide, which converts to cobalt monoxide CoO at 900 °C. Cobalt reacts with many nonmetals, such as molten sulfur, boron, phosphorus, arsenic, carbon. It also reacts with halogens, giving cobalt(III) halides. Cobalt however does not react with hydrogen gas or nitrogen gas even at high temperatures. As such, annealed cobalt alloys are sometimes used to store hydrogen.


Cobalt is a hard, lustrous, silver-gray metal. Like iron, nickel and gadolinium (under 20 °C), cobalt is ferromagnetic


Cobalt is sold by various chemical suppliers.

Old Alnico magnets contain cobalt, which can be extracted through various methods.

In European Union, most cobalt compounds are classified as "Substance of very high concern" in the EU and their sale is regulated.


Cobalt metal can be made by reducing cobalt oxides with carbon, hydrogen or aluminium. Very pure cobalt can also be obtained electrolytically.


  • Make cobalt blue
  • Make various colorful cobalt coordination complexes
  • Make lithium cobalt oxide battery



Cobalt and its compounds display toxicity. It is an essential trace element only in minute quantities. The LD50 value for soluble cobalt salts has been estimated to be between 150 and 500 mg/kg. Cobalt is known to cause contact dermatitis.

Cobalt is the active center of coenzymes called cobalamins, the most common example of which is vitamin B12. As such it is an essential trace dietary mineral for all animals.


Cobalt should be stored in closed containers away from corrosive vapors.

Cobalt powder should be kept in sealed containers or bottles, preferably in an oxygen free environment, to prevent oxidation.


Cobalt should be taken to disposal centers.


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