Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. Like nickel, cobalt in the Earth's crust is found only in chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.
Identity
- Atomic mass
- 58.933 u
- Category
- Transition metals
- Group
- 9
- Period
- 4
- Block
- d
- Oxidation States
- +2, +3
- Phase (25 °C)
- Solid
- Appearance
- hard lustrous gray metal
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d7
- Short configuration
- [Ar] 3d7 4s2
- Electrons per shell
- 2, 8, 15, 2
- Electronegativity (Pauling)
- 1.88
- Electron affinity
- 63.9 kJ/mol
- 1st ionization energy
- 760 kJ/mol
- Ionization energies
- 760 · 1648 · 3232 · 4950 · 7670… kJ/mol
Physical properties
- Melting point
- 1494.8 °C (1768 K)
- Boiling point
- 2926.8 °C (3200 K)
- Density
- 8.9 g/cm³
- Molar heat capacity
- 24.8 J/(mol·K)
Chemical behavior of cobalt
Cobalt is a group 9 transition metal with symbol Co and atomic number 27. It is a hard, lustrous bluish-gray metal, corrosion resistant and ferromagnetic. It shows mainly +2 and +3 oxidation states, with +2 the most common in simple salts. It is a key element for lithium-ion batteries, magnetic alloys, intense blue pigments and industrial catalysts.
Reactivity
2 Co + 3 Cl2 -> 2 CoCl3
Cobalt reacts with chlorine to yield cobalt(III) chloride, a brown solid that is the most stable compound.
Co + 2 HCl (acid), -> CoCl2 + H2
Cobalt reacts with dilute hydrochloric acid, releasing hydrogen and forming cobalt(II) chloride.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Cobalt(II) chloride | CoCl2 | Pink solid; used in indicators and as a catalyst |
| Cobalt aluminate | Co(AlO2)2 | Intense blue pigment; used in ceramics, glass and dyes |
| Cobalt(III) oxide | Co2O3 | Brown solid; getter and catalyst precursor |
Electron shell distribution
View in the atom builderIsotopes
Natural cobalt consists of a single stable isotope: cobalt-59 (100 %). More than twenty radioisotopes are known, the most important being cobalt-60, with a half-life of 5.27 years. Cobalt-60 emits penetrating gamma rays and is used in radiotherapy sources, industrial radiography and food irradiation.
Occurrence and production
Cobalt is a trace element in the Earth crust (about 25 ppm). It appears in minerals such as Cobaltite, skutterudite and erythrite, usually associated with copper and nickel ores. The main reserves are in the Democratic Republic of the Congo, Australia, Cuba, Zambia and Russia. Most production comes as a by-product of copper and nickel refining.
History and origin of the name
Cobalt was isolated in 1735 by Georg Brandt in Sweden from cobalt minerals used to color glass. Its name comes from the German Kobold (goblin), because miners blamed evil spirits for the toxic fumes and poor metal yields of these ores. It was the first transition metal recognized as a new element in modern times.
Applications and relevance of cobalt
Uses of the element
Its major use is in lithium-ion batteries and in superalloys and hard metals for cutting tools, turbines and wear-resistant parts. It also forms part of samarium-cobalt permanent magnets, used when a stable magnetic field is needed under demanding conditions.
Uses of its compounds
Oxides feed battery cathodes and catalysts; cobalt blue colors ceramics and glass; cobalt-60 sources sterilize medical material and food; vitamin B12 is an essential medicine and supplement.
Health, safety and environment
Biological role
Cobalt is essential in trace amounts as the central atom of vitamin B12, needed for red blood cells and the nervous system. Deficiency causes anemia and neurological disorders. But eating cobalt metal or its salts does not provide B12, whose complex structure is only made by certain microorganisms.
Safety
Compact metal has low toxicity, but cobalt dust and inorganic compounds can irritate skin, eyes and airways, and prolonged high exposure affects lungs and heart. Respiratory protection and gloves are recommended with powders. Cobalt-60 sources require strict radiological control.
Environment
Cobalt does not degrade and can accumulate in soils and sediments near mining areas. Responsible extraction, effluent control and battery recycling are key to limiting dispersion and recovering this strategic metal.
Frequently asked questions about Cobalt
Why is cobalt essential if it can be toxic?
Because life uses it in one specific form: vitamin B12, which carries a single cobalt atom at its center. The body needs only micrograms per day for red blood cells and nerves. Inorganic salts or metal cannot replace the vitamin, and high doses damage heart, thyroid and lungs.
Why does cobalt produce such intense blue pigments?
Some cobalt compounds return a deep stable blue, as in cobalt aluminate (cobalt blue). The color comes from light interacting with cobalt electrons in the crystal. It is prized in ceramics, glass, enamels and art for strength and lightfastness.
Why is cobalt used in industrial catalysts?
Cobalt catalysts speed up conversion of synthesis gas (carbon monoxide plus hydrogen) into liquid hydrocarbons. This Fischer-Tropsch process can make fuels and feedstocks. Performance depends on formulation, temperature and pressure.
Why is cobalt important yet controversial in batteries?
Lithium cobalt oxide and nickel-manganese-cobalt cathodes store much energy per kilogram for mobiles and EVs. But cobalt is scarce, costly and geographically concentrated, raising mining concerns. Hence the push to cut content, recycle and develop lower-cobalt chemistries.
Why is cobalt used in permanent magnets?
Cobalt helps retain magnetism at relatively high temperatures. Samarium-cobalt magnets keep a stable field where other magnets weaken. They are used in sensors, motors, instruments and aerospace equipment, although cost decides whether they beat neodymium magnets.
How is cobalt used in cutting tools?
In hard metals, cobalt binds tungsten carbide particles, holding ceramic grains together and adding toughness. This gives tools that withstand intense machining stress. Proportion and grain size are tuned to balance hardness and fracture resistance.
What is the difference between cobalt-60 and alloy cobalt?
Cobalt-60 is a radioactive isotope emitting gamma radiation, unlike stable cobalt-59 in alloys and batteries. Its radiation is used in controlled sterilization and medical uses. Both are cobalt with different neutron numbers.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.