Cobalt – facts and properties
- Symbol
- Co
- Atomic number
- 27
- Atomic mass
- 58.933 u
- Period
- 4
- Group
- 9
- Category
- transition metal
- Electron configuration
- [Ar] 3d⁷ 4s²
- Electronegativity
- 1.88
- Atomic radius
- 152 pm
- Valence electrons
- 9
- State at standard conditions
- solid
- Melting point
- 1495 °C
- Boiling point
- 2927 °C
- Density
- 8.86 g/cm³
- Discovered
- 1735
Cobalt, also spelled Kobalt in everyday German, is a bluish-silvery, hard metal with atomic number 27. For thousands of years it has coloured glass and ceramics a deep blue. Today it is mostly found in the batteries of smartphones and electric cars. In the human body it is part of vitamin B12. Because much of the world's cobalt comes from the Democratic Republic of Congo, it is also an example of how chemistry, economics and ethics are linked.
Position in the periodic table
Occurrence and extraction
Cobalt occurs in the Earth's crust only in compounds, and it is much rarer than iron. It usually accompanies copper and nickel ores, for example in cobaltite, a cobalt arsenic sulfide, or in the ores of the copper belt of central Africa. Almost all cobalt is therefore made as a by-product of copper and nickel mining. The Democratic Republic of Congo supplies by far the largest share of world production, and other producers include Indonesia, Russia and Australia.
The name comes from the miners of the Ore Mountains in Germany. They found ores that looked like silver ore but gave no silver when smelted and released poisonous arsenic fumes. They blamed a mischievous mine spirit, the kobold. In 1735 the Swedish chemist Georg Brandt showed that the substance that coloured these ores blue was a new metal, which disproved the kobold.
Today cobalt is recovered from ore concentrates by roasting them or leaching them with acid, then separating the metal salts from the solution. The metal itself is usually obtained at the end by electrolysis or by reducing the oxides.
Properties and reactions
Cobalt is in period 4 and group 9, and its electron configuration is [Ar] 3d⁷ 4s². Its electronegativity is 1.88. The metal has a density of 8.86 g/cm³, melts at 1495 °C and boils at 2927 °C. It is hard and fairly stable in air. Like iron and nickel, cobalt is ferromagnetic, and it keeps its magnetism up to very high temperatures, up to about eleven hundred degrees Celsius, higher than any other element.
Cobalt is a base metal that dissolves slowly in acids. In compounds it appears mainly as cobalt(II) and cobalt(III). Cobalt(II) salts dissolve in water with a pink colour, because a complex with six water molecules forms. Anhydrous cobalt(II) chloride, by contrast, is blue, a change once used in cobalt chloride paper to show moisture. Because of the health risks, other indicators have largely replaced it. Around 1900 Alfred Werner used stable cobalt(III) complexes to work out how complex compounds are built, which won him the Nobel Prize.
Uses in everyday life and technology
The most important use today is rechargeable batteries. Many lithium-ion batteries contain cobalt in the cathode, in phones often as pure lithium cobalt oxide, in car batteries together with nickel and manganese or aluminium.
Cobalt alloys are heat-resistant and wear-resistant. They are found in the hot parts of jet engines, in drills and cutting tools, and in dental and joint implants. Strong permanent magnets such as alnico and samarium-cobalt magnets also contain cobalt. The radioactive isotope cobalt-60 gives off high-energy gamma radiation and is used to treat tumours and to sterilise medical equipment.
For humans, cobalt is essential as part of vitamin B12. At the centre of this molecule sits a cobalt ion. Only micro-organisms make vitamin B12; animals take it up and store it, so it is found mainly in animal foods such as meat, fish, eggs and dairy products, and people on a vegan diet should take a supplement.
Mining in the DR Congo is criticised. Part of the cobalt comes from small, often unregulated mines where, according to human rights organisations, the work is dangerous and children are among the workers. For many families small-scale mining is also the main source of income, so aid organisations favour better working conditions over a boycott. In the EU, battery makers must check where their cobalt comes from. Battery recycling and low-cobalt batteries are meant to reduce this dependence.
Molecules containing this element
- Cobalt(II) oxide CoO
- Cobalt(III) hydroxide Co(OH)3
- Lithium cobalt(III) oxide LiCoO2
Quick question
What is the atomic number of cobalt (Co)?
FAQ
What is cobalt used for?
Mostly for lithium-ion batteries, and also for heat-resistant alloys in jet engines, for magnets, for catalysts and as a pigment for glass and ceramics. The radioactive cobalt-60 is used in medicine to irradiate tumours.
Why is cobalt mining in the DR Congo controversial?
A large share of cobalt comes from the Democratic Republic of Congo. Alongside large industrial mines there are small mines where, according to reports by human rights organisations, people work under dangerous conditions, sometimes including children.
Is cobalt healthy or toxic?
Both, depending on the amount. The body needs cobalt as a component of vitamin B12. Larger amounts are toxic, though, and can damage the heart and the thyroid. Cobalt dust irritates the airways and can trigger asthma, and cobalt is one of the most common contact allergens after nickel. Soluble cobalt salts such as cobalt chloride are classed as probably carcinogenic when inhaled and as toxic to reproduction. You should therefore never experiment with cobalt salts yourself.