ElementsWhat is cobalt?
Cobalt is a hard, shiny, bluish-grey metal that can be made into a magnet. Its compounds have been used for many centuries to colour glass and pottery deep blue.
What is cobalt used for?
Rechargeable batteries, strong magnets, hard-wearing alloys for jet engines, and blue pigments.
Where the name comes from
From the German word kobold, a goblin. Miners blamed goblins for ores that gave no useful metal.
Key facts
- Symbol
- Co
- Atomic number
- 27
- Atomic mass
- 58.93319 u
- Family
- Transition metal
- Period
- 4
- Group
- 9
- Block
- d
- State at room temperature
- Solid
- Melting point
- 1,495 °C (1,768 K, 2,723 °F)
- Boiling point
- 2,927 °C (3,200 K, 5,300 °F)
- Density
- 8.86 g/cm³
- Electrons per shell
- 2, 8, 15, 2
- Electron configuration
- [Ar]4s2 3d7
- Common oxidation states
- +3, +2
- Electronegativity (Pauling)
- 1.88
- First ionisation energy
- 7.881 eV
- Electron affinity
- 0.661 eV
- Atomic radius (van der Waals)
- 192 pm
- Discovered
- 1735
Place in the periodic table
Cobalt is element number 27. It sits in period 4 (row 4) of the periodic table, in group 9 (column 9), and belongs to the d-block. Just before it comes iron (26), and just after it comes nickel (28). In the same column, rhodium is below it. Elements in one column tend to behave alike.
Family: Transition metals.
Physical properties
At room temperature cobalt is a solid. It melts at 1,495 °C and boils at 2,927 °C, so it stays liquid over a range of 1,432 degrees. Its density is 8.86 grams per cubic centimetre, so a one-litre block would weigh about 8.86 kilograms. By density it ranks 34th among the 96 elements with a measured value.
Inside the atom
An atom of cobalt has 27 protons in its nucleus and, when neutral, 27 electrons around it. The electrons are arranged in 4 shells: 2, 8, 15, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Ar]4s2 3d7.
Its electronegativity on the Pauling scale is 1.88. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 7.881 electronvolts (about 760 kilojoules per mole). One mole of cobalt weighs 58.93319 grams, so a single gram contains about 1.02 × 1022 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains cobalt with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.