Chemical element · reference page

27Co

Cobalt

Cobalt is element 27 (Co), located in period 4, group 9 and the d block. At room temperature its reference phase is solid. Its electron configuration is [Ar] 3d7 4s2.

Atomic number27

Standard atomic weight58.933194(3)

Group9

Period4

Blockd

Electron configuration[Ar] 3d7 4s2

Electronic structure

What controls its chemistry

The d subshell participates in the chemistry of cobalt. Transition metals can use electrons of similar energies in bonding, which is why variable oxidation states, coordination compounds and catalytic behaviour are common across the d block.

Its electron configuration, [Ar] 3d7 4s2, should be read together with the oxidation state of the compound: removing electrons changes which d orbitals are occupied and can alter colour, magnetism, bonding and reactivity.

Chemical forms

Compounds, ions and materials

For cobalt, important chemical forms include metals and alloys, oxides, halides and coordination compounds. Variable oxidation states are common because s and d electrons have similar energies, and ligands around the metal ion can strongly alter colour, magnetism and reactivity.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 8.9 g/cm³. The melting point is 1 768 K (1 495 °C). The boiling point is 3 200 K (2 927 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where cobalt is found and used

In practice, cobalt is encountered in batteries, superalloys and pigments.

In practice

Examples of real uses

Batteries

Cobalt is used in selected battery chemistries, electrode materials or electrolytes. The exact role depends on the compound and cell design.

Superalloys

Cobalt is used in alloys engineered to retain mechanical strength and resist oxidation at very high temperatures.

Pigments

Cobalt compounds provide stable colours or optical properties in paints, ceramics, plastics or glass.

Periodic position

What its neighbours tell us

Cobalt is in period 4 and group 9. In the neighbourhood shown here, it lies between iron and nickel in the same period; rhodium is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationtransition metal

Phase at room temperatureSolid

Electron shells2 · 8 · 15 · 2

Density8.9 g/cm³

Melting point1 768 K (1 495 °C)

Boiling point3 200 K (2 927 °C)

Pauling electronegativity1.88

First ionisation energy760.4 kJ/mol

Electron affinity63.9 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of cobalt is associated with Georg Brandt. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 58.933194(3). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Cobalt has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with cobalt depend on chemical form, dose and route of exposure. Pure cobalt, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.