Atomic number 27

Cobalt

Co
+ phase
− phase

Shells Orbitals 3d 4s

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27 electrons in 4 shells: 2, 8, 15, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 4
Group: 9
Block: d
Standard atomic weight: 58.933(1) Da
Electron configuration: [Ar] 3d⁷ 4s²
Phase at 25 °C: solid
Melting point: 1495 °C
Boiling point: 2927 °C
Density: 8.86 g/cm³
Electronegativity (Pauling): 1.88
Discovered: 1735, Georg Brandt

Neighbours in the table

Fe Ni

Georg Brandt, a Swedish chemist, wanted to prove that the blue some minerals gave glass came from an unknown element, not bismuth. Los Alamos and CIAAW’s note on PubChem’s record date the find to 1735; Jefferson Lab’s text in the same record says 1739. The name comes from the German Kobold, a goblin that miners blamed for their trouble. The ore held arsenic that injured their health and gave no metal by the usual methods.

For cobalt the tally is 27 electrons, split 2, 8, 15 and 2 among shells, [Ar] 3d⁷ 4s². The 4s pair filled before the seven 3d electrons. OpenStax’s reason: a 3d orbital keeps less of its density near the nucleus than 4s, so the inner electrons shield it more and it sits higher in energy. In its ions the pair goes first, and Co²⁺ keeps seven d electrons, Co³⁺ six. Jefferson Lab gives +3 and +2 as the common oxidation states; Los Alamos lists −3 to +5.

Los Alamos calls the metal hard and brittle, like iron and nickel to look at. Over a wide temperature range it is a mixture of two allotropes, two packings of its atoms. The sluggish change between them partly explains the scatter in its reported properties. OpenStax names Co²⁺ among the coloured hydrated ions that stay stable in water.

Where it is found

PubChem’s element record gives 25 milligrams of cobalt in a kilogram of crust and 2 × 10⁻⁵ milligrams in a litre of seawater. Every natural atom is cobalt-59, the one stable isotope. Los Alamos names the minerals cobaltite, smaltite and erythrite; Jefferson Lab writes the first as CoAsS and the last as Co₃(AsO₄)₂, the miners’ arsenic still in the formulas.

Both say the ores lie with those of nickel, silver, lead, copper and iron, and cobalt is usually won as a by-product of refining them. Los Alamos adds meteorites and the Pacific floor near Hawaii, where cobalt-rich deposits may lie at fairly shallow depths. The US Geological Survey puts world reserves at 11 million tons, 6 million of them in Congo (Kinshasa).

The Survey estimates 2024 world mine production at 290,000 metric tons of cobalt content. Congo (Kinshasa) mined 220,000 tons, about 76 per cent, and Indonesia 28,000, about 10 per cent. The United States mined 300 tons, and most of its roughly 1 million tons of resources lie in Minnesota. China leads refining, largely of partly refined cobalt it imports from Congo (Kinshasa) and Indonesia.

Why it matters

In the US Geological Survey’s figures for 2024, superalloys, mainly aircraft gas turbine engines, took 51 per cent of US cobalt consumption. Cemented carbides for cutting tools took 9 per cent. Los Alamos names Stellite, cobalt with chromium and tungsten, for heavy-duty cutting tools and dies; ATSDR adds artificial hip and knee joints. With aluminium, nickel and iron it forms Alnico, an alloy of unusual magnetic strength that goes into powerful permanent magnets.

The Survey calls the lithium-ion battery the largest use worldwide, and China, the leading consumer, puts most of its cobalt there. OpenStax writes the cathode reaction as Li_1−xCoO₂ + xLi⁺ + xe⁻ giving LiCoO₂, the lithium ions and electrons coming from LiC₆ at the anode. In the textbook x is usually 0.5 or less, and the cell gives about 3.7 V. The Survey notes that their cobalt content is being reduced, and cobalt-free substitutes of iron and phosphorus hold a significant share in China.

Cobalt salts have coloured porcelain, glass, pottery, tiles and enamels a permanent blue for centuries; Los Alamos names cobalt the principal ingredient of Thénard’s blue. Cobalt-60, an artificial isotope, is a gamma source used in radiotherapy, as a tracer and, on OpenStax’s page, to find flaws in structural metal. In the body the element is a constituent of vitamin B12.

Properties

Properties of cobalt Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	27
Standard atomic weight	58.933(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1495 °C	°C	Los Alamos: 1495 °C
Boiling point	2927 °C	°C	Los Alamos: 2927 °C
Density	8.86 g/cm³	g/cm³
Electronegativity (Pauling)	1.88
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷
Phase at 25 °C	solid

Safety

Cobalt in a turbine blade or a magnet is a noncombustible, insoluble solid on NIOSH’s card; the hazards PubChem lists concern dust and fume breathed in, and repeated skin contact. The EU classification gives elemental cobalt the signal word Danger: allergic skin reactions, asthma symptoms if inhaled, cancer, damage to fertility, and suspected genetic defects. ECHA’s 2,768 reports add harmful if swallowed on 65.2% and fatal if inhaled on 23.4%. The sheets to read are the PubChem Laboratory Chemical Safety Summary and NIOSH’s card for cobalt metal dust and fume.

Questions

What is cobalt-60 used for?

Cobalt-60 comes from neutron bombardment of cobalt-59. It emits a beta particle and becomes nickel-60, its only decay route on the IAEA's evaluation, with a half-life of 1925.28 days, about 5.27 years. Two gamma rays come with the decay, 1.17 and 1.33 MeV on IUPAC's note in PubChem's record. Those rays treat cancer from outside the body, sterilise syringes, irradiate food and find flaws in metal. Half of any source is gone every 5.27 years.

Does the human body need cobalt?

Only inside vitamin B12, which OpenStax lists among coordination compounds, metal ions bonded to electron-pair donors. The anatomy text lists cobalt as a trace mineral from fish, nuts, leafy greens and whole grains, with no deficiency of its own. B12 is a co-enzyme for DNA synthesis, so new cells, red cells included, depend on it, and poor absorption of it causes pernicious anaemia. Sheep get supplemental cobalt against swayback.

Why do cobalt compounds have such strong colours?

Cobalt's ions keep a partly filled d subshell: seven electrons in Co²⁺, six in Co³⁺. Ligands, the groups bound to the metal, approach along an octahedron's axes and repel those electrons unevenly, splitting the d orbitals in two. The gap often matches a visible photon, so one colour is absorbed and the eye sees its complement. Which colour depends on the ligands and on their arrangement: the isomers of [Co(NH₃)₄Cl₂]NO₃ are violet with the chlorides adjacent, green with them opposite.

Why is cobalt magnetic?

OpenStax's physics text names five ferromagnetic elements: iron, cobalt, nickel, gadolinium and dysprosium. Their atoms form domains, where the atomic dipoles all point one way, held parallel by a quantum-mechanical coupling that room-temperature heat cannot break in the first three. A field turns the domains its way and aligned ones grow; heat cannot misalign a whole domain, so the metal stays magnetised. Los Alamos puts cobalt's permeability, its readiness to carry a magnetic field, at about two thirds of iron's.