ChemSphere
27Co58.933

Cobalt

Transition metal · Period 4 · Group 9 · d-block

The blue of Ming porcelain and the heart of vitamin B12

Atomic mass
58.933 u
State at 25 °C
Solid
Electronegativity
1.88
Melting point
1,494.9 °C
Boiling point
2,926.9 °C
Configuration
[Ar] 3d7 4s2
Building atom…
27 protons 32 neutrons 27 electrons

Cobalt, explained

Video lesson in production

Bhavya’s explainer for this element will play here.

Overview

Cobalt ([Ar]3d7 4s2) forms compounds mainly in the +2 and +3 oxidation states. Cobalt(II) salts are pink in water, where each ion is surrounded by six water molecules, but turn deep blue when four chloride ions crowd around instead; the change in geometry alters how the d orbitals split and which light is absorbed. Cobalt(III) forms exceptionally stable, slow-reacting complexes, which is why Alfred Werner chose cobalt ammines to work out the theory of coordination compounds that won him the 1913 Nobel Prize.

Cobalt is a hard, silvery, ferromagnetic metal, and it stays magnetic to about 1,115 °C, higher than any other element. Those properties suit it to jet-engine superalloys and powerful samarium–cobalt magnets. Its biggest market today is lithium-ion batteries, where cobalt stabilizes the layered oxide cathodes of phones and many electric cars. About 70 percent of the world's supply comes from the Democratic Republic of the Congo, raising serious concerns about working conditions in small-scale mines.

Discovery

Cobalt compounds colored Egyptian glass and Chinese porcelain blue for thousands of years before anyone knew the element existed. Around 1735 the Swedish chemist Georg Brandt showed that the blue came from a previously unknown metal, not from bismuth as had been assumed. The name comes from the German Kobold, a goblin: miners blamed goblins for ores that looked valuable but gave off poisonous arsenic fumes when smelted and yielded no copper or silver.

Where it’s found
Cobalt is produced mostly as a by-product of copper and nickel mining, above all in the Democratic Republic of the Congo, from minerals such as carrollite, heterogenite and cobaltite.
What it’s used for
  • Lithium-ion battery cathodes
  • Jet-engine superalloys
  • Samarium–cobalt magnets
  • Blue glass and ceramics
  • Cobalt-60 radiotherapy
  • Industrial catalysts
In living things
Humans need cobalt only as part of vitamin B12, which is essential for making red blood cells and maintaining nerves. Ruminants such as sheep need dietary cobalt so that their gut microbes can make B12 for them.
Safety
Inhaled cobalt dust can cause asthma and lung disease, and cobalt metal is classified by IARC as probably carcinogenic to humans. Cobalt-60 is an intense gamma-ray source that must be heavily shielded.

How it compares

Where Cobalt sits among all 118 elements on the trends that shape the table.

  • Electronegativity1.88Pauling

    #40 of 100 elements with data

  • Ionization energy760kJ/mol

    #38 of 104 elements with data

  • Atomic radius135pm

    #59 of 95 elements with data

  • Electron affinity63.9kJ/mol

    #43 of 109 elements with data

  • Density8.9g/cm³

    #48 of 114 elements with data

  • Melting point1,770K

    #31 of 110 elements with data

Density
8.9 g/cm³
Atomic radius
135 pm
Covalent radius
111 pm
Molar heat
24.81 J/(mol·K)
Appearance
hard lustrous gray metal
Discovered by
Georg Brandt

Inside the atom

27 electrons arranged in 4 shells: 2 · 8 · 15 · 2.

Orbital diagram

1s2 2s2 2p6 3s2 3p6 4s2 3d7 · shorthand [Ar] 3d7 4s2

  1. 1s2
  2. 2s2
  3. 2p6
  4. 3s2
  5. 3p6
  6. 4s2
  7. 3d7

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Cobalt bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. The biggest leap, from electron 25 to 26 (×4.3), is where the next electron has to come from a filled shell much closer to the nucleus.

Signature reactions

The chemistry Cobalt is known for.

  1. Complex formation · Equilibrium

    Chloride complex color change

    Adding chloride turns pink octahedral hexaaquacobalt(II) into the blue tetrahedral tetrachlorocobaltate(II) ion. Because the forward reaction is endothermic, heating the solution turns it blue and cooling turns it pink again, a vivid Le Chatelier demonstration.

    [Co(H2O)6]2+(aq)+4Cl−(aq)[CoCl4]2−(aq)+6H2O(l)

    Conditions: concentrated HCl; warming favors the blue form

  2. Decomposition · Hydrate

    Dehydration of cobalt chloride

    Pink cobalt chloride hexahydrate loses its water of crystallization and turns blue. The change reverses in damp air, which is the principle behind cobalt chloride moisture-test paper.

    CoCl2·6H2O(s)CoCl2(s)+6H2O(g)

    Conditions: gentle heating

  3. Nuclear · Neutron activation

    Making cobalt-60

    Natural cobalt, which is entirely cobalt-59, captures a neutron to become radioactive cobalt-60, with a half-life of 5.27 years. Its gamma rays sterilize medical equipment and food and treat cancer.

    59Co+10n60Co

    Conditions: neutron irradiation in a nuclear reactor

Where you’ll meet it

  • Blue and white porcelain

    Blue and white porcelain

    Chinese blue-and-white porcelain, including Ming ware, is painted with cobalt oxide under the glaze.

    Photo: Wikimedia Commons
  • Erythrite

    Erythrite

    Erythrite, a hydrated cobalt arsenate, forms crimson to pink crystals called cobalt bloom.

    Photo: Wikimedia Commons
  • Lithium-ion battery

    Lithium-ion battery

    Many lithium-ion cells use cathodes of lithium cobalt oxide or nickel–manganese–cobalt oxide.

    Photo: Wikimedia Commons
  • Cobalt blue

    Cobalt blue

    Cobalt blue, cobalt(II) aluminate, is a stable, intense pigment prized by painters since the 19th century.

    Photo: Wikimedia Commons

Research

No research on Cobalt yet

When Bhavya publishes a project that studies Cobalt, it will be linked here.

See all research