Transition metalElement 27 · d-block

Cobalt (Co)

Cobalt (Co) is a transition metal with atomic number 27 and a standard atomic weight of 58.933. At room temperature it is a solid that melts at 1,495 °C and boils at 2,927 °C.

27CoCobalt58.933

Atomic number

protons

Atomic weight

58.933

CIAAW 2024

Melting point

1,495 °C

1,768.15 K

Boiling point

2,927 °C

3,200.15 K

Density

8.86 g/cm³

near room temperature

Electronegativity

1.88

Pauling scale

State at 25 °C

Solid

Discovered

1735

Sweden

Cobalt at a glance

Identity

Symbol	Co
Atomic number (Z)	27
Atomic weight	58.933 ± 0.001 (CIAAW 2024)
Category	Transition metal
Group	9
Period	4
Block	d-block
CAS number	7440-48-4

Atomic properties

Electron configuration	[Ar] 3d⁷ 4s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²
Electrons per shell	2, 8, 15, 2
Oxidation states	+2, +3 Less common: -3, -1, 0, +1, +4, +5
Electronegativity (Pauling)	1.88
First ionisation energy	7.881 eV (760.4 kJ/mol) 2nd: 17.084 eV · 3rd: 33.5 eV
Electron affinity	0.662 eV
Atomic radius (empirical)	135 pm
Covalent radius	111 pm
Van der Waals radius	200 pm

Physical properties

State at 25 °C	Solid
Melting point	1,495 °C · 2,723 °F · 1,768.15 K
Boiling point	2,927 °C · 5,300.6 °F · 3,200.15 K
Density	8.86 g/cm³
Specific heat capacity	0.421 J/(g·K)
Thermal conductivity	100 W/(m·K)
Crystal structure	Hexagonal

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	⁵⁹Co 100%
Stable isotopes	4
Known isotopes	32
Abundance in Earth's crust	25 mg/kg
Abundance in seawater	0.00002 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Cobalt electron configuration

Cobalt has 27 electrons. Written in short form, its ground-state configuration is [Ar] 3d⁷ 4s² — the electrons of the noble gas argon, plus 3d⁷ 4s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s².

3d↑↓↑↓↑↑↑

4s↑↓

Discovery and name

Discovered

1735 — Georg Brandt · Sweden.

Name

German Kobold, a goblin blamed by miners for troublesome ores.

Known for

Used in superalloys for jet engines, in lithium-ion battery cathodes and in strong magnets; cobalt blue has coloured glass and ceramics for centuries. Vitamin B₁₂ contains cobalt.

Isotopes of cobalt

Naturally occurring cobalt is a mix of 1 isotope. Each has 27 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
⁵⁹Co	27	32	100%	stable

32 isotopes of cobalt are known in our data set (half-lives as published in nuclear data tables).

Cobalt compared with group 9

The other elements in group 9, from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Cobalt	27	58.933	1,495 °C	2,927 °C	8.86 g/cm³	1.88
Rhodium	45	102.91	1,963 °C	3,695 °C	12.4 g/cm³	2.28
Iridium	77	192.22	2,446 °C	4,428 °C	22.562 g/cm³	2.2
Meitnerium	109	[278]	—	—	37.4 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of cobalt?

The standard atomic weight of cobalt is 58.933 ± 0.001 (CIAAW 2024), so one mole weighs about 58.933 grams.

How many protons, neutrons and electrons does cobalt have?

A neutral cobalt atom has 27 protons and 27 electrons. Its most common isotope, cobalt-59, has 32 neutrons.

What is the electron configuration of cobalt?

[Ar] 3d⁷ 4s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²).

What are the melting and boiling points of cobalt?

Cobalt melts at 1,495 °C (2,723 °F) and boils at 2,927 °C (5,300.6 °F).

Is cobalt a metal?

Cobalt is a metal. It is in group 9, period 4, and is a solid at room temperature.

Who discovered cobalt?

1735 — Georg Brandt (Sweden).