Cobalt

Cobalt (Co) is a transition metal with atomic number 27 and an atomic mass of 58.933. Its electron configuration is [Ar] 3d7 4s2.

Cobalt key data

Symbol	Co
Atomic number	27
Atomic mass (standard atomic weight)	58.933
Molar mass	58.933 g/mol
Protons	27
Neutrons (most abundant isotope, cobalt-59)	32
Electrons (neutral atom)	27
Category	Transition metal
Period	4
Group	9
Block	d-block
Noble gas notation	[Ar] 3d7 4s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d7 4s2
Electrons per shell	2, 8, 15, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of cobalt

From PubChem, the US National Institutes of Health. Public domain.

Melting point	1768 K (1494.9 °C), 30th of 103 by melting point
Boiling point	3200 K (2926.9 °C), 35th of 93 by boiling point
Density	8.86 g/cm³, 34th of 96 by density
State at room temperature	Solid
Electronegativity	1.88 (Pauling), 40th of 95 by electronegativity
Van der Waals radius	192 pm, joint 69th of 99 by atomic radius
First ionisation energy	7.881 eV (760.4 kJ/mol), 36th of 102 by first ionisation energy
Electron affinity	0.661 eV (63.8 kJ/mol), 33rd of 57 by electron affinity
Common oxidation states	+3, +2
Discovered	1735

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Cobalt electron configuration

The full electron configuration of cobalt is 1s2 2s2 2p6 3s2 3p6 3d7 4s2. With the noble-gas core collapsed it is [Ar] 3d7 4s2, and the outermost subshell is 4s2.

Bohr model of cobalt (Co): a nucleus and four shells holding 2, 8, 15 and 2 electrons from the inside out. — Bohr model of cobalt (Co). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of cobalt (Co), 1s2 2s2 2p6 3s2 3p6 3d7 4s2: one box per orbital, filled by Hund’s rule, showing three unpaired electrons, in 3d. — Orbital diagram of cobalt (Co). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one cobalt atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 58.933 / (6.02214076 × 10²³)
= 9.7861 × 10⁻²³ g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 58.933
= 1.0219 × 10²² atoms

Cobalt on the periodic table

Cobalt is in period 4, group 9, in the d-block.

The other elements of group 9 are rhodium, iridium and meitnerium.