Cobalt (Co)

Element 27 · transition metal · group 9, period 4

Electron shells, 2 + 8 + 15 + 2. The outer shell is highlighted - it is the only one that takes part in bonding.

Cobalt is element 27, a transition metal in group 9 and period 4. Its electron configuration is [Ar] 3d7 4s2. The shells fill 2, 8, 15, 2. Courses differ here. Counting only the outer s subshell gives 2; counting the inner d subshell with it gives 9, which is the group number. It commonly forms ions with a charge of +2, +3.

27 protons
2/9 valence e−
+2, +3
d-block

Properties

Atomic number	27
Symbol	Co
Standard atomic weight	58.933 u
Group	9
Period	4
Block	d-block
Category	transition metal
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²
Noble-gas notation	[Ar] 3d⁷ 4s²
Electrons per shell	2, 8, 15, 2
Valence electrons	2 or 9
Common ionic charge	+2, +3
Electronegativity (Pauling)	1.88
Protons / electrons	27 / 27

Electron configuration of cobalt

Orbitals fill from the lowest energy upward. Writing that out for cobalt gives 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s², which is 1s2 2s2 2p6 3s2 3p6 3d7 4s2 in plain text.

Everything up to Ar is the same as in that noble gas, so it is usually abbreviated to [Ar] 3d⁷ 4s². The part outside the bracket is what does the chemistry.

Subshell	Electrons	Capacity
1s	2	2
2s	2	2
2p	6	6
3s	2	2
3p	6	6
3d	7	10
4s	2	2

Cobalt ion charge

Cobalt commonly forms ions with a charge of +2, +3.

Common questions

How many valence electrons does cobalt have?

There is no single agreed answer for cobalt. Courses differ here. Counting only the outer s subshell gives 2; counting the inner d subshell with it gives 9, which is the group number. Check which convention your course uses before answering.

What is the electron configuration of cobalt?

1s2 2s2 2p6 3s2 3p6 3d7 4s2, or [Ar] 3d7 4s2 written from the previous noble gas. Orbitals fill lowest energy first.

What is the charge of cobalt?

Cobalt commonly forms ions with a charge of +2, +3. Transition metals have several accessible oxidation states, which is why more than one charge is listed.

How many protons, electrons and neutrons does cobalt have?

A neutral cobalt atom has 27 protons and 27 electrons. Its standard atomic weight is 58.933, so the most common isotope has about 32 neutrons.

See the ionic charge of every element →

How this page was produced. The configuration was built by filling orbitals in energy order from atomic number 27 alone, not looked up. The shell counts, the valence count and the charge all follow from it. Nothing on this page is written per element.