What is the electron configuration of cobalt?

Cobalt (Co, Z=27) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s². The abbreviated form using the argon noble-gas core is [Ar] 3d⁷ 4s².

How many unpaired electrons does cobalt have?

Cobalt has 3 unpaired electrons in its 3d subshell. Following Hund's rule, the 7 electrons in 3d fill as ↑↓ ↑↓ ↑↓ ↑ ↑ — three paired orbitals and two singly occupied orbitals, giving 3 unpaired electrons.

Why does cobalt use [Ar] in its abbreviated configuration?

[Ar] represents the filled 1s² 2s² 2p⁶ 3s² 3p⁶ core (18 electrons) shared by cobalt and all Period 4 elements with Z > 18. Writing [Ar] 3d⁷ 4s² is compact shorthand that highlights only the valence-region electrons.

Is cobalt paramagnetic or diamagnetic?

Cobalt is paramagnetic. With 3 unpaired electrons in its 3d subshell, cobalt is attracted to magnetic fields. This unpaired-electron paramagnetism is distinct from the bulk ferromagnetism of solid cobalt metal, which arises from cooperative magnetic ordering.

What are cobalt's common oxidation states?

Cobalt most commonly forms Co²⁺ (losing both 4s electrons) and Co³⁺ (losing both 4s and one 3d electron). Co²⁺ is the more stable aqueous ion; Co³⁺ is a strong oxidizer in most simple salts but is stabilized in coordination complexes and in vitamin B12.

How many valence electrons does cobalt have?

Cobalt has 9 valence electrons: 7 in 3d and 2 in 4s. The d and s electrons beyond the argon core are considered valence electrons for transition metals.

Where does cobalt sit in the periodic table?

Cobalt is in Period 4, Group 9, d-block. Its neighbors are iron (Fe, Z=26) on the left and nickel (Ni, Z=28) on the right. All three are part of the iron-group transition metals known for magnetic properties.

Cobalt Electron Configuration

Name: Cobalt Electron Configuration
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Author: Nham Vu

Reference for cobalt's electron configuration ([Ar] 3d⁷ 4s²), orbital box diagram showing Hund's rule on the 3d subshell, subshell breakdown, and element facts.

Z = 27 Co Cobalt

Cobalt — Electron Configuration

Atomic number 27 · Transition metal · Period 4, Group 9 · d-block

[Ar] 3d⁷ 4s² 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s² 27 electrons 9 valence e⁻ 3 unpaired e⁻

Subshell Breakdown

Subshell	Type	Electrons	Max	Notation	Core
1s	s orbital, n=1	2	2	1s²	[Ar] core
2s	s orbital, n=2	2	2	2s²	[Ar] core
2p	p orbitals, n=2	6	6	2p⁶	[Ar] core
3s	s orbital, n=3	2	2	3s²	[Ar] core
3p	p orbitals, n=3	6	6	3p⁶	[Ar] core
3d	d orbitals, n=3	7	10	3d⁷	valence
4s	s orbital, n=4	2	2	4s²	valence
Total		27

Full Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²

All 7 subshells written explicitly.

Noble-Gas Shorthand

[Ar] 3d⁷ 4s²

[Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons).

Valence Region

3d⁷ 4s²

9 electrons beyond the argon core; 3 are unpaired.

3d-Block Neighbors — Iron Group

Element	Z	Configuration	Unpaired e⁻
Manganese (Mn)	25	[Ar] 3d⁵ 4s²	5
Iron (Fe)	26	[Ar] 3d⁶ 4s²	4
Cobalt (Co) ←	27	[Ar] 3d⁷ 4s²	3
Nickel (Ni)	28	[Ar] 3d⁸ 4s²	2
Copper (Cu)	29	[Ar] 3d¹⁰ 4s¹	1

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)

Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)

Shell 3 (n=3) — 3s² 3p⁶ 3d⁷ 15 / 18 electrons (83%)

Shell 4 (n=4) — 4s² 2 / 32 electrons (6%)

Shell 3 holds 3s² + 3p⁶ + 3d⁷ = 15 electrons (max 18). Shell 4 has only 4s² filled; the 4p, 4d, and 4f subshells are empty in the ground state.

Orbital Box Diagram

Each box = one orbital. Arrows show electron spin: ↑ (spin up, m_s=+½) and ↓ (spin down, m_s=−½).

2 electrons — paired — [Ar] core

6 electrons, 3 orbitals — all paired — [Ar] core

2 electrons — paired — [Ar] core

6 electrons, 3 orbitals — all paired — [Ar] core

Valence subshells (beyond [Ar] core)

4s² — 2 electrons, paired (filled first by Aufbau, lost first in ionization)

mₗ=−2

mₗ=−1

mₗ= 0

—

mₗ=+1

—

mₗ=+2

3d⁷ — first 3 orbitals paired (↑↓), last 2 singly occupied (↑) — 3 unpaired electrons

Hund's rule: each 3d orbital receives one spin-up electron before any gets a second. With 7 electrons in 5 orbitals, three orbitals are fully paired and two hold a single electron each, leaving 3 unpaired electrons.

4s before 3d: 4s fills before 3d during Aufbau, but during ionization the 4s electrons are removed first (giving Co²⁺: [Ar] 3d⁷, Co³⁺: [Ar] 3d⁶).

Paramagnetism: the 3 unpaired electrons make cobalt paramagnetic. Bulk cobalt metal is also ferromagnetic due to cooperative spin alignment.

Quantum Numbers — Valence Electrons (3d and 4s)

e⁻ #	n	l	mₗ	mₛ	Subshell	Paired?
19	4	0	0	+½	4s	Yes
20	4	0	0	−½	4s	Yes
21	3	2	-2	+½	3d	Yes
22	3	2	-2	−½	3d	Yes
23	3	2	-1	+½	3d	Yes
24	3	2	-1	−½	3d	Yes
25	3	2	0	+½	3d	Yes
26	3	2	0	−½	3d	Yes
27	3	2	1	+½	3d	No (unpaired)

Showing valence electrons 19–27. Electrons 1–18 fill the [Ar] core (same quantum numbers as argon). n = principal; l = azimuthal (l=2 → d); mₗ = magnetic; mₛ = spin. Highlighted row (e⁻ #27) is the lone unpaired electron in mₗ=+1.

Summary