Element 27 · Transition metal
Cobalt (Co) — Element 27
German kobold — goblin
Cobalt (Co) is a transition metal in Period 4, Group 9 of the periodic table with atomic number 27, standard atomic weight 58.933 u, and ground-state electron configuration [Ar] 3d⁷ 4s². At standard temperature and pressure it is classified as a solid with 9 valence electrons and common oxidation states +2, +3.
Batteries, magnets, blue glass
What are the atomic properties and physical constants of Cobalt?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 27
- Standard atomic mass
- 58.933 u
- Chemical family
- Transition metal
- Ground-state configuration
- [Ar] 3d⁷ 4s²
- Electrons per shell
- 2, 8, 15, 2
- Valence electrons
- 9
- Common ion charges / oxidation states
- +2, +3
- Phase at STP
- Solid
- Pauling electronegativity
- 1.88 Pauling
- First ionization energy
- 760 kJ/mol
- Atomic radius
- 152 pm
- Melting point
- 1,495 °C
- Boiling point
- 2,927 °C
- Discovery year
- 1735
- Table coordinate
- Period 4, Group 9
- Radioactivity
- Has stable isotope(s)
How are electrons arranged across Cobalt’s shells?
A neutral Cobalt atom contains 27 electrons distributed across 4 occupied principal energy shells ([Ar] 3d⁷ 4s²). The table below compares Cobalt’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Co | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 15 e⁻ | 18 e⁻ |
| n = 4 | N | 2 e⁻ | 32 e⁻ |
Frequently Asked Questions about Cobalt (Co)
What is the atomic mass and atomic number of Cobalt (Co)?
Cobalt (Co) has atomic number Z = 27 (27 protons in its nucleus) and an IUPAC standard atomic weight of 58.933 u. It sits in Period 4, Group 9 and is classified as a transition metal.
What is the ground-state electron configuration of Cobalt?
The neutral ground-state electron configuration of Cobalt (Z = 27) is [Ar] 3d⁷ 4s². Across its 4 occupied principal shells (n = 1 to n = 4), the electrons per shell are 2, 8, 15, 2, with 9 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Cobalt?
Cobalt commonly forms +2, +3 oxidation states or ion charges in compounds. Its Pauling electronegativity is 1.88 and its first ionization energy is 760 kJ/mol.
When was Cobalt discovered and what is it used for?
Cobalt was discovered in 1735 (German kobold — goblin). Batteries, magnets, blue glass
How does Cobalt compare to neighboring elements?
Other transition metal elements: