Cobalt (Co): Bohr Model, Protons, Neutrons & Electrons
Cobalt is element number 27, a transition metal in group 9, period 4. Its atom has 27 protons, 32 neutrons and 27 electrons arranged in 4 shells: 2, 8, 15, 2.
Protons, neutrons and electrons in cobalt
The atomic number of cobalt is 27, so every cobalt atom has 27 protons. A neutral atom has the same number of electrons, 27. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For cobalt-59, that is 59 − 27 = 32 neutrons.
How to draw the Bohr model of cobalt
- Draw the nucleus and write 27 p⁺ and 32 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Draw the second shell and put 8 electrons on it.
- Draw the third shell and put 15 electrons on it.
- Draw the fourth shell and put 2 electrons on it.
- Check: 2 + 8 + 15 + 2 = 27 electrons, the same as the number of protons.
From potassium onwards the shells don’t simply fill as 2, 8, 8: the fourth shell starts before the third is full, so the real arrangement above (2, 8, 15, 2) is the one to draw.
Electron configuration
The electron configuration of cobalt is [Ar] 4s² 3d⁷. As a transition metal it fills an inner d-subshell, and its common ion is Co²⁺.
Key facts about cobalt
| Symbol | Co |
|---|---|
| Atomic number | 27 |
| Relative atomic mass | 58.933 |
| Group · period · block | group 9 · period 4 · d-block |
| Type | Transition Metal |
| State at 25 °C | solid |
| Electron shells | 2, 8, 15, 2 |
| Electron configuration | [Ar] 4s² 3d⁷ |
| Valence electrons | varies (outer shell: 2) |
| Common ion | Co²⁺ |
| Electronegativity (Pauling) | 1.88 |
| First ionisation energy | 7.88 eV (760 kJ/mol) |
| Atomic radius (empirical) | 135 pm |
Where cobalt sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does cobalt have?
Cobalt has 27 protons and 27 electrons (atomic number 27). Its most common isotope, cobalt-59, has 59 − 27 = 32 neutrons.
What is the electron configuration of cobalt?
[Ar] 4s² 3d⁷. In shells that is 2, 8, 15, 2.
How many valence electrons does cobalt have?
Cobalt is a transition metal, so its valence electrons are spread over two shells; its outermost shell holds 2.
Where is cobalt on the periodic table?
In group 9, period 4, in the d-block. It is a transition metal.
