Cobalt (Co)
Cobalt is element 27, a transition metal in group 9, period 4. A neutral atom has 27 protons, 32 neutrons and 27 electrons, 2 of them valence electrons.
Transition metal- Atomic number
- 27
- Atomic mass
- 58.933 u
- Valence electrons
- 2*
- Protons · neutrons
- 27 · 32
- Electrons per shell
- 2, 8, 15, 2
- At room temperature
- solid
Bohr model of cobalt
The Bohr model draws cobalt as a nucleus of 27 protons and 32 neutrons, with its 27 electrons on 4 circular shells: 2 in shell 1, 8 in shell 2, 15 in shell 3, 2 in shell 4. Press Build the atom to watch the shells fill from the inside out. The outer ring (the N shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of cobalt
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷
Short form (noble gas notation): [Ar] 4s² 3d⁷. The [Ar] stands for the electrons of argon, the noble gas before it.
How many valence electrons does cobalt have?
Cobalt has 2 valence electrons.
Cobalt is a transition metal, so its valence electrons are less clear-cut than for main-group elements. It has 2 electrons in its outermost shell (shell 4), and electrons in its inner d subshell can also take part in bonding.
Protons, neutrons and electrons
| Protons | 27 | equal to the atomic number |
|---|---|---|
| Electrons | 27 | equal to the protons in a neutral atom |
| Neutrons | 32 | mass number 59 − 27 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Cobalt properties
| Symbol | Co |
|---|---|
| Category | Transition metal |
| Group · period · block | 9 · 4 · d |
| Atomic mass | 58.933 u |
| State at 25 °C | solid |
| Melting point | 1,768 K (1,490 °C, 2,720 °F) |
| Boiling point | 3,200 K (2,930 °C, 5,300 °F) |
| Density | 8.86 g/cm³ |
| Electronegativity (Pauling) | 1.88 |
| Atomic radius | 192 pm |
| First ionization energy | 7.881 eV |
| Electron affinity | 0.661 eV |
| Oxidation states | +3, +2 |
| Common ion charge | +3, +2 |
| Discovered | 1735 by Georg Brandt |
Where cobalt sits on the periodic table
Group 9: Rhodium, Iridium, Meitnerium.
Questions about cobalt
How many valence electrons does cobalt have?
Cobalt has 2 valence electrons in its outer shell; inner d or f electrons can also bond.
How many protons, neutrons and electrons does cobalt have?
A neutral cobalt atom has 27 protons and 27 electrons. Its most common form has about 32 neutrons (mass number 59 − 27 protons).
What is the electron configuration of cobalt?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷, written in short form as [Ar] 4s² 3d⁷.
What does the Bohr model of cobalt look like?
A nucleus of 27 protons and 32 neutrons, circled by 4 electron shells holding 2, 8, 15, 2 electrons.
What is the atomic number of cobalt?
27. The atomic number is the number of protons in the nucleus, and it is what makes an atom cobalt.