Cobalt at a glance
- Atomic number
- 27
- Symbol
- Co
- Atomic mass
- 58.933 u
- Category
- Transition metal
- Group / period
- 9 / 4
- Electron configuration
- [Ar] 3d 7 4s 2
- State at 25 °C
- Solid
- Melting point
- 1495 °C
- Boiling point
- 2927 °C
- Density
- 8.86 g/cm³
- Electronegativity
- 1.88
- 1st ionization energy
- 760 kJ/mol
- Oxidation states
- +2, +3
- Discovered
- 1735
Atomic structure
A neutral cobalt atom has 27 protons and 27 electrons, and its most common isotope carries about 32 neutrons. One atom weighs about 9.79 × 10⁻²⁶ kg, so a mole of cobalt (6.022 × 10²³ atoms) weighs 58.933 g. That nucleus holds 1.19 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 15, N 2), which gives the configuration [Ar] 3d 7 4s 2. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. It has only one stable isotope, Co-59. No other element has a stable isotope with mass number 59, so Co-59 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) cobalt is a solid. It melts at 1495 °C and boils at 2927 °C, so it stays liquid across a 1432-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 8.86 g/cm³, 8.86 times as dense as water. Its atoms have a radius of about 135 pm (Iron next door is 140 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.88 on the Pauling scale. Pulling off its outermost electron takes 760 kJ/mol. A bond between cobalt and chlorine would be polar covalent, with an electronegativity difference of 1.3. It releases 64 kJ/mol when it gains an extra electron. Its common oxidation states are +2, +3, so in compounds you will usually meet it as Co²⁺ and Co³⁺. Group 9 also holds Rhodium, Iridium and Meitnerium, which share its outer-shell pattern.
Where it comes from
It makes up 25 mg/kg of Earth's crust, the 30th most abundant element there. In the human body it accounts for about 0.000002 % of body mass. It was first identified in 1735, making it the 14th element to be discovered (counting the 11 known since antiquity together).