Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[Ar] 4s² 3d⁷
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 8, 15, 2
Number of electrons in each shell, from the nucleus outwards.
1,495 °C (1,768 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
2,927 °C (3,200 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
8.86 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.88 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
7.881 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
0.662 eV
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
126 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+3, +2
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Cobalt (Co) is a chemical element with atomic number 27. It sits in group 9 and period 4 of the periodic table, in the d-block. It is a transition metal. At room temperature it is a solid. Its standard atomic weight is 58.933.
Origin of the name
The name comes from the German Kobold, a goblin of the mines. German silver miners cursed a mineral that gave no metal when smelted and gave off poisonous fumes, in fact from the arsenic it contained, and blamed it on the goblins. The Swedish chemist Georg Brandt showed that the beautiful blue this ore gives to glass came from a new metal, not from bismuth, as had been believed.
Where is cobalt found?
Cobalt is almost always mined as a by-product of copper and nickel. Most of the world’s cobalt comes from the Copperbelt of the Democratic Republic of the Congo. It is also found in meteorites, alloyed with iron and nickel.
What is cobalt used for?
A great deal of cobalt now goes into the electrodes of lithium-ion batteries. It also makes strong permanent magnets, heat-resistant alloys for jet engines and hard cutting tools, and gives glass and porcelain the deep colour known as cobalt blue. Radioactive cobalt-60 is a source of gamma rays used to treat cancer and to preserve food.
Interesting facts
The Egyptians coloured glass blue with cobalt more than 3000 years ago.
Vitamin B12, which people need to stay healthy, has an atom of cobalt at its centre.
A solution of cobalt chloride works as invisible ink: the writing is almost colourless until the paper is warmed.