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] 3d⁵ 4s¹
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, 13, 1
Number of electrons in each shell, from the nucleus outwards.
1,907 °C (2,180 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.
2,671 °C (2,944 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).
7.15 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.66 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.
6.767 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.
0.676 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.
139 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.
+6, +3, +2
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Chromium (Cr) is a chemical element with atomic number 24. It sits in group 6 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 51.996.
Origin of the name
The name comes from the Greek chrōma, ‘colour’, because chromium compounds come in so many colours. In 1797 the French chemist Louis-Nicolas Vauquelin found the new element in ‘Siberian red lead’, the orange-red mineral crocoite from the Urals, and a year later he obtained the metal itself.
Where is chromium found?
Chromium is mined as the ore chromite. South Africa is by far the largest producer: its Bushveld Complex holds most of the world’s known chromium.
What is chromium used for?
Most chromium goes into alloys, above all stainless steel: chromium is what stops it rusting. A thin layer of chromium plating gives car parts and other metal objects their hard, mirror-like shine. Chromium salts tan leather, lead chromate is the pigment chrome yellow, and bricks made of chromite line furnaces because they withstand great heat.
Interesting facts
The red of rubies and the green of emeralds both come from traces of chromium; Vauquelin himself noticed it.
Compounds of hexavalent chromium, such as chromates, are poisonous and cause cancer.
The bronze weapons of the Terracotta Army were long thought to have been chrome-plated by the ancient Chinese. A study in 2019 showed that the chromium came from the lacquer on them.