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 3d⁶
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, 14, 2
Number of electrons in each shell, from the nucleus outwards.
1,538 °C (1,811 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.
2,861 °C (3,134 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).
7.874 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.83 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.
7.902 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.
0.153 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.
132 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.
Iron (Fe) is a chemical element with atomic number 26. It sits in group 8 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 55.845.
Origin of the name
The English word iron goes back to Old English īsern, from Proto-Germanic *īsarną, which was most likely borrowed from Celtic *īsarnom; German Eisen comes from the same root. The symbol Fe stands for Latin ferrum, a word of unclear origin. It lives on in Italian ferro, French fer and Spanish hierro.
Where is iron found?
By mass, iron is the most common element on Earth, about 32% of the planet, because the Earth’s core is mostly iron. In the crust it ranks fourth, after oxygen, silicon and aluminium. Pure iron is almost never found at the surface, since it oxidises quickly; iron meteorites are the main exception. Iron is mined from ores such as hematite Fe₂O₃, magnetite Fe₃O₄ and siderite FeCO₃.
What is iron used for?
Iron makes up more than 90% of all the metal produced in the world. Pure iron is fairly soft, so almost all of it is alloyed with carbon: steel contains up to about 2% carbon, cast iron more. Steel goes into rails, bridges, building frames, ship hulls and cars; adding chromium makes it stainless. Iron(II) sulfate, FeSO₄, is a medicine for iron-deficiency anaemia.
Interesting facts
An adult’s body holds 3–4 g of iron, most of it in haemoglobin, the protein that carries oxygen in the blood.
Above 770 °C, its Curie point, iron loses its ferromagnetism, and a magnet barely attracts it.
The Iron Pillar of Delhi has stood for about 1,600 years: it is 7.2 m tall, weighs more than 6 tonnes and has hardly rusted.