Iron is a chemical element with the symbol Fe (from Latin ferrum 'iron') and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, just ahead of oxygen (32. 1% and 30. 1%, respectively), forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust, being mainly deposited by meteorites in its metallic state, with its ores also being found there.

Extracting usable metal from iron ores requires kilns or furnaces capable of reaching 1,500 °C (2,730 °F) or higher, about 500 °C (932 °F) higher than that required to smelt copper. Humans started to master that process in Eurasia during the 2nd millennium BCE and the use of iron tools and weapons began to displace copper alloys—in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. In the modern world, iron alloys, such as steel, stainless steel, cast iron and special steels, are by far the most common industrial metals, due to their mechanical properties and low cost. The iron and steel industry is thus very important economically, and iron is the cheapest metal, with a price of a few dollars per kilogram or pound.

Pristine and smooth pure iron surfaces are a mirror-like silvery-gray. Iron reacts readily with oxygen and water to produce brown-to-black hydrated iron oxides, commonly known as rust. Unlike the oxides of some other metals that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing more fresh surfaces for corrosion. High-purity irons (e. g. electrolytic iron) are more resistant to corrosion.

The body of an adult human contains about 4 grams (0. 005% body weight) of iron, mostly in hemoglobin and myoglobin. These two proteins play essential roles in vertebrate metabolism, respectively oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are iron(II) and iron(III). Iron shares many properties of other transition metals, including the other group 8 elements, ruthenium and osmium. Iron forms compounds in a wide range of oxidation states, −4 to +7. Iron also forms many coordination compounds; some of them, such as ferrocene, ferrioxalate, and Prussian blue have substantial industrial, medical, or research applications.

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Atomic properties

Standard atomic weight55.845 ±0.002
Atomic mass55.8452 u

Atomic radii

Radius (empirical)126 pm
Radius (calculated)156 pm
Covalent radius132 ±3 pm
Van der Waals radius194 pm

Atomic shell

Electron configurationAr 3d6 4s2
Ionization energy(1st) 7.9024681 eV
(2nd) 16.1992 eV
(3rd) 30.651 eV
(4th) 54.91 eV
(5th) 75 eV
Shell model

Physical properties

Density7.874 g·cm−3
Molar volume7.09·10-6 m3·mol−1
Speed of sound4,910 m·s−1 (293.1 K)


Melting point1,811 K
Boiling point3,134 K
Liquid range1,323 K
Transition temperature2 K


Melting enthalpy13.8 kJ·mol-1
Enthalpy of vaporization347 kJ·mol-1
Binding energy415 kJ·mol-1

Heat and conductivity

Specific heat capacity449 J·kg−1·K−1
Thermal conductivity80 W·m-1·K-1
Expansion coefficient1.18·10-5 K-1
Work function4.5 eV


Mohs hardness4
Brinell hardness490 NM·m-2
Vickers hardness608 NM·m-2

Elastic properties

Young’s modulus211 GPa
Shear modulus82 GPa
Bulk modulus170 GPa
Poisson’s ratio0.29

Electrical properties

Electrical conductivity1·107 S·m-1
Resistance1·10-7 Ωm



Optical properties

Reflectivity65 %

Chemical properties

Oxidation state2, 3, 4, 5, 6
Standard potential-0.44 V (Fe2+ + 2e- → Fe)


Pauling scale1.83
Sanderson scale2.2
Allred-Rochow scale1.64
Ghosh-Gupta scale3.583 eV
Nagle scale1.4
Pearson absolute negativity4.06 eV

Other properties

Natural occurrenceprimordial
Crystal structureBody-centered cubic
Goldschmidt Classificationsiderophile
Superconductorwith transition tempperature under special conditions
Price/kg0.424 USD

Natural abundances

1.1·106 ppb ≈ 1.1·1016 M☉
1·106 ppb ≈ 1.98·1015 Mt
2.2·108 ppb ≈ 220 kg
Earth’s crust
6.3·107 ppb ≈ 1.74·109 Mt
3 ppb ≈ 4.11 Gt
Flowing water
670 ppb ≈ 10.7 Gt
Human body
60,000 ppb ≈ 4.19 g