Iron,  26Fe
Pure iron chips with a high purity iron cube
Appearancelustrous metallic with a grayish tinge
Standard atomic weight Ar, std(Fe)55.845(2)[1]
Iron in the periodic table
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon


Atomic number (Z)26
Groupgroup 8
Periodperiod 4
Element category  Transition metal
Electron configuration[Ar] 3d6 4s2
Electrons per shell
2, 8, 14, 2
Physical properties
Phase at STPsolid
Melting point1811 K ​(1538 °C, ​2800 °F)
Boiling point3134 K ​(2862 °C, ​5182 °F)
Density (near r.t.)7.874 g/cm3
when liquid (at m.p.)6.98 g/cm3
Heat of fusion13.81 kJ/mol
Heat of vaporization340 kJ/mol
Molar heat capacity25.10 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)172818902091234626793132
Atomic properties
Oxidation states−4, −2, −1, +1,[2] +2, +3, +4, +5,[3] +6, +7[4] (an amphoteric oxide)
ElectronegativityPauling scale: 1.83
Ionization energies
  • 1st: 762.5 kJ/mol
  • 2nd: 1561.9 kJ/mol
  • 3rd: 2957 kJ/mol
  • (more)
Atomic radiusempirical: 126 pm
Covalent radiusLow spin: 132±3 pm
High spin: 152±6 pm
Color lines in a spectral range
Spectral lines of iron
Other properties
Natural occurrenceprimordial
Crystal structurebody-centered cubic (bcc)
Body-centered cubic crystal structure for iron

a=286.65 pm
Crystal structureface-centered cubic (fcc)
Face-centered cubic crystal structure for iron

between 1185–1667 K
Speed of sound thin rod5120 m/s (at r.t.) (electrolytic)
Thermal expansion11.8 µm/(m·K) (at 25 °C)
Thermal conductivity80.4 W/(m·K)
Electrical resistivity96.1 nΩ·m (at 20 °C)
Curie point1043 K
Magnetic orderingferromagnetic
Young's modulus211 GPa
Shear modulus82 GPa
Bulk modulus170 GPa
Poisson ratio0.29
Mohs hardness4
Vickers hardness608 MPa
Brinell hardness200–1180 MPa
CAS Number7439-89-6
Discoverybefore 5000 BC
Main isotopes of iron
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
55Fesyn2.73 yε55Mn
59Fesyn44.6 dβ59Co
60Fetrace2.6×106 yβ60Co
| references

Iron (n/) is a chemical element with symbol Fe (from Latin: ferrum) 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, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Pure iron is very rare on the Earth's crust, basically being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE[not verified in body], and iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel, inox, and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost.

Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts readily with oxygen and water to give 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 fresh surfaces for 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.[8]

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, −2 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.



Molar volume vs. pressure for α iron at room temperature

At least four allotropes of iron (different atom arrangements in the solid) are known, conventionally denoted α, γ, δ, and ε.

Low-pressure phase diagram of pure iron

The first three forms are observed at ordinary pressures. As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic (bcc) crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic (fcc) crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope.[9]

The physical properties of iron at very high pressures and temperatures have also been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed (hcp) structure, which is also known as ε-iron. The higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure.

Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K. It is supposed to have an orthorhombic or a double hcp structure.[10] (Confusingly, the term "β-iron" is sometimes also used to refer to α-iron above its Curie point, when it changes from being ferromagnetic to paramagnetic, even though its crystal structure has not changed.[9])

The inner core of the Earth is generally presumed to consist of an iron-nickel alloy with ε (or β) structure.[11]

Melting and boiling points

The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus;[12] however, they are higher than the values for the previous element manganese because that element has a half-filled 3d subshell and consequently its d-electrons are not easily delocalized. This same trend appears for ruthenium but not osmium.[13]

The melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data (as of 2007) still varies by tens of gigapascals and over a thousand kelvin.[14]

Magnetic properties

Magnetization curves of 9 ferromagnetic materials, showing saturation. 1. Sheet steel, 2. Silicon steel, 3. Cast steel, 4. Tungsten steel, 5. Magnet steel, 6. Cast iron, 7. Nickel, 8. Cobalt, 9. Magnetite[15]

Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall magnetic field.[16] This happens because the orbitals of those two electrons (dz2 and dx2y2) do not point toward neighboring atoms in the lattice, and therefore are not involved in metallic bonding.[9]

In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometers across,[17] such that the atoms in each domain have parallel spins, but different domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.

Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field. This effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, and electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists even after the external field is removed -- thus turning the iron object into a (permanent) magnet.[16]

Similar behavior is exhibited by some iron compounds, such as the ferrites and the mineral magnetite, a crystalline form of the mixed iron(II,III) oxide Fe
(although the atomic-scale mechanism, ferrimagnetism, is somewhat different). Pieces of magnetite with natural permanent magnetization (lodestones) provided the earliest compasses for navigation. Particles of magnetite were extensively used in magnetic recording media such as core memories, magnetic tapes, floppies, and disks, until they were replaced by cobalt-based materials.


Iron has four stable isotopes: 54Fe (5.845% of natural iron), 56Fe (91.754%), 57Fe (2.119%) and 58Fe (0.282%). 20-30 artificial isotopes have also been created. Of these stable isotopes, only 57Fe has a nuclear spin (−​12). The nuclide 54Fe theoretically can undergo double electron capture to 54Cr, but the process has never been observed and only a lower limit on the half-life of 3.1×1022 years has been established.[18]

60Fe is an extinct radionuclide of long half-life (2.6 million years).[19] It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide 60Ni.[18] Much of the past work on isotopic composition of iron has focused on the nucleosynthesis of 60Fe through studies of meteorites and ore formation. In the last decade, advances in mass spectrometry have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work is driven by the Earth and planetary science communities, although applications to biological and industrial systems are emerging.[20]

In phases of the meteorites Semarkona and Chervony Kut, a correlation between the concentration of 60Ni, the granddaughter of 60Fe, and the abundance of the stable iron isotopes provided evidence for the existence of 60Fe at the time of formation of the Solar System. Possibly the energy released by the decay of 60Fe, along with that released by 26Al, contributed to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of 60Ni present in extraterrestrial material may bring further insight into the origin and early history of the Solar System.[21]

The most abundant iron isotope 56Fe is of particular interest to nuclear scientists because it represents the most common endpoint of nucleosynthesis.[22] Since 56Ni (14 alpha particles) is easily produced from lighter nuclei in the alpha process in nuclear reactions in supernovae (see silicon burning process), it is the endpoint of fusion chains inside extremely massive stars, since addition of another alpha particle, resulting in 60Zn, requires a great deal more energy. This 56Ni, which has a half-life of about 6 days, is created in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the supernova remnant gas cloud, first to radioactive 56Co, and then to stable 56Fe. As such, iron is the most abundant element in the core of red giants, and is the most abundant metal in iron meteorites and in the dense metal cores of planets such as Earth.[23] It is also very common in the universe, relative to other stable metals of approximately the same atomic weight.[23][24] Iron is the sixth most abundant element in the Universe, and the most common refractory element.[25]

Although a further tiny energy gain could be extracted by synthesizing 62Ni, which has a marginally higher binding energy than 56Fe, conditions in stars are unsuitable for this process. Element production in supernovas and distribution on Earth greatly favor iron over nickel, and in any case, 56Fe still has a lower mass per nucleon than 62Ni due to its higher fraction of lighter protons.[26] Hence, elements heavier than iron require a supernova for their formation, involving rapid neutron capture by starting 56Fe nuclei.[23]

In the far future of the universe, assuming that proton decay does not occur, cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into 56Fe nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting all stellar-mass objects to cold spheres of pure iron.[27]

Other Languages
Afrikaans: Yster
Alemannisch: Eisen
አማርኛ: ብረት
Ænglisc: Īsen
العربية: حديد
aragonés: Fierro
ܐܪܡܝܐ: ܦܪܙܠܐ
armãneashti: Heru
অসমীয়া: লো
asturianu: Fierro
Avañe'ẽ: Itakandua
azərbaycanca: Dəmir
تۆرکجه: دمیر
বাংলা: লোহা
Bahasa Banjar: Wasi
Bân-lâm-gú: Thih
башҡортса: Тимер
беларуская: Жалеза
беларуская (тарашкевіца)‎: Жалеза
भोजपुरी: लोहा
Bislama: Aien
български: Желязо
བོད་ཡིག: ལྕགས།
bosanski: Željezo
brezhoneg: Houarn
буряад: Түмэр
català: Ferro
Чӑвашла: Тимĕр
Cebuano: Yero
čeština: Železo
corsu: Ferru
Cymraeg: Haearn
dansk: Jern
Deutsch: Eisen
ދިވެހިބަސް: ދަގަނޑު
Diné bizaad: Béésh (Fe)
eesti: Raud
Ελληνικά: Σίδηρος
эрзянь: Кшни
español: Hierro
Esperanto: Fero
estremeñu: Hierru
euskara: Burdina
فارسی: آهن
Fiji Hindi: Loha
føroyskt: Jarn
français: Fer
Frysk: Izer
furlan: Fier
Gaeilge: Iarann
Gaelg: Yiarn
Gàidhlig: Iarann
galego: Ferro
Gĩkũyũ: Iron
ગુજરાતી: લોખંડ
गोंयची कोंकणी / Gõychi Konknni: लोखण
客家語/Hak-kâ-ngî: Thiet
хальмг: Төмр
հայերեն: Երկաթ
हिन्दी: लोहा
hrvatski: Željezo
Bahasa Hulontalo: Wuwate
Ido: Fero
Bahasa Indonesia: Besi
interlingua: Ferro
Interlingue: Ferre
íslenska: Járn
italiano: Ferro
עברית: ברזל
ಕನ್ನಡ: ಕಬ್ಬಿಣ
ქართული: რკინა
қазақша: Темір
kernowek: Horn
Kiswahili: Chuma
коми: Кӧрт
Kongo: Kibende
Kreyòl ayisyen: Fè (non)
kurdî: Hesin
Кыргызча: Темир
лакку: Мах
Latina: Ferrum
latviešu: Dzelzs
Lëtzebuergesch: Eisen
лезги: Ракь
lietuvių: Geležis
Ligure: Færo
Limburgs: Iezer
lingála: Ebendé
Livvinkarjala: Raudu
la .lojban.: tirse
lumbaart: Fer
magyar: Vas
मैथिली: लोहा
македонски: Железо
Malagasy: Vy
മലയാളം: ഇരുമ്പ്
Malti: Ħadid
Māori: Rino
मराठी: लोखंड
Bahasa Melayu: Besi
Mìng-dĕ̤ng-ngṳ̄: Tiék
မြန်မာဘာသာ: သံ (သတ္တု)
Nederlands: IJzer (element)
Nedersaksies: Iezer
नेपाली: फलाम
नेपाल भाषा: नँ
Napulitano: Fierro
Nordfriisk: Stälj
Norfuk / Pitkern: Aiyen
norsk: Jern
norsk nynorsk: Jern
occitan: Fèrre
ଓଡ଼ିଆ: ଲୁହା
oʻzbekcha/ўзбекча: Temir
ਪੰਜਾਬੀ: ਲੋਹਾ
پنجابی: لوہا
Papiamentu: Heru
پښتو: وسپنه
Patois: Ayan
Перем Коми: Кӧрт
Piemontèis: Fer
Plattdüütsch: Iesen
polski: Żelazo
português: Ferro
Qaraqalpaqsha: Temir
Ripoarisch: Eisen
română: Fier
Runa Simi: Khillay
русиньскый: Желїзо
русский: Железо
саха тыла: Тимир
ᱥᱟᱱᱛᱟᱲᱤ: ᱢᱮᱬᱦᱮᱫ
संस्कृतम्: अयः
sardu: Ferru
Scots: Airn
Seeltersk: Iersen
shqip: Hekuri
sicilianu: Ferru
සිංහල: යකඩ
Simple English: Iron
سنڌي: لوھ
slovenčina: Železo
slovenščina: Železo
Soomaaliga: Xadiid
کوردی: ئاسن
српски / srpski: Гвожђе
srpskohrvatski / српскохрватски: Željezo
Basa Sunda: Beusi
suomi: Rauta
svenska: Järn
Tagalog: Bakal
தமிழ்: இரும்பு
Taqbaylit: Uzzal
татарча/tatarça: Тимер
తెలుగు: ఇనుము
ไทย: เหล็ก
тоҷикӣ: Оҳан
Türkçe: Demir
українська: Залізо
اردو: لوہا
ئۇيغۇرچە / Uyghurche: تۆمۈر
Vahcuengh: Diet
vèneto: Fero
vepsän kel’: Raud
Tiếng Việt: Sắt
Volapük: Ferin
Võro: Raud
walon: Fier
West-Vlams: Yzer (element)
Winaray: Puthaw
ייִדיש: אייזן
Yorùbá: Iron
Zazaki: Asın
žemaitėška: Gelžės