Iron is a chemical element with atomic number 26, symbol Fe.

Atomic number26
Period4th period
BlockBlock d
Element familyTransition metal
Electronic configuration[Ar] 4 S2 3D6
Electrons by energy level2, 8, 14, 2
Atomic mass55.845 ± 0.002 u
Atomic radius (calc)140 pm (156 pm)
Covalence radius132 ± 3 pm
(low spin)

152 ± 6 pm(high spin)
Oxidation state+2, +3, +4, +6
Electronegativity (Pauling)1,83
1re: 7,902 4 eV2nd: 16.187 7 eV
3rd: 30.652 eV4th: 54.8 eV
5th: 75.0 eV6th: 99.1 eV
7th: 124.98 eV8th: 151.06 eV
9th: 233.6 eV10th: 262.1 eV
11th: 290.2 eV12th: 330.8 eV
13 eV: 361.0 eV14th: 392.2 eV
15th: 457 eV16e: 489.256 eV
17th: 1,266 eV18th: 1,358 eV
19th: 1,456 eV20th: 1,582 eV
21st: 1,689 eV22nd: 1,799 eV
23rd: 1,950 eV24th: 2,023 eV
25th: 8,828 eV26th: 9,277.69 eV
Ordinary stateFerromagnetic solid
Allotrope in the standard stateIron α (centered cubic)
Other allotropesIron γ (face-centered cubic), δ iron (centered cubic)
Density7.874 g·cm-3 at (20 °C)
Crystal systemCentered cubic
Hardness (Mohs)4
Coloursilvery white; gray reflections
Melting point1,538 °C
Boiling point2,861 °C
Fusion energy13.8 kJ·mol-1
Vaporization energy349.6 kJ·mol-1
Molar volume7.09×10−6 m3·mol-1
Vapour pressure7.05 Pa
Speed of sound4,910 m·s-1 at 20 °C
Mass heat440 J·kg-1· K-1
Electrical conductivity9.93×106 S·m-1
Thermal conductivity80.2 W·m-1· K-1
Solubilityground. in H2SO4 diluted, HCl
CAS No.7439-89-6
No ECHA100.028.270
No EC231-096-4
SI & CNTP units, unless otherwise stated.

The simple body is the most common metal and ferromagnetic material in everyday life, most often in the form of various alloys. Pure iron is a ductile transition metal, but the addition of very small amounts of additional elements significantly changes its mechanical properties. Combined with carbon and other addition elements, it forms steels, whose sensitivity to thermomechanical treatments makes it possible to further diversify the properties of the material.


Iron belongs to the group of elements at the origin of transition metals, it shows characteristic analogies with ruthenium, osmium, cobalt and nickel.


The French word “iron” comes from the Latin ferrum, with the same meaning. It is “uncountable” (in the sense that, when it designates metal, it is not used in the plural or directly with a numeral).

The Latin word ferrum was traditionally attached to the family of meaning firmus (“firm, solid”). But today we see in the Spanish hierro and the English iron (same meaning) a Celtic loan and we rather attach it to the radical of aes (“airin” and other metals) and its reconstructed Indo-European root: *ḫeṷis or *ai̯os- [“metal”, (copper, bronze or iron)].

Nuclear physico-chemistry, isotopes, frequency

Iron-56 is the heaviest stable nuclide resulting from the fusion of silicon by α reactions during stellar nucleosynthesis, which actually results in nickel-56, which is unstable and gives 56Fe by two successive β+ decays; elements of higher atomic number are synthesized by more energetic reactions occurring rather during the explosion of supernovas.

Nuclear properties

The iron-56 nucleus has the lowest mass per nucleon of all nuclides but not the highest binding energy, due to a slightly higher proportion of protons than nickel-62, which has the highest binding energy per nucleon.

Iron-56 results from the natural decay of nickel-56, an unstable isotope produced in the heart of massive stars by the fusion of silicon-28 during cascade alpha reactions that stop at nickel precisely because the latter has the highest nuclear binding energy per nucleon: continue fusion, to produce, for example, zinc-60, would consume energy instead of releasing it.


Iron has 28 known isotopes, with mass numbers ranging from 45 to 72, as well as six nuclear isomers. Of these isotopes, four are stable, 54 Fe, 56 Fe, 57 Fe and 58 Fe. 56Fe is by far the most abundant (91.754%), followed by 54 Fe (5.845% possibly slightly radioactive with a half-life greater than 3.1 × 10-22 years), 57Fe (2.119%) and 58 Fe (0.282%). The standard atomic mass of iron is 55.845(2) u.

The most stable of the iron radioisotopes is 60Fe with a half-life of 1.5 million years, followed by 55 Fe (2.7 years), 59 Fe (just under 44.5 days) and 52Fe (8.5 hours).

Occurrence and natural abundance

Iron is the most abundant metal in meteorites as well as in the core of planets, such as Earth’s.

Mineral iron is present in nature in pure form or more rarely in the form of alloy with nickel (5 to 18%) of meteorite origin but also in the form of terrestrial iron called “telluric”. Too rare and especially disseminated, it is artificially manufactured by blacksmith and steelmaker man and massively in some Caucasian civilizations for more than three millennia from its main minerals. Chemical and mineral combinations involving iron are plethoric, but true relatively pure ores with high iron content are much less common and often very localized in iron mines mostly known from high antiquity.

Iron is the 6th most abundant element in the Universe, it is formed as the “final element” of nuclear fusion, by fusion of silicon in massive stars. While it makes up about 5% (by mass) of the Earth’s crust, the Earth’s core is thought to be largely an iron-nickel alloy, making up 35% of the Earth’s mass as a whole. The element is perhaps, in fact, the most abundant element on Earth or at least comparable (in just 2nd position) by mass to oxygen, but only the 4th most abundant element in the Earth’s crust.

Convection currents in the outer layer of the Earth’s core (outer core), of the mainly iron-nickel liquid “alloy”, are thought to be the source of the Earth’s magnetic field.

Functions in the biosphere

Iron plays a major role as a trace element or micronutrient for many species, and as an element regulating the magnitude and dynamics of oceanic primary productivity, making it an essential component of marine biogeochemical cycles and marine carbon sinks.

Recent data show that the oceanic iron cycle initially thought to be linked to iron-rich dust inputs is actually much more complex, and closely coupled biogeochemically with major nutrients (carbon, nitrogen). It was shown in 2017 that in iron-poor areas of Antarctica, particulate iron from glacier planing rocks is an alternative source of iron that phytoplankton can exploit. Studies have shown that some phytoplankton do seem to benefit from a high level of CO2, but to assimilate this CO2 they also need iron; it has been speculated since the late twentieth century that seeding the ocean with iron could help limit climate change. However, it is discovered that in most phytoplankton species, this iron is only assimilable in the presence of carbonates. Problem: the latter are destroyed by acidification induced by the solubilization of CO2 in water.

Single body

Iron reveals a metallic polymorphism. Allotropy nevertheless applies a basic change in the procession of physical properties (expansion, resistivity, specific heat related to the crystallochemical structure, etc.).

Physical properties

It is a metal that, depending on the temperature, has an obvious metallic polymorphism. Allotropy distinguishes:

  • Under normal conditions of temperature and pressure, i.e. at low temperatures or “at low temperatures”, a crystalline solid with a centered cubic structure (iron α, a structure called ferrite in steel). Iron α is highly ferromagnetic: the magnetic moments of the atoms align under the influence of an external magnetic field and retain their new orientation after the disappearance of this field. Its Curie temperature is 770 °C. Its heat capacity is 0.5 kJ kg−1 °C−1. At room temperature, it has a hardness between 4 and 5 on the Mohs scale. Its density is around 7.86 g cm−3 at 20 °C. Alpha iron is characterized by an atomic sublimation heat equivalent to 99.6 kcal/atom-gram at room temperature (298 K);
  • Iron β beta is a face-centered cubic structure obtained above the Curie point, around 770 °C or 1,042 K. The ferromagnetism of α iron disappears without atomic rearrangement;
  • From high temperatures at ambient pressure, from 912 °C, α iron becomes a face-centered cubic iron (γ iron, called structure or austenite in steel), the transformation involves an internal energy variation of about 0.22 kcal/atom-gram at 1,184 K. Iron γ is paramagnetic;
  • Above 1,394 °C or 1,665 K, it again becomes a centered cubic mesh mineral (iron δ); This transformation implies a variation of internal energy of about 0.27 kcal / atom-gram;
  • The transformation into iron ε (compact hexagonal structure) occurs at room temperature at 130 kilobars.

The pure substance melts at 1,538 °C with a latent heat of fusion of the order of 3.7 kcal/atom-gram. The boiling of iron, characterized by a latent boiling heat of the order of 84.18 kcal/atom-gram appears around 2,860 °C, in practice for a simple body more or less impure between 2,750 °C and 3,000 °C.

Chemical properties

Iron is insoluble in water and bases. It is attacked by acids.

Iron chemistry

Iron has essentially three degrees of oxidation:

  • 0 in the single-body iron and its alloys;
  • +II in ferrous compounds (ferrous ion Fe2+ in ionic compounds);
  • +III in ferric compounds (ferric ion Fe3+ in ionic compounds).

In carbides, the degree of oxidation of iron is not uniquely definable. There are three known formulas Fe 3 C, Fe 5 C 2 and Fe7C3.

Metal oxidation

Iron, combined with oxygen, oxidizes, under three iron oxides conditions:

  • Iron(II) oxide FeO (“ferrous oxide“);
  • Iron(III) oxide Fe2O3 (“ferric oxide“);
  • Iron(II,III) oxide Fe3O4 (“magnetic oxide“).

In the open air in the presence of moisture, it corrodes forming rust, consisting of hydrated oxides and ferric oxyhydroxides, which can be written Fe2O3· nH2O and FeO(OH)· nH2O respectively. Since rust is a porous material, the oxidation reaction can spread to the core of the metal, unlike, for example, aluminum, which forms a thin layer of impermeable oxide.

Mössbauer spectroscopy provides a powerful tool for distinguishing different degrees of iron oxidation. With this technique, it is possible to make a quantitative analysis in the presence of a mixture of iron phases.

Iron ions in an aqueous solution

Iron in rocks make river red
The reddish-orange coloration of this river is due to the ferric ion, Fe(III) or Fe3+, in the rocks

The reddish-orange coloration of this river is due to the ferric ion, Fe(III) or Fe3+, in the rocks.

In an aqueous solution, the chemical element is present in ionic form with two main valences:

  • Fe2+ (iron(II) ion, formerly called ferrous). Depending on the chemical environment in solution, it can take on different colors. The solution obtained by dissolving Mohr salt, for example, has a pale green color. Such a solution is stable for pH below 6. For a pH above this value, iron(II) hydroxide Fe(OH)2precipitates;
  • Fe3+ (iron(III) ion, formerly called ferric). Iron(III) chloride solutions are orange, and iron(III) nitrate solutions are colorless. These solutions must have a pH below 2 because iron(III) hydroxide Fe(OH)3 is poorly soluble.


A number of ions lead to the precipitation of iron ions in solution. The hydroxide ion HO is one of these (see above). The sulfide ion S 2 forms iron(II) sulfide FeS, iron(III) sulfide and Fe2S3 for pH not too acidic. It is indeed necessary that a reasonable amount of sulfide ions is present, which is not the case at acidic pH since the sulfide ion is then in its diacidal form, hydrogen sulfide H2S.

Iron ion redox

The reference potentials of iron pairs are:

Fe2+ / Fe: E° = −0.44 V

Fe3+ / Fe2+: E° = +0.77 V

This indicates that metallic iron is not stable in an aqueous medium. It oxidizes all the faster the lower the pH.

This also indicates that in the presence of dissolved oxygen (E°(O 2 / H2O) = 1.3 V), iron(II) ions are not stable either.

These reference potentials change with the ions present in solution, especially if the stability constants of the complexes corresponding to Fe(II) and Fe(III) are significantly different.

Redox is a way of titrating iron(II) ions, for example by cerium(IV) ions (couple Ce 4+ / Ce3+) or by permanganate ions MnO 4 (couple MnO4 / Mn2+ in sulfuric acid medium).

Although the reduction of iron ions to metallic iron is possible, it is rarely practiced from an aqueous solution.

Complexation of iron ions

Many iron complexes in aqueous solution are easily formed, by simply adding the ligand (at the right pH). Among the most common complexes are those involving ligands:

  • Cyanide ion CN

For Fe(II): Fe(CN)64−, hexacyanoferrate(II) ion, diamagnetic, yellow;

For Fe(III): Fe(CN)63−, hexacyanoferrate(III) ion, paramagnetic, orange;

These complexes are used to prepare Prussian blue;

  • Fluoride ion F

For Fe(III): FeF2+, colorless fluorofer(III) ion

In analytical chemistry, this complex is used to mark the color of iron(III) ions;

  • 1,10-phenantroline (O-phen for short)

For Fe(II): Fe(ophen)32+, red, triorphophenantrolinefer(II) ions

For Fe(III): Fe(ophen)33+, green, triorphophenantrolinefer(III) ions

The redox couple consisting of these two complexes is used as an indicator of redox titration;

  • Thiocyanate ions SCN

For Fe(III): Fe(SCN)2+, blood red, thiocyanatofer(III) ion

This complex makes it possible to highlight small quantities of iron(III) ion in solution thanks to its characteristic color.

Organometallic chemistry

The first organometallic complex isolated as such, in 1951, was an iron complex: ferrocene. It consists of an iron(II) ion with two cyclopentadienyl ions C 5 H5. Many other complexes have been produced since then, either derived from ferrocene or of a completely different nature.


Most of the iron in the crust is combined with oxygen, forming iron oxide ores, such as hematite (Fe 2 O 3), magnetite (Fe 3 O 4) and limonite (Fe 2O 3· nH2O). Magnetic oxide or magnetite Fe3O4 has been known since Greek antiquity. It takes its name from Mount Magnetos (the great mountain), a Greek mountain particularly rich in this mineral.

About one meteorite in twenty includes taenite, the only alloy of iron-nickel mineral (iron 35-80%), and kamacite (iron 90-95%). Although rare, iron meteorites are a source of nickel-plated iron, this meteoric iron arrived on the earth’s surface being at the origin of the steel industry in the etymological sense; The other natural source of slightly nickel-plated iron metal are deposits of telluric iron or native iron of mineralogists which are rarer.

The red color of the surface of Mars is due to a regolith rich in amorphous hematite; The Red Planet is something of a “rusty planet”.

90% of the world’s iron ore deposits are retained in a thin layer very rich in Fe(II), the ribboned iron layer. In early life, at the Archean aeon c. −2 to −4 Ga, cyanobacteria lived in Fe(II) oceans. When they begin to photosynthesize, the oxygen produced is dissolved and reacts with Fe(II) to form Fe(III) oxides that precipitate at the bottom of the oceans. After consumption of Fe(II), oxygen concentrates in the oceans and then in the atmosphere, it then constitutes a poison for proto-life. Thus, the deposits of ribboned iron are systematically found between the geological layers of the crystalline massifs (schists, gneiss, etc.) and the dolomitic limestone layers (corals) constituting the pre-Alpine massifs.

History of iron metallurgy

Iron was known from the Chalcolithic through telluric iron sites and especially iron meteorites often already alloyed of high quality, and it is not certain that its metallurgy remained confidential as is often estimated until the twelfth century BC, an era that marks, precisely, the beginning of the “Iron Age”: around the fifteenth century BC the Hittites, in Anatolia, had developed a fairly good mastery of iron work, their tradition determining its origin in the Caucasus region, and this technique also seems to have been known quite early in northern India, especially in Uttar Pradesh.

In the Hellenistic world iron is the attribute of Hephaestus, the Greek god of metallurgy and volcanoes. Among the Romans, always forged by Vulcan, Italic avatar of Hephaestus, it is a princely attribute of Mars. Alchemists named iron after Mars, God of war in Roman mythology.

Until the middle of the Middle Ages, Europe refined iron by means of low furnaces, which do not produce pig iron; the blast furnace technique, which produces pig iron from charcoal and iron ore, was developed in China in the middle ofthe fifth century BC. It is common in Western Europe from the middle of the fifteenth century.

The West is independently reinventing technology more than a thousand years after China. According to the ancient doxographer Theophrastus, it was Delas, a Phrygian, who invented iron.

The minute changes in the pieces of solid metal obtained by the physical labor of the blacksmith (hammering, heating, surface alloys, etc.) are very little important to the chemist. Iron chemistry largely forgets the extremely fine appreciation of blacksmiths or forge ponds during the long technical history of iron.

Iron industry

Iron ore mining

The main iron ore-producing countries in 2013 were:

CountryOre (million tonnes)% world iron oreIron content (million tonnes)% world iron content
1China1 450,045,9436,029,5
6South Africa71,532,345,73,1
8United States53,01,732,82,2
World Total3 1601001 480100

The world’s leading iron ore producers in 2008 were:

  • BHP Billiton and Rio Tinto (39.6% of the estimated global market in 2008, in the event of a merger);
  • Vale (ex-CVRD) (Brazil) (35.7%);
  • Rio Tinto (24% alone);
  • BHP Billiton (16% alone);
  • Fortescue (5.4%);
  • Kumba (5.2%);
  • Other (LKAB, SNIM, CVG Ferrominera, Hierro Peru, Kudremukh, CAP) (13.7%).

In 2007, China produced one-third of the world’s steel and 50% of iron ore exports.


Most iron-based metals are magnetic. This property simplifies their sorting. In the second half of the twentieth century, the low cost of scrap made electric steel mills more competitive than blast furnaces.


Iron is obtained industrially by reducing the iron oxides contained in the ore by carbon monoxide (CO); this can be achieved since the Iron Age, and until the nineteenth century in some parts of the world, by reducing the ore with charcoal in a low furnace or low hearth. One obtains, without going through a liquid phase, a heterogeneous mass of iron, steel, or even cast iron, mixed with slag, called “magnifying glass”, “cutter” or “iron sponge”. In order to make the metal suitable for the elaboration of objects, the “magnifying glass” can be broken and sorted by type of carbon content or more simply be directly compacted at the forge.

It is with the development of mills and hydraulic power that the technical line of the blast furnace was able to develop and was globally imposed to the detriment of that of the low furnace. The main difference in this process is that the reduction of iron oxides is done at the same time as melting. The metal is produced in the liquid phase as cast iron which has absorbed some of the carbon in the coke and melts more easily than iron (lower melting temperature of at least 200°C). But the cast iron will then have to be transformed into iron.

It is also by adding silica to the limestone gangue ore, or limestone to the siliceous gangue ore, that we passed to the blast furnace: a precise proportion of silica and limestone gives an easily fusible slag that separates naturally from the liquid pig iron. For a long time blast furnaces ran on charcoal. Coke, harder and more abundant, made it possible to make much higher blast furnaces but producing a sulfur-laden pig iron.

To obtain a forgivable metal, it is necessary to refine the cast iron. This step, carried out in a steel mill, essentially consists of decarburizing the cast iron to obtain a lower carbon alloy: iron or steel. The cast iron is transformed into steel at the converter. In this tank, oxygen is blown on or into the cast iron to remove carbon.

If the removal of carbon by combustion with oxygen is the main step in the refining of pig iron, the steel mill will also:

  • Remove sulphur from the coke loaded into the blast furnace; by injecting calcium carbide, magnesium and/or soda, sulfur forms sulphides that float among the slag of the pig iron; This slag will then be removed using a scraper;
  • Burn the silicon dissolved in the cast iron; This combustion is the first chemical reaction that occurs in a converter; it is immediately followed by carbon combustion;
  • Remove phosphorus from the ore; like sulfur, this other embrittling element, is carried out by reaction with lime in the converter, to form P2O5 which, going into the slag, will be removed by separation from the liquid iron; the dephosphorization reaction is the third and last chemical reaction sought in the converter.

In some cases, the abundance of natural gas or the difficulty of adapting iron ore to the blast furnace has led to the adoption of the so-called “direct reduction” route. The principle is to reduce the iron present in the ores without going through the melting step (as in the blast furnace), using reducing gases obtained from hydrocarbons or coal. A large number of processes have been developed. In 2010, 5% of the steel produced was made from iron obtained by direct reduction.


Pure fragments (more than 99.97%) of iron, refined by electrolysis, next to a cube of 1 cm3 of high purity iron (99.9999%), for comparison (this is distorted due to polishing of the cube alone).

Iron is practically not used in its pure state (except to solve certain weldability problems, especially on stainless steels). Cast iron and steel (1,000 Mt) are the main alloys:

  • Cast iron contains 2.1% to 6.67% carbon;
  • Steel contains 0.025% to 2.1% carbon, iron being the main element in its composition;
  • Below 0.025% carbon, we speak of “industrial irons”.

The addition of various additional elements makes it possible to obtain special castings and steels, but the element with the greatest impact on the properties of these alloys remains carbon.

Stainless steels owe their corrosion resistance properties to the presence of chromium which, by oxidizing, will form a thin protective film.


The name “wire” does not mean pure iron wire, the wires are actually made of mild steel, very malleable.

Metallic iron and its oxides have been used for decades to fix analog or digital information on appropriate media (magnetic tapes, audio and video cassettes, floppy disks). However, the use of these materials is now supplanted by compounds with better permittivity, for example in hard drives.

Iron biochemical use

Iron is an indispensable element in the human body. In the very first years of a child’s life, the need for dietary iron is very important, under the penalty of dietary deficiency (iron deficiency anemia). In addition, an iron overdose is also harmful to health. Indeed, too much iron would increase the risk of hepatitis, cancer, and could be involved in Parkinson’s disease.

Bioinorganic complex

Blood hemoglobin is a metalloprotein consisting of an iron(II) complex. This complex allows red blood cells to carry oxygen from the lungs to the body’s cells. The solubility of oxygen in the blood is indeed insufficient to effectively supply the cells. This complex consists of a Fe(II) cation complexed by the four nitrogen atoms of a porphyrin and by the nitrogen of a histidine residue belonging to the protein chain. The sixth site of iron complexation is either vacant or occupied by an oxygen molecule.

It is notable that iron(II) binds a molecule of oxygen without being oxidized. This is due to the encumberment of iron by the protein.

In food

Iron is a trace mineral and is one of the essential mineral salts found in food, but can be toxic in some forms. Iron deficiency is a source of anemia and can affect the cognitive and socio-emotional development of the child’s brain or exacerbate the effects of certain poisonings (lead poisoning for example).

Iron is essential for the transport of oxygen and the formation of red blood cells in the blood. It is an essential constituent of mitochondria, since it is part of the heme composition of cytochrome c. It also plays a role in making new cells, hormones and neurotransmitters. The iron contained in plants (so-called “non-heme” iron) Fe 3+ or ferric iron is less well absorbed by the body than that contained in raw foods of animal origin (“heme” iron) Fe2+ or ferrous iron.

Cooking meats transforms some of the heme iron into less bioavailable, non-heme iron. However, iron absorption is promoted if it is consumed with certain nutrients, such as vitamin C or lemon juice. Putting lemon juice on your food is therefore an excellent culinary habit if you lack iron; on the other hand, a vitamin C supplement is useless if you do not suffer from vitamin C deficiency (extreme deficiency is scurvy), even if it cannot lead to hypervitaminosis since vitamin C is water-soluble (and therefore its surplus is eliminated through sweating and the urinary tract). Like beef, insects are a good source of iron.

On the other hand, its absorption is inhibited by the consumption of tea and/or coffee because tannins (polyphenols) are iron chelators. This is why it is recommended that people at risk (adolescents, pregnant women, women of childbearing age, vegetarians) and tea or coffee drinkers drink it one hour before the meal or two hours after.

The accumulation of iron in the body leads to cell death. Inserm researchers suspect, because of this, that excess iron could be involved in the degeneration of neurons in patients with Parkinson’s disease.

For menopausal women and adult men, the recommended daily allowance of iron is 10 mg; This nutritional requirement is 16 to 18 mg for the woman from puberty to menopause.

In pharmacies

Iron is used as a medicine. It is used in cases of iron deficiency (called “iron deficiency”) that can cause asthenia or even iron deficiency anemia. It can be given orally or as an injection.

Global production of iron

World iron ore production amounted to 2.4 billion tonnes in 2010, largely accounted for by China (37.5%), followed by Australia (17.5%), Brazil (15.4%), India (10.8%), Russia (4.2%) and Ukraine (3.0%); global iron ore reserves are estimated at 180 billion tonnes, containing 87 billion tonnes. of tonnes of iron, and are held mainly by Ukraine (16.7%), Brazil (16.1%) and Russia (13.9%). China produced 60% of the world’s metallic iron in 2010 (about 600 million out of 1 billion tons) and 45% of the world’s steel (about 630 million out of 1.4 billion tons), ahead of Japan (8.2% of the iron and 7.9% of the steel produced in the world).

References (sources)