Atomic number 26

Iron

Fe
+ phase
− phase

Shells Orbitals 3d 4s

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26 electrons in 4 shells: 2, 8, 14, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 4
Group: 8
Block: d
Standard atomic weight: 55.845(2) Da
Electron configuration: [Ar] 3d⁶ 4s²
Phase at 25 °C: solid
Melting point: 1538 °C
Boiling point: 2861 °C
Density: 7.874 g/cm³
Electronegativity (Pauling): 1.83
Discovered: Prehistoric times

Neighbours in the table

Mn Co

Iron has four shells, holding 2, 8, 14 and 2 of its 26 electrons, written [Ar] 3d⁶ 4s². From scandium onward each added electron enters the 3d subshell, which makes iron a transition metal. The 4s orbital filled first even though it belongs to the outer shell. A 3d orbital keeps less of its electron density near the nucleus than the 4s does, so it lies higher in energy. Ionisation runs the other way: the 4s pair is easier to strip, so Fe²⁺ has lost both 4s electrons and keeps all six in 3d.

Iron(II) and iron(III) are its two common ions, and OpenStax puts its oxidation states at +2 to +6. Partway along the first transition series it stops being possible to remove every valence electron. Both common ions are stable in water.

The solid is a metallic crystal, nuclei in a sea of delocalised electrons. Those electrons leave easily, which is why iron conducts heat and electricity, and because it bends rather than shatters it serves as a building material. Los Alamos lists four allotropic forms, alpha, beta, gamma and omega, changing over at 700, 928 and 1530 °C, the last just short of the 1538 °C melting point. The alpha form is magnetic; the change to beta removes the magnetism while the lattice stays the same.

Where it is found

On PubChem’s element record iron is 5.63 × 10⁴ milligrams of every kilogram of crust, close to 5.6%. Los Alamos ranks it fourth by weight among the crust’s elements. The core, 2150 miles in radius, is believed to be largely iron with about 10 per cent hydrogen, and iron is a principal component of the meteorites called siderites.

Almost none of it lies uncombined; on Earth iron occurs almost entirely in oxidised forms. It is mined as the oxides hematite, Fe₂O₃, and magnetite, Fe₃O₄. Early people got uncombined metal from the sky instead. The rare iron artefacts surviving from about 4000 to 2500 BC contain iron-nickel alloys that occur only in meteorites, and smelting took thousands more years to develop.

Mines lifted about 2.5 billion metric tons of usable ore in 2024, holding 1.6 billion tons of iron, on the US Geological Survey’s estimate. Australia produced 930 million tons of it. Michigan, Minnesota and Utah, with eight open-pit mines, shipped 98% of the United States’ 48 million tons; US resources are mainly low-grade Lake Superior taconite.

Why it matters

Iron ores lie almost everywhere and carbon reduces their compounds easily, which is why iron reached toolmakers early. In a blast furnace, coke burns at the bottom to carbon dioxide. The gas rises through white-hot coke and is reduced to carbon monoxide, which reduces the iron oxides higher up to the metal. Limestone decomposes to calcium oxide, which binds the ore’s silica into a slag that floats on the iron and shields it from oxidation.

The world made 1.3 billion tons of pig iron and 1.9 billion tons of raw steel in 2024, China alone 990 million tons of steel. Furnace iron carries too much carbon, which would leave it brittle. Commercial oxygen removes much of it during steelmaking.

Rust is a galvanic cell on the surface. Iron is oxidised to Fe²⁺ at a standard potential of −0.44 V while oxygen is reduced to water at +1.23 V, a cell of +1.67 V. In humid air the iron(II) goes on to a hydrated iron(III) oxide, rust, which unlike copper’s patina flakes away and bares fresh metal, so the corrosion continues.

Each of haemoglobin’s four globin chains carries a haem pigment holding one Fe²⁺. Each ion binds one oxygen molecule, so one haemoglobin carries four. A red cell holds about 300 million of them, room for 1.2 billion oxygen molecules.

Properties

Properties of iron Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	26
Standard atomic weight	55.845(2) Da	Da	IUPAC/CIAAW 2024
Melting point	1538 °C	°C	Los Alamos: 1538 °C
Boiling point	2861 °C	°C	PubChem: 3134 K
Density	7.874 g/cm³	g/cm³
Electronegativity (Pauling)	1.83
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
Phase at 25 °C	solid

Safety

Iron as most people handle it, a nail, a pan or a gate, meets no GHS hazard criteria on PubChem’s record, the verdict of 74.1% of its reports. The two statements that remain, flammable solid and self-heating, match what the sheet says of finely divided powder: it can ignite on its own and gives off hydrogen with moisture. Inhaled iron dust irritates the lungs, a workplace matter rather than a kitchen one. The PubChem Laboratory Chemical Safety Summary is the sheet for anyone handling the powder.

Questions

Why is iron magnetic?

Inside the metal are regions called domains, each holding many atoms whose magnetic dipoles are held parallel by a coupling between the atoms. It is quantum-mechanical, and room-temperature heat cannot break it in iron. An outside field lets the aligned domains grow at the expense of the rest, and since heat cannot misalign whole domains, their combined field is large. The iron then stays magnetised: a permanent magnet. Only four other elements, cobalt, nickel, gadolinium and dysprosium, share it.

What is the difference between iron and steel?

Steel is iron with much of its carbon removed and other metals added. Pig iron holds about 3 per cent carbon on Los Alamos's figure, with sulfur, silicon, manganese and phosphorus alongside. Steelmaking takes the surplus out, leaving between 0.04 and 2.5 per cent, then adds manganese, chromium, nickel, tungsten, molybdenum or vanadium for particular properties. Chromium is what keeps a stainless steel from rusting; tungsten keeps a steel hard when it runs hot; vanadium adds strength and springiness.

Why does the body need iron?

Iron sits in haem, the pigment that lets haemoglobin bind oxygen in the lungs. When iron runs short the body cannot make enough haem, red cells or haemoglobin fall, less oxygen reaches the tissues, and the result is the fatigue of iron-deficiency anaemia, which OpenStax calls the most common type. Less than a fifth of the iron in food is absorbed, and haem iron from meat, poultry and fish is taken up better than iron from plants.

How do you stop iron from rusting?

Rust needs water and oxygen together, and paint simply keeps both off the metal. Galvanising seals the iron under zinc while the layer is intact; about half of all zinc produced goes to protecting iron and other metals. Stainless steel guards itself: its chromium collects near the surface and corrodes into a passivating oxide film. A more active metal attached to the iron, a sacrificial anode, corrodes in its place while the iron only conducts electrons.

Why is the symbol for iron Fe?

The symbol abbreviates ferrum, the Latin word, which CIAAW's note on PubChem's record says means firmness. The English name has a separate root, the Anglo-Saxon iron, whose origin is unknown. Nobody discovered it; iron was in use before written history, and the Old Testament names it many times. A wrought-iron pillar about 7.25 m tall and 40 cm across has stood in the weather in Delhi since about AD 400, and Los Alamos reports minimal corrosion.

Compounds containing iron

Fe₂O₃ Iron(III) oxide 5 atoms in one formula unit of an ionic lattice.