Chemical element · reference page

26Fe

Iron

Iron is element 26 (Fe), located in period 4, group 8 and the d block. At room temperature its reference phase is solid. Its electron configuration is [Ar] 3d6 4s2.

Atomic number26

Standard atomic weight55.845(2)

Group8

Period4

Blockd

Electron configuration[Ar] 3d6 4s2

Electronic structure

What controls its chemistry

The d subshell participates in the chemistry of iron. Transition metals can use electrons of similar energies in bonding, which is why variable oxidation states, coordination compounds and catalytic behaviour are common across the d block.

Its electron configuration, [Ar] 3d6 4s2, should be read together with the oxidation state of the compound: removing electrons changes which d orbitals are occupied and can alter colour, magnetism, bonding and reactivity.

Chemical forms

Compounds, ions and materials

For iron, important chemical forms include metals and alloys, oxides, halides and coordination compounds. Variable oxidation states are common because s and d electrons have similar energies, and ligands around the metal ion can strongly alter colour, magnetism and reactivity.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 7.874 g/cm³. The melting point is 1 811 K (1 538 °C). The boiling point is 3 134 K (2 861 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where iron is found and used

In practice, iron is encountered in metallurgy and steel, construction and biology.

In practice

Examples of real uses

Metallurgy and steel

Iron is used in iron and steel processing, either as an alloying element, a refining aid or part of the ore-to-metal chemistry.

Construction

Iron is used in structural materials, coatings, cement chemistry or alloys chosen for mechanical durability.

Biology

Iron ions or compounds participate in biological structures, enzymes, signalling or metabolism.

Periodic position

What its neighbours tell us

Iron is in period 4 and group 8. In the neighbourhood shown here, it lies between manganese and cobalt in the same period; ruthenium is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationtransition metal

Phase at room temperatureSolid

Electron shells2 · 8 · 14 · 2

Density7.874 g/cm³

Melting point1 811 K (1 538 °C)

Boiling point3 134 K (2 861 °C)

Pauling electronegativity1.83

First ionisation energy762.5 kJ/mol

Electron affinity14.79 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

Use of iron is recorded from about 5000 BC. This predates modern chemistry, so early use, chemical identification and the later atomic-number definition are separate stages.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 55.845(2). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Iron has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with iron depend on chemical form, dose and route of exposure. Pure iron, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.