Iron
Iron is an abundant transition metal and the principal metallic ingredient of steel.
Iron links bridges and buildings with the oxygen transport in your blood.
Explore Iron's atom- Atomic number
- 26
- Standard atomic weight
- 55.845(2)
- Family
- Transition metal
- Period
- 4
- Group
- 8
- Standard state
- Solid
In everyday life
Steel contains iron; haemoglobin uses iron-containing groups to carry oxygen.
Something to remember
Rust forms when iron reacts in the presence of oxygen and water.
Meet its electrons
[Ar] 4s2 3d6
Published neutral-atom ground-state configuration; excited atoms and ions can differ.
Electrons per shell: 2 · 8 · 14 · 2.
The playground shows an illustrative shell model. Real electrons are described by quantum orbitals, rather than tiny planets traveling along fixed paths.
A connection to explore
Carbon changes the properties of iron to make a wide range of steels.
Meet Carbon (C)Properties and conditions
- Standard atomic weight
- 55.845(2)CIAAW 2024 standard atomic weight (dimensionless). Parentheses give uncertainty in the last quoted digits. Source flags: none.CIAAW standard atomic weights ↗ · Retrieved 2026-10-15
- Standard state
- SolidPubChem classification, including explicit predictions where stated. Reference temperature and pressure are not supplied in the table.PubChem periodic table ↗ · Retrieved 2026-10-15
- First ionisation energy
- 7.902 eVEnergy to remove the first electron from an isolated neutral gas-phase atom. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electronegativity
- 1.83 Pauling scaleA dimensionless relative measure; values depend on the chosen scale. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Melting point
- 1811 KPubChem reported value. Pressure, allotrope and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Boiling point
- 3134 KPubChem reported value. Pressure and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Density
- 7.874 g/cm³PubChem reported value. Temperature, pressure, allotrope and measurement uncertainty are not specified in the table; gas densities are especially condition-dependent.PubChem periodic table ↗ · Retrieved 2026-10-15
- Oxidation states
- +3, +2Selected PubChem oxidation states; formal electron bookkeeping, not a list of stable free-ion charges or every possible compound.PubChem periodic table ↗ · Retrieved 2026-10-15
- Discovery
- AncientPubChem discovery attribution. Ancient knowledge and competing priority claims cannot always be represented by a single year.PubChem periodic table ↗ · Retrieved 2026-10-15
- Atomic radius
- 194 pmPubChem tabulated radius. Radius definitions depend on bonding and convention; measurement definition and uncertainty are not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electron affinity
- 0.163 eVPubChem reported gas-phase electron affinity. Sign conventions and electronic states matter; uncertainty is not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
Isotopes, abundance and decay
Isotopes share a proton count but have different neutron counts. This selection includes naturally tabulated compositions and a reference isotope for other elements. It is not a list of every known isotope.
Explore 4 isotopes of Fe
| Isotope | Relative atomic mass | Natural fraction | Half-life | Decay |
|---|---|---|---|---|
| Fe-54 | 53.93960899(53) | 0.05845(35) | No decay observed | ;2β+ ? |
| Fe-56 | 55.93493633(49) | 0.91754(36) | No decay observed | No decay tabulated |
| Fe-57 | 56.93539284(49) | 0.02119(10) | No decay observed | No decay tabulated |
| Fe-58 | 57.93327443(53) | 0.00282(4) | No decay observed | No decay tabulated |
Fractions use NIST’s terrestrial isotopic compositions, which can vary by sample. Parentheses denote uncertainty in the last digits. Half-lives and decay branches use NUBASE2020; # marks estimates, >/< mark limits. “No decay observed” follows that evaluation and does not prove permanent stability. Decay branch intensities are percentages; SF means spontaneous fission, EC electron capture, IT an isomeric transition.
Follow the science
Use these original references to explore the data and context. Atomic weights can vary with isotope composition; values for elements without stable isotopes may describe a representative isotope rather than a standard atomic weight.