Iron(Fe)
Iron is a transition metal central to steel and to biological oxygen transport as part of haem.
KEY PROPERTIES
Electron configuration & shells
[Ar] 3d⁶ 4s²
Shell model is schematic — see lesson on atomic structure for orbitals. Oxidation states: +2, +3. Common ions: Fe²⁺, Fe³⁺.
Measured properties
Values at stated conditions; missing data shown as “—”. Radii distinguished by type; densities at stated phase/temperature.
- Electronegativity (Pauling)
- 1.83
- First ionization energy
- 763 kJ·mol⁻¹ (first ionization, gas phase)
- Atomic radius
- 156 pm (calculated)
- Covalent radius
- 132 pm (covalent)
- Van der Waals radius
- 194 pm (van der Waals)
- Density
- 7.874 g·cm⁻³ (solid at 20 °C, 1 atm)
- Melting point
- 1811 K (1 atm)
- Boiling point
- 3134 K (1 atm)
Standard conditions: 1 atm unless noted. Values with null are not established or not comparable across allotropes. Radii are distinguished by type. Densities at stated phase/temperature.
| Mass number | Abundance | Stability | Half-life |
|---|---|---|---|
| 56 | trace/synthetic | stable | — |
Isotopic abundances are natural terrestrial values where applicable; synthetic isotopes shown as trace.
Occurrence & uses
Fourth most abundant element in Earth's crust; occurs as hematite and magnetite.
- Steel
- Construction
- Biological oxygen transport
BIOLOGICAL ROLE
Biologically relevant — see uses/occurrence for context.
Discovery
Ancient — known since antiquity — ancient
Safety
Compounds featuring Fe
Reactions involving Fe
RELATED CONTENT
SOURCES
- [nist-atomic-weights]nist-atomic-weights (2022)/#nist-atomic-weights
- [nist-2022]nist-2022 (2022)/#nist-2022
- [iupac-gold]iupac-gold (2022)/#iupac-gold
Last reviewed: 2026-08-22 • Reviewer: dr-example