Transition metalElement 26 · d-block

Iron (Fe)

Iron (Fe) is a transition metal with atomic number 26 and a standard atomic weight of 55.845. At room temperature it is a solid that melts at 1,538 °C and boils at 2,861 °C.

26FeIron55.845

Atomic number

protons

Atomic weight

55.845

CIAAW 2024

Melting point

1,538 °C

1,811.15 K

Boiling point

2,861 °C

3,134.15 K

Density

7.87 g/cm³

near room temperature

Electronegativity

1.83

Pauling scale

State at 25 °C

Solid

Discovered

Prehistoric

Iron at a glance

Identity

Symbol	Fe
Atomic number (Z)	26
Atomic weight	55.845 ± 0.002 (CIAAW 2024)
Category	Transition metal
Group	8
Period	4
Block	d-block
CAS number	7439-89-6

Atomic properties

Electron configuration	[Ar] 3d⁶ 4s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
Electrons per shell	2, 8, 14, 2
Oxidation states	+2, +3 Less common: -4, -2, -1, 0, +1, +4, +5, +6, +7
Electronegativity (Pauling)	1.83
First ionisation energy	7.902 eV (762.5 kJ/mol) 2nd: 16.199 eV · 3rd: 30.651 eV
Electron affinity	0.151 eV
Atomic radius (empirical)	140 pm
Covalent radius	116 pm
Van der Waals radius	204 pm

Physical properties

State at 25 °C	Solid
Melting point	1,538 °C · 2,800.4 °F · 1,811.15 K
Boiling point	2,861 °C · 5,181.8 °F · 3,134.15 K
Density	7.87 g/cm³
Specific heat capacity	0.449 J/(g·K)
Thermal conductivity	80.4 W/(m·K)
Crystal structure	Body-centred cubic

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	⁵⁶Fe 91.754%, ⁵⁴Fe 5.845%, ⁵⁷Fe 2.119%, ⁵⁸Fe 0.282%
Stable isotopes	4
Known isotopes	32
Abundance in Earth's crust	56,300 mg/kg
Abundance in seawater	0.002 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Iron electron configuration

Iron has 26 electrons. Written in short form, its ground-state configuration is [Ar] 3d⁶ 4s² — the electrons of the noble gas argon, plus 3d⁶ 4s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s².

3d↑↓↑↑↑↑

4s↑↓

Discovery and name

Discovered

Prehistoric — Known since antiquity.

Name

Anglo-Saxon iren; the symbol Fe is from Latin ferrum.

Known for

The most-used metal: steel is iron with a little carbon. Earth's core is mostly iron, and haemoglobin uses iron to carry oxygen in the blood.

Isotopes of iron

Naturally occurring iron is a mix of 4 isotopes. Each has 26 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
⁵⁶Fe	26	30	91.754%	stable
⁵⁴Fe	26	28	5.845%	stable
⁵⁷Fe	26	31	2.119%	stable
⁵⁸Fe	26	32	0.282%	stable

32 isotopes of iron are known in our data set (half-lives as published in nuclear data tables).

Iron compared with group 8

The other elements in group 8, from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Iron	26	55.845	1,538 °C	2,861 °C	7.87 g/cm³	1.83
Ruthenium	44	101.07	2,333 °C	4,147 °C	12.1 g/cm³	2.2
Osmium	76	190.23	3,033 °C	5,008 °C	22.587 g/cm³	2.2
Hassium	108	[269]	—	—	40.7 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of iron?

The standard atomic weight of iron is 55.845 ± 0.002 (CIAAW 2024), so one mole weighs about 55.845 grams.

How many protons, neutrons and electrons does iron have?

A neutral iron atom has 26 protons and 26 electrons. Its most common isotope, iron-56, has 30 neutrons.

What is the electron configuration of iron?

[Ar] 3d⁶ 4s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²).

What are the melting and boiling points of iron?

Iron melts at 1,538 °C (2,800.4 °F) and boils at 2,861 °C (5,181.8 °F).

Is iron a metal?

Iron is a metal. It is in group 8, period 4, and is a solid at room temperature.

Who discovered iron?

Iron has been known since ancient times.