Iron

Iron (Fe) is a transition metal with atomic number 26 and an atomic mass of 55.845. Its electron configuration is [Ar] 3d6 4s2.

Iron key data

Symbol	Fe
Atomic number	26
Atomic mass (standard atomic weight)	55.845
Molar mass	55.845 g/mol
Protons	26
Neutrons (most abundant isotope, iron-56)	30
Electrons (neutral atom)	26
Category	Transition metal
Period	4
Group	8
Block	d-block
Noble gas notation	[Ar] 3d6 4s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d6 4s2
Electrons per shell	2, 8, 14, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of iron

From PubChem, the US National Institutes of Health. Public domain.

Melting point	1811 K (1537.9 °C), 26th of 103 by melting point
Boiling point	3134 K (2860.9 °C), 38th of 93 by boiling point
Density	7.874 g/cm³, 41st of 96 by density
State at room temperature	Solid
Electronegativity	1.83 (Pauling), 41st of 95 by electronegativity
Van der Waals radius	194 pm, 67th of 99 by atomic radius
First ionisation energy	7.902 eV (762.4 kJ/mol), 34th of 102 by first ionisation energy
Electron affinity	0.163 eV (15.7 kJ/mol), 55th of 57 by electron affinity
Common oxidation states	+3, +2
Discovered	Ancient

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Iron electron configuration

The full electron configuration of iron is 1s2 2s2 2p6 3s2 3p6 3d6 4s2. With the noble-gas core collapsed it is [Ar] 3d6 4s2, and the outermost subshell is 4s2.

Bohr model of iron (Fe): a nucleus and four shells holding 2, 8, 14 and 2 electrons from the inside out. — Bohr model of iron (Fe). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of iron (Fe), 1s2 2s2 2p6 3s2 3p6 3d6 4s2: one box per orbital, filled by Hund’s rule, showing four unpaired electrons, in 3d. — Orbital diagram of iron (Fe). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one iron atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 55.845 / (6.02214076 × 10²³)
= 9.2733 × 10⁻²³ g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 55.845
= 1.0784 × 10²² atoms

Iron on the periodic table

Iron is in period 4, group 8, in the d-block.

The other elements of group 8 are ruthenium, osmium and hassium.