Iron
Transition metal · Period 4 · Group 8 · d-block
Forged in dying stars, carried in every red blood cell
- Atomic mass
- 55.845 u
- State at 25 °C
- Solid
- Electronegativity
- 1.83
- Melting point
- 1,537.9 °C
- Boiling point
- 2,860.9 °C
- Configuration
- [Ar] 3d6 4s2
Iron, explained
Video lesson in production
Bhavya’s explainer for this element will play here.
Overview
Iron ([Ar]3d6 4s2) forms compounds mainly in the +2 (ferrous) and +3 (ferric) states, and it switches between them easily. That reversible electron transfer makes iron useful to living cells and to industrial catalysts alike. Iron also matters on a cosmic scale: iron-56 sits near the peak of the nuclear binding-energy curve, so fusing iron-group nuclei releases no energy. When a massive star's core fills with iron, fusion can no longer hold it up, and it collapses into a supernova. That is why iron is so common in the universe, and why it makes up about a third of Earth's mass, most of it in the core.
Pure iron is fairly soft, but adding up to about 2 percent carbon turns it into steel, the most-used metal on Earth, with roughly 1.9 billion tonnes made each year. Iron is ferromagnetic below 770 °C, which makes it the core material of transformers, motors and magnets. Its great weakness is rust: unlike the thin, sealed oxide films on chromium or titanium, hydrated iron(III) oxide is porous and flakes away, exposing fresh metal to further attack.
Discovery
Iron has been known since prehistory. The earliest iron objects, such as Egyptian beads from about 3200 BCE, were hammered from meteorites. Smelting iron from ore spread across Anatolia and the Near East in the second millennium BCE, beginning the Iron Age around 1200 BCE. The symbol Fe comes from the Latin ferrum; the English word iron derives from the Old English īsern, from an older Germanic root.
- Where it’s found
- The fourth most abundant element in the crust (about 5 percent), iron is mined as hematite (Fe2O3) and magnetite (Fe3O4), mostly in Australia, Brazil and China.
- What it’s used for
- Steel construction
- Vehicles and machinery
- Transformer and motor cores
- Cast-iron cookware
- Ammonia synthesis catalyst
- Iron supplements
- In living things
- Iron is essential to nearly all life: hemoglobin and myoglobin carry oxygen on iron atoms, and iron-containing cytochromes and iron–sulfur proteins move electrons in respiration. Iron deficiency is the most common nutritional disorder in the world.
- Safety
- Metallic iron is harmless, though fine iron powder can burn. Accidental overdose of iron supplements is dangerous, especially for young children.
How it compares
Where Iron sits among all 118 elements on the trends that shape the table.
- Electronegativity1.83Pauling
#42 of 100 elements with data
- Ionization energy763kJ/mol
#35 of 104 elements with data
- Atomic radius140pm
#52 of 95 elements with data
- Electron affinity14.8kJ/mol
#78 of 109 elements with data
- Density7.87g/cm³
#56 of 114 elements with data
- Melting point1,810K
#27 of 110 elements with data
- Density
- 7.87 g/cm³
- Atomic radius
- 140 pm
- Covalent radius
- 116 pm
- Molar heat
- 25.1 J/(mol·K)
- Appearance
- lustrous metallic with a grayish tinge
- Discovered by
- 5000 BC
Inside the atom
26 electrons arranged in 4 shells: 2 · 8 · 14 · 2.
Orbital diagram
1s2 2s2 2p6 3s2 3p6 4s2 3d6 · shorthand [Ar] 3d6 4s2
- 1s2
- 2s2
- 2p6
- 3s2
- 3p6
- 4s2
- 3d6
Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Iron bonds.
Ionization energies
Signature reactions
The chemistry Iron is known for.
Redox · Corrosion
Rusting of iron
Iron is oxidized by oxygen in the presence of water, producing hydrated iron(III) oxide, the familiar flaky reddish-brown rust. Salt speeds the process by making the water a better electrolyte. Corrosion costs the world economy trillions of dollars a year.
Conditions: moist air, room temperature
Redox · Metal extraction
Blast furnace reduction
Carbon monoxide, made by burning coke, strips oxygen from iron ore as it descends through the furnace. Molten iron collects at the bottom and is tapped off as pig iron, which is then refined into steel.
Conditions: above 1500 °C in the lower furnace
Redox · Single displacement
Thermite reaction
Aluminium, a more reactive metal, takes oxygen from iron oxide and releases so much heat that the iron comes out molten, above 2,500 °C. Railway workers use the reaction to weld rails together in the field.
Conditions: ignition with a magnesium fuse
Where you’ll meet it
Iron Pillar of Delhi
This 7-meter iron column has stood outdoors in Delhi for about 1,600 years with remarkably little rust.
Photo: Wikimedia Commons
Banded iron formation
Banded iron formations record the oxygenation of Earth's oceans billions of years ago and supply much of today's iron ore.
Photo: Wikimedia Commons
Hemoglobin
Each hemoglobin molecule carries four iron atoms that bind oxygen in the lungs and release it in the tissues.
Photo: Wikimedia Commons
Research
No research on Iron yet
When Bhavya publishes a project that studies Iron, it will be linked here.