Iridium (Ir)

Iridium (symbol Ir) is the chemical element with atomic number 77: a transition metal, solid at 0 °C, with an atomic mass of 192.22 u. Discovered in 1803 (Smithson Tennant).

Iridium is the second densest element and the metal that best resists corrosion. Rare on Earth but more abundant in meteorites, it helped prove that a giant asteroid struck Earth 66 million years ago. You'll find it in premium spark plugs and hydrogen electrolyzers.

Key facts

Atomic number: 77
Symbol: Ir
French name: Iridium
Atomic mass: 192.22 u
Molar mass: 192.22 g/mol
Family: Transition metals
Position: period 6, group 9, d-block
State at 0 °C: Solid
Melting point: 2,445.9 °C
Boiling point: 4,129.9 °C
Density: 22.56 g/cm³
Electronegativity (Pauling): 2.2
First ionization energy: 865 kJ/mol
Atomic radius: 135 pm
Electron configuration: [Xe] 4f¹⁴ 5d⁷ 6s²
Abundance in Earth's crust: 0.001 ppm (80th most abundant element)
Indicative price: 140,000 $/kg (order of magnitude)
Radioactive: no
Discovery: 1803, Smithson Tennant

Origin of the name

From the Greek iris, rainbow, for its colorful salts.

What does iridium look like?

A very hard, brittle, silvery-white metal with a slight yellowish tinge and a density of 22.56 g/cm³.

Where is iridium found?

About 1 part per billion of Earth's crust, because most of the planet's iridium sank with iron into its core. It occurs with platinum in deposits such as South Africa's Bushveld Complex.

Natural sources

Osmiridium
Iridosmine
Native platinum

How is iridium produced?

A by-product of platinum and nickel mining. It is separated from the other platinum-group metals through a complex series of dissolutions and precipitations. World production is only a few tonnes a year.

Top producers

South Africa
Russia
Zimbabwe

What is iridium used for?

Electrode tips of long-life spark plugs.
Anodes of PEM electrolyzers producing green hydrogen.
Crucibles for growing laser and electronics crystals.
Iridium-192, a radiation source for industrial radiography and brachytherapy.
OLED screens (phosphorescent iridium compounds).

In everyday life

An “iridium” car spark plug
A smartphone's OLED screen
A fountain-pen nib tip

Role in living things

No biological role; the metal is very low in toxicity.

Hazards of iridium

The metal is inert. The iridium-192 used in industrial radiography is an intense radioactive source: lost or stolen sources have already caused serious accidents.

Cosmic origin

Neutron stars: Created when two neutron stars collide — the “forge” of gold.

History of iridium

Discovered in 1803 by Smithson Tennant in the insoluble residue of platinum, at the same time as osmium. He named it after Iris, goddess of the rainbow, because of the varied colors of its salts.

Did you know?

In 1980, Luis and Walter Alvarez found a thin, iridium-rich clay layer dating back 66 million years: the signature of the Chicxulub asteroid.
The 1889 standard meter and kilogram were made of a 90% platinum – 10% iridium alloy.
In 2014, iridium was observed in the +9 oxidation state, a record for any element.

Isotopes of iridium

Natural iridium is a mix of two stable isotopes, 191 and 193. Iridium-192, made in reactors, is one of the most widely used gamma sources in industry and hospitals.

iridium-191: 37.3% of natural iridium, stable
iridium-192: radioactive, half-life 73.8 days. A gamma-ray source for radiographing welds and treating certain cancers (brachytherapy).
iridium-193: 62.7% of natural iridium, stable

Related elements

Same family (transition metals): Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Yttrium (Y), Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Technetium (Tc), Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Silver (Ag), Cadmium (Cd), Hafnium (Hf), Tantalum (Ta), Tungsten (W), Rhenium (Re), Osmium (Os), Platinum (Pt), Gold (Au), Mercury (Hg), Rutherfordium (Rf), Dubnium (Db), Seaborgium (Sg), Bohrium (Bh), Hassium (Hs).

Same column (group 9): Cobalt (Co), Rhodium (Rh), Meitnerium (Mt).