Iridium

Iridium is the most corrosion-resistant metal known and is almost indestructible under normal conditions. It holds a special place in science: a thin layer of iridium in rock worldwide marks the boundary between the Cretaceous and Palaeogene geological periods: the layer left by the asteroid impact that killed the non-avian dinosaurs.

• Atomic Number7777 protons, 77 electrons
• Atomic Mass192.22 u77× heavier than hydrogen
• State at Room TempSolidSolid
• Density22.42 g/cm³
• Melting / Boiling2445.8°C / 4427.9°C
• Discovered1803

What is Iridium?

Iridium is a hard, brittle platinum group metal with 77 protons and one of the highest densities of any element (22.56 g/cm³). Its name comes from the Latin iris meaning rainbow, because of the many colours of its compounds. Discovered by Smithson Tennant in 1803. Used in the international standard kilogram (the Le Grand K platinum-iridium cylinder) until 2019.

Discovered in 1803 by Smithson Tennant in the same residue from platinum dissolution that yielded osmium. Named after Iris, the Greek goddess of the rainbow, because of the many colours of its salt solutions.

Fact The Cretaceous-Palaeogene (K-Pg) boundary: the layer in rock that marks the mass extinction 66 million years ago, contains up to 160 times more iridium than normal rock. Because iridium is rare in Earth's crust but common in asteroids, this iridium anomaly (discovered by Luis and Walter Alvarez in 1980) is the key evidence that a massive asteroid impact caused the extinction event that ended the dinosaurs.

Where you find Iridium

On Earth

By-product of platinum group metal mining. South Africa and Russia are primary sources.

How we use Iridium

• Spark plug electrodes, iridium tips last far longer than platinum.. Spark plugs
• Crucibles for growing crystals at extreme temperatures.. Laboratory equipment
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Did you know? The Cretaceous-Palaeogene (K-Pg) boundary: the layer in rock that marks the mass extinction 66 million years ago, contains up to 160 times more iridium than normal rock. Because iridium is rare in Earth's crust but common in asteroids, this iridium anomaly (discovered by Luis and Walter Alvarez in 1980) is the key evidence that a massive asteroid impact caused the extinction event that ended the dinosaurs.

How it was discovered

Discovered in 1803 by Smithson Tennant in the same residue from platinum dissolution that yielded osmium. Named after Iris, the Greek goddess of the rainbow, because of the many colours of its salt solutions.

Deeper dive: iridium properties and applications

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Moving to 78 protons on the periodic table takes us to the next element.