Iridium (Ir)
“The one metal even aqua regia cannot touch — and the tell-tale fingerprint of the asteroid that ended the dinosaurs.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- The most corrosion-resistant metal known — no acid, alkali, seawater or even aqua regia attacks it at room temperature, which is why the original international metre bar was cast in platinum–iridium.
- Density about 22.56 g/cm³, a whisker behind osmium for the elemental record, and it refuses to melt until near 2,450 °C — hard, stiff, and almost impossible to machine without shattering.
- Electron configuration [Xe] 4f¹⁴ 5d⁷ 6s² — period 6, group 9, d-block; its partly filled 5d shell lets it wear oxidation states from −3 up to +9, the +9 state being the highest verified for any element.
- Scarce in the crust because most iridium sank into the core with iron as the young Earth melted through; the crust’s share was topped up later by meteorites — which is exactly why an iridium spike fingerprints an asteroid impact.
A hard, brittle, silvery-white metal of startling heft — a matchbox brimful of it would weigh over half a kilogram. Its mirror-bright surface shrugs off every ordinary assault: acids, seawater and even aqua regia, the acid mixture that dissolves gold and platinum, leave iridium untouched.
Where you meet it
✦ Uses
- Long-life spark plugs for car and CNG engines — an iridium-alloy tip survives tens of millions of ignitions that would burn an ordinary steel electrode away.
- Crucibles, dies and rods for growing single crystals of sapphire for LED lamps, because nothing corrodes iridium even at white heat.
- Geology’s dinosaur-detection kit: the 66-million-year-old K–Pg boundary clay is identified — at Gubbio in Italy and worldwide — by the iridium anomaly the Alvarez team measured in 1980.
- Extreme-duty parts: fountain-pen nib points, electrical contacts, compass bearings and the electrodes of high-performance ignition systems where wear is unforgivable.
◍ Everyday examples
- Many a tuned-up CNG car in Bangladesh rides on iridium spark plugs, whose tips keep sparking cleanly for a hundred thousand kilometres and more.
- The thin dark band in cliff exposures of the 66-million-year-old boundary clay — the famous “dinosaur extinction layer” — is trailed by geologists precisely for its iridium spike.
- The platinum jewellery sold in Dhaka showrooms is routinely hardened with iridium, a platinum-group cousin, so rings keep their polish for a lifetime.
History & name
In 1803 in London, Smithson Tennant poured aqua regia over raw platinum and studied the stubborn black grains that others dismissed as charcoal; by 1804 he had shown the residue held two new metals, osmium and iridium. He named the second one after Iris, the Greek goddess of the rainbow, because its compounds flash vivid, unexpected colours. A century and three-quarters later the metal turned detective: in 1980 the team of Luis and Walter Alvarez found that the 66-million-year-old clay layer marking the dinosaurs’ extinction is freakishly rich in iridium — scarce in Earth’s crust but plentiful in asteroids — the crucial clue that a giant impact ended the Cretaceous.