Iridium
transition metalProperties
| Property | Value |
|---|---|
| Atomic Mass | 192.22 amu |
| Category | transition metal |
| Group | 9 |
| Period | 6 |
| Electron Configuration | [Xe] 4f14 5d7 6s2 |
| Electronegativity | 2.2 (Pauling) |
| Oxidation States | 6, 4, 3, 1, -1 |
| Melting Point | 2719 K (2445.8 °C) |
| Boiling Point | 4701 K (4427.9 °C) |
| Density | 22.56 g/cm³ |
| Discovered By | Smithson Tennant (1803) |
About Iridium
Iridium runs neck-and-neck with osmium for densest element (22.56 vs 22.59 g/cm³) and is the most corrosion-resistant metal known — aqua regia barely touches it below 100 °C. It's also vanishingly rare in the crust at about 0.001 ppm, because most of the planet's siderophile iridium followed iron down into the core during differentiation. That scarcity is why Walter and Luis Alvarez's 1980 finding of an iridium-rich layer at the K-Pg boundary was such a clean signal: there's almost no terrestrial source for the spike, but chondritic meteorites carry hundreds of times more, so an asteroid impact fits. In the lab, iridium's main jobs lean on its inertness: spark-plug electrodes that survive a billion combustion cycles, crucibles for pulling sapphire and YAG single crystals from melts above 2,000 °C, and the Ir-based catalyst in the Cativa process that makes most of the world's acetic acid.
Fun Fact
A thin layer of iridium deposited 66 million years ago in rocks worldwide provided the key evidence that an asteroid impact killed the dinosaurs — iridium is rare on Earth but abundant in meteorites.
Common Uses
- Spark-plug center electrodes in long-life and aviation engines
- Crucibles for Czochralski growth of sapphire, YAG, and GGG crystals
- Cativa-process Ir catalyst for methanol carbonylation to acetic acid
- Pt-Ir alloy electrodes in pacemakers and neural stimulation arrays
- OLED phosphors based on cyclometalated Ir(III) complexes