Iridium
Atomic Data
| Atomic Number | 77 |
| Symbol | Ir |
| Atomic Weight | 192.22 u |
| Density (STP) | 22.56 g/cm³ |
| Melting Point | 2445.85 °C (2719 K) |
| Boiling Point | 4129.85 °C (4403 K) |
| Electronegativity | 2.2 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d7 6s2 |
| Oxidation States | +3, +4 |
| Phase at STP | Solid |
| Category | Transition Metal |
| Period / Group | 6 / 9 |
| CAS Number | 7439-88-5 |
Electron Configuration
[Xe] 4f14 5d7 6s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 7 | 75 |
| P | 6 | 6s | 2 | 77 |
| Total | 77 | 77 | ||
Isotopes of Iridium
Iridium has two naturally occurring stable isotopes. The most abundant is ¹⁹³Ir, comprising 62.7% of all naturally occurring Iridium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Iridium-191 | ¹⁹¹Ir | 77 | 114 | 37.3 | Stable |
| Iridium-193 | ¹⁹³Ir | 77 | 116 | 62.7 | Stable |
Abundance & Occurrence
Iridium is present in Earth's crust at approximately 0.001 ppm by mass and at approximately 0.2 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Iridium dust inhalation: Iridium metal dust and iridium compound dusts are respiratory irritants; fine particles generated during machining or grinding should not be inhaled: use ventilation and respiratory protection.
- Iridium-192: radiation hazard: Ir-192 (t½ = 74 days, gamma emitter) is one of the most widely used industrial and medical radiography sources; improper handling or orphaned sources have caused serious radiation injuries and deaths: strict inventory and shielding are mandatory.
- Iridium halides: Iridium halides are moderately irritating to skin, eyes, and the respiratory tract; handle with appropriate gloves and ventilation.
- General toxicity: Bulk iridium metal has low systemic toxicity; the principal safety concern in most applications is the radiation hazard from Ir-192 sources.
Iridium in the Real World
Real-World Uses
- Automotive spark plug tips: Iridium-alloy tips on spark plug centre and ground electrodes provide exceptional wear resistance and maintain a sharp edge for consistent spark generation, used in long-life spark plugs for modern gasoline engines.
- Crucibles for crystal growth: Iridium crucibles withstand temperatures above 2200 °C and resist chemical attack from many molten oxides, used to grow high-purity single crystals of gadolinium gallium garnet, calcium fluoride, and other specialty materials for optical and scintillator applications.
- OLED phosphorescent emitters: Iridium(III) organometallic complexes are the most widely used phosphorescent emitters in OLED displays and lighting, enabling efficient red, green, and blue light emission in smartphones and OLED TVs through triplet harvesting.
- International prototype kilogram (historical): The international prototype of the kilogram (IPK), the physical mass standard from 1889 to 2019, was a cylinder of 90% platinum-10% iridium; iridium provided hardness and oxidation resistance to the standard.
- Radioisotope brachytherapy: Iridium-192 (t½ = 73.8 d) sealed sources are the dominant radioisotope for high-dose-rate (HDR) brachytherapy of cervical, prostate, breast, and oesophageal cancers worldwide.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is iridium used for?
Iridium is used in spark plug electrodes for aircraft and high-performance engines: its extreme hardness and high melting point mean iridium-tipped plugs last far longer than conventional ones. The international prototype kilogram (until 2019) was made of a platinum-iridium alloy. Iridium crucibles are used for growing single crystals of high-temperature materials. Iridium-192 is used in brachytherapy for cancer treatment.
Is iridium the rarest element on Earth?
Iridium is among the rarest elements in Earth's crust, with an abundance of only about 0.001 parts per million. Most of Earth's iridium, like other platinum-group metals, is believed to have sunk to the core during Earth's formation. Interestingly, iridium is far more abundant in meteorites: the thin iridium-rich layer found in rocks worldwide at the Cretaceous-Palaeogene boundary is the key evidence for the asteroid impact that killed the non-avian dinosaurs.
How was iridium discovered?
Iridium was discovered in 1803 by British chemist Smithson Tennant alongside osmium, from the residue remaining after platinum ore was dissolved in aqua regia. He named it iridium from the Latin 'iris' (rainbow) because its salts form compounds of many vivid colours: iridescent is ultimately derived from the same root.
What is the Cretaceous-Palaeogene boundary and why does iridium matter?
The Cretaceous-Palaeogene (K-Pg) boundary, about 66 million years ago, marks one of the greatest mass extinctions in Earth's history: including the end of the non-avian dinosaurs. In 1980, Luis and Walter Alvarez discovered that a thin clay layer at this boundary worldwide contains iridium at concentrations 30–130 times higher than typical Earth crust levels. Because iridium is rare on Earth's surface but common in asteroids, they proposed that a large asteroid impact had distributed iridium globally: a hypothesis confirmed by the discovery of the Chicxulub crater.