Iridium
Iridium is a chemical element with symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is generally credited with being the second densest element (after osmium) based on measured density, although calculations involving the space lattices of the elements show that iridium is denser. It is also the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.
Identity
- Atomic mass
- 192.217 u
- Category
- Transition metals
- Group
- 9
- Period
- 6
- Block
- d
- Oxidation States
- +1, +3, +4, +6
- Phase (25 °C)
- Solid
- Appearance
- silvery white
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d7
- Short configuration
- [Xe] 4f14 5d7 6s2
- Electrons per shell
- 2, 8, 18, 32, 15, 2
- Electronegativity (Pauling)
- 2.20
- Electron affinity
- 151 kJ/mol
- 1st ionization energy
- 880 kJ/mol
- Ionization energies
- 880 · 1600 kJ/mol
Physical properties
- Melting point
- 2445.8 °C (2719 K)
- Boiling point
- 4129.9 °C (4403 K)
- Density
- 22.56 g/cm³
- Molar heat capacity
- 25.1 J/(mol·K)
Chemical behavior of iridium
Iridium has the electron configuration [Xe] 4f¹⁴ 5d⁷ 6s². It is the second densest known element (22.56 g/cm³), practically tied with osmium, and holds the record as the metal with the greatest resistance to chemical corrosion in the entire periodic table. It is not attacked by any pure acid or by aqua regia at normal or moderate temperatures, resisting even molten metals and acidic salts. Its usual oxidation states are +3 and +4, although in organometallic chemistry it spans from −3 to +9. It is a hard, brittle metal with a yellowish-white luster and a melting point above 2,440 °C.
Reactivity
Ir + fused alkali salts → soluble iridates
It is inert to all liquid acids, but dissolves by oxidative fusion with sodium hydroxide and sodium peroxide, or potassium chlorate, above 600 °C.
Ir + O₂ (heat) → IrO₂
At temperatures above 600 °C in air it slowly oxidizes at the surface forming a dark layer of iridium dioxide; above 1,100 °C the oxide dissociates back to metallic iridium.
[Ir(CO)Cl(PPh₃)₂] + H₂ ⇌ [Ir(CO)Cl(H)₂(PPh₃)₂]
Vaska's complex undergoes reversible oxidative addition with molecular hydrogen at room temperature, a fundamental model in homogeneous catalysis.
Ir + 3 F₂ → IrF₆
It reacts with fluorine gas at 250 °C producing volatile yellow iridium hexafluoride.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Iridium dioxide | IrO₂ | Highly durable electrocatalytic electrode for water electrolysis and green hydrogen generation |
| Vaska's complex | IrCl(CO)[P(C₆H₅)₃]₂ | Historic organometallic compound that established the concept of reversible oxidative addition of gas molecules in chemical catalysis |
| Iridium(III) chloride | IrCl₃ | Key precursor salt for synthesizing phosphorescent OLED diode phosphors and Cativa process catalysts |
| Crabtree's catalyst | [C₈H₁₂IrP(C₆H₁₁)₃(C₅H₅N)]PF₆ | Extremely active hydrogenation catalyst for sterically hindered alkenes |
Electron shell distribution
View in the atom builderIsotopes
Natural iridium is made up of two stable isotopes: iridium-191 (¹⁹¹Ir, with 114 neutrons and 37.3% abundance) and iridium-193 (¹⁹³Ir, with 116 neutrons and 62.7%). Among its radioisotopes, iridium-192 (¹⁹²Ir, half-life of 73.8 days) stands out: a powerful gamma emitter produced in nuclear reactors that constitutes the most widely used radiation source in the world for non-destructive industrial radiography of welds and for high-dose-rate brachytherapy against tumors.
Occurrence and production
It is one of the scarcest elements in the Earth's crust, with barely 0.001 parts per million (about 30 to 40 times rarer than gold). As a strongly siderophile element, the vast majority of the planet's iridium sank into the metallic core during Earth's formation. Chondritic asteroids, however, preserve concentrations thousands of times higher. In the crust it is recovered as a byproduct of nickel and platinum sulfide mining in Sudbury (Canada), Norilsk (Russia) and the Bushveld complex (South Africa).
History and origin of the name
It was discovered in 1803 by the British chemist Smithson Tennant in the very same insoluble platinum residue in aqua regia that gave rise to osmium. When neutralizing and dissolving the precipitates with alkalis and acids, Tennant observed an extraordinary variety of intense colors in the solutions depending on the oxidation state of the new element (dark red, bright green, blue, yellow). Fascinated by this polychromy, he called it 'iridium' after Iris, the messenger goddess of the rainbow in Greek mythology.
Applications and relevance of iridium
Uses of the element
Thanks to its chemical inertness and physical unalterability, the 90% platinum / 10% iridium alloy was the material chosen in 1889 to make the International Kilogram Prototype and the standard Meter kept in Paris. It is used in single-crystal iridium crucibles to melt refractory oxides and grow garnet laser crystals (YAG), in long-life aviation spark plug electrodes and in tips for deep brain stimulation electrodes.
Uses of its compounds
Iridium dioxide is the most efficient and durable anodic catalyst for PEM electrolyzers producing green hydrogen. Cyclometalated iridium complexes provide the red and green phosphorescent emission of extremely high quantum efficiency in smartphone OLED displays.
Health, safety and environment
Biological role
It has no known biological role in living organisms. The bulk solid metal is completely biocompatible and insoluble, which is why it is safely implanted in medical brain electrodes and cardiac pacemakers without inducing tissue inflammation.
Safety
In metallic form it is completely harmless. Fine iridium dust is flammable and its soluble halide salts can be irritating. The greatest risk comes from abandoned industrial sources of radiological iridium-192, whose intense gamma radiation can cause lethal acute radiation burns if not confined with lead shielding.
Environment
Its direct ecological impact is insignificant due to its extreme scarcity and the total recycling imposed by its extraordinarily high economic value across all its technology supply chains.
Frequently asked questions about Iridium
How did iridium help prove that a meteorite wiped out the dinosaurs?
Iridium is an extremely rare metal on Earth's surface because most of it sank toward the core while the planet formed. Asteroids, however, contain much higher amounts. In 1980, geologists discovered a thin clay layer from 66 million years ago enriched in iridium all around the world, demonstrating the colossal impact of the bolide that changed Earth's climate.
Why is iridium used in some equipment to produce hydrogen?
Some electrolyzers split water into hydrogen and oxygen. On the side where oxygen forms, the environment is very corrosive and many materials deteriorate quickly. Iridium dioxide withstands these conditions and helps accelerate the reaction, so it is used in certain equipment. Its main drawback is the scarcity and cost of iridium, which drive the search for alternatives.
What does industrial radiography with iridium-192 radioactive sources consist of?
Iridium-192 is produced by bombarding iridium-191 pellets with neutrons in nuclear reactors. It emits penetrating gamma rays with a half-life of nearly seventy-four days. Housed inside a portable tungsten capsule, it is used in remote pipelines, naval shipyards and refineries to radiograph welds of thick steel pipes looking for cracks or internal bubbles invisible to the eye, with no need for electrical connections or X-ray generators.
What is the Mössbauer effect and what is it used for?
The Mössbauer effect allows certain nuclei to emit and absorb gamma rays without losing an appreciable amount of energy to recoil. This makes it possible to measure very small changes in the radiation energy and obtain information about the chemical environment of the nucleus. The phenomenon was discovered in 1957 and is applied in physics and materials science; certain iridium isotopes can be used in studies of this kind.
Why were the official meter and kilogram standards made of iridium?
In the 19th century, scientists needed reference standard bars and cylinders that measured exactly one meter and weighed exactly one kilogram without changing over the centuries. They chose an alloy of platinum with ten percent iridium because this metal adds extraordinary hardness, barely expands with temperature changes and never oxidizes in air.
How do iridium phosphorescent complexes light up smartphone OLED displays?
In organic light-emitting diodes (OLED), electrical recombination generates singlet excitons (25%) and triplet excitons (75%). Conventional fluorescent emitters only harvest the singlets, wasting the rest of the energy as heat. Iridium organometallic complexes have enormous spin–orbit coupling thanks to their heavy nucleus, which allows harvesting the triplet states through fast phosphorescence, reaching 100% internal quantum efficiency in high-brightness red and green pixels.
Why are iridium crucibles indispensable for growing laser and telecommunications crystals?
To grow single crystals of refractory oxides such as yttrium aluminum garnet (YAG) or lithium tantalate by the Czochralski method, the material must be melted above 2,000 °C in atmospheres containing oxygen. Iridium is the only known metal that withstands these extreme temperatures without melting or dissolving in the molten oxide, resisting surface oxidation without contaminating the crystalline purity of telecommunications optical crystals.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.