- Iridium (Ir) is a rare, lustrous, and extremely dense transition metal with atomic number 77, positioned in Group 9 of the periodic table alongside cobalt and rhodium.
- Its electron configuration is [Xe] 4f¹⁴ 5d⁷ 6s², giving it nine valence electrons available for bonding. Each atom of iridium contains seventy-seven protons, seventy-seven electrons, and approximately one hundred and fourteen to one hundred and twenty-four neutrons depending on the isotope.
- Naturally occurring iridium is composed of two stable isotopes, iridium-191 (¹⁹¹Ir) and iridium-193 (¹⁹³Ir), with ¹⁹³Ir being slightly more abundant.
- Iridium was discovered in 1803 by the English chemist Smithson Tennant, during his investigations of the insoluble residues left after dissolving crude platinum ores in aqua regia. Alongside osmium, Tennant identified this new element and named it iridium after the Latin word iris, meaning “rainbow,” due to the colorful salts it forms.
- Physically, iridium is one of the densest elements, with a density of 22.56 g/cm³, only slightly less than osmium. It is a hard, brittle, silvery-white metal with a high melting point of 2,446 °C (4,435 °F) and a boiling point of 4,428 °C (8,002 °F). Iridium is highly corrosion-resistant and remains unaffected by air, water, and most acids, even at high temperatures, making it one of the most durable metals known.
- Chemically, iridium is relatively inert but can form compounds in oxidation states ranging from –3 to +9, the widest range for any element, though +3 and +4 are the most common. Examples include iridium trichloride (IrCl₃) and iridium dioxide (IrO₂). Iridium complexes are important in homogeneous catalysis, particularly in hydrogenation and carbon–carbon bond-forming reactions.
- Iridium’s applications rely on its rarity, strength, and resistance to corrosion. It is used in high-performance alloys with platinum and osmium to make durable equipment such as spark plugs, crucibles, and electrodes that must withstand extreme conditions. In the electronics industry, iridium is employed in electrical contacts and thin films. It is also used in medical devices, including pacemaker electrodes, and in radioisotope thermoelectric generators, where the isotope iridium-192 is applied in cancer radiotherapy and industrial radiography.
- Iridium has also gained significance in geoscience. A thin global layer of iridium-rich clay at the Cretaceous–Paleogene (K–Pg) boundary provides strong evidence for the asteroid impact that contributed to the extinction of the dinosaurs about 66 million years ago. This “iridium anomaly” has made the element a key tracer in planetary science.
- Biologically, iridium has no essential role in living organisms. While metallic iridium is considered inert and safe, soluble iridium compounds may be toxic and should be handled with care.
- Environmentally, iridium is one of the rarest elements in Earth’s crust, with an abundance of only about 0.001 parts per million. It is typically found in alluvial deposits associated with platinum-group metals, often in South Africa and Russia, which are the leading sources. Meteorites also contain elevated levels of iridium, supporting its extraterrestrial association. Because of its scarcity and value, recycling iridium from used catalysts and industrial materials is an important part of its supply chain.
