Iridium
Iridium is a chemical element with the symbol Ir and atomic number 77. It is a hard, silvery-white transition metal in the platinum group and is considered to be the second-densest metal (after osmium). Iridium is one of the rarest elements in the Earth's crust. Here are some key aspects of iridium:
- Physical Properties: Iridium is known for its high density and corrosion resistance. It has a very high melting point (around 2,446°C or 4,435°F), making it solid at room temperature.
- Chemical Properties:
- Corrosion Resistance: Iridium is remarkably resistant to corrosion, even at high temperatures, and is not attacked by most acids. It only dissolves in molten salts, such as sodium cyanide.
- Alloys: Due to its hardness and high melting point, iridium is often alloyed with other metals to improve their properties.
- Occurrence: Iridium is found in the Earth's crust in very small quantities and is often alloyed with other platinum-group metals. It is also found in meteorites in much higher abundance than in the Earth's crust.
- Applications:
- High-Temperature Materials: Iridium's high melting point makes it suitable for high-temperature applications, such as crucibles for growing large high-quality single crystals.
- Electrical Contacts and Electrodes: Due to its corrosion resistance, iridium is used in spark plugs, electrodes for the chlor-alkali process, and other equipment subjected to harsh conditions.
- Radiation Therapy: Iridium-192, a radioactive isotope of iridium, is used in brachytherapy for the treatment of certain types of cancer.
- Scientific Research: The standard meter bar, which was used to define the meter until 1960, was made of a platinum-iridium alloy.
- Historical Significance: The discovery of high iridium concentrations in geological layers at the Cretaceous-Paleogene boundary provided key evidence for the asteroid impact theory of dinosaur extinction.
- Safety: While metallic iridium is generally considered non-toxic, its compounds should be handled with care as they can be highly toxic or corrosive.
Iridium's rarity, combined with its remarkable physical and chemical properties, makes it a valuable material for a variety of specialized applications, particularly in high-technology and scientific fields.