Iridium Facts – Ir or Atomic Number 77

Iridium is a dense, hard, and corrosion-resistant metal belonging to the platinum group metals (PGMs). It has the element symbol Ir and atomic number 77. Iridium is one of the rarest elements on Earth. Its remarkable resistance to heat and chemical attack makes it valuable in various high-temperature and corrosive applications. You encounter iridium in daily life in OLED screens, spark plugs, and some pen nibs. Here is a collection of useful and interesting iridium facts.

Discovery, Naming, and Isolation

Iridium was discovered in 1803 by English chemist Smithson Tennant. Tennant isolated the element alongside osmium while examining the residues left after dissolving platinum ore in aqua regia (a mixture of nitric acid and hydrochloric acid). Tennant named the element “iridium” after the Latin word “iris,” meaning rainbow. The name reflects the colorful salts iridium forms, which exhibit a variety of hues.

Isolating iridium is challenging due to its high melting point and chemical inertness. Early attempts involved tedious processes of dissolving platinum ores, precipitating iridium compounds, and reducing them to the metal. Modern isolation methods involve complex extraction and refining techniques using various chemicals and high temperatures.

Appearance and Properties

Iridium is a silvery-white shiny metal with a slight yellow cast. The metal exhibits a face-centered cubic crystal structure. Iridium is the most corrosion-resistant metal, resisting attack by acids. However, it is susceptible to reactions with oxidants and sulfur, particularly at higher temperatures. The metal has an extremely high density, second only to osmium and even exceeding it under certain conditions. Iridium is incredibly hard, with a Mohs hardness of 6.5. Working the metal is extremely difficult, due to its hardness, brittleness, very high melting point, and lack of malleability. Powder metallurgy is common, since machining or welding the metal is not viable.

Element Group

Iridium belongs to group 9 of the periodic table and resides in period 6. It is part of the d-block elements, a transition metal, and is closely associated with other platinum group metals such as platinum, rhodium, and osmium.

Natural and Synthetic Isotopes

Natural iridium consists of two stable isotopes: Ir-191 and Ir-193. Ir-193 is the more abundant isotope (62.7%). There are at least 37 synthetic radioisotopes, with mass numbers ranging from 164 to 202. Synthetic isotopes of iridium comes from nuclear reactions and are used in medical and industrial applications, particularly in radiography and cancer treatment.

Abundance and Sources

The original source of iridium is likely the r-process in neutron stars and supernovae. Iridium is extremely rare in the Earth’s crust, with an average concentration of about 0.001 parts per million. It sometimes occurs as a pure native element or in natural alloys, such as osmiridium and iridosmium. More often, the element occurs in platinum ores and nickel deposits. Iridium’s relative abundance is higher in meteorites, volcanoes, and hydrothermal vents. The primary sources of iridium are South Africa, Russia, and Canada, with extraction primarily as a byproduct of nickel and platinum mining.

Purification

Purifying iridium involves several steps, including extraction from ore, chemical separation from other platinum group metals, and refining using high-temperature and chemical methods. One common method is the chloride process, where iridium is converted into iridium tetrachloride and then reduced to pure metal using hydrogen gas.

Uses of Iridium

Iridium’s unique properties make it valuable in various applications, including:

• Catalysis: Iridium compounds are catalysts in the automotive industry and in chemical synthesis, particularly in hydrogenation reactions.
• Electronics: Iridium occurs in electrical contacts, white OLEDs, spark plugs, and other components requiring resistance to high temperatures and corrosion.
• Medical Devices: Radioactive iridium isotopes find use in brachytherapy, a type of cancer treatment.
• Scientific Equipment: Iridium crucibles and other laboratory equipment are important in high-temperature experiments due to their stability and resistance to chemical attack.
• Aerospace: Iridium alloys are common in aerospace applications, including engine components and space missions, where extreme conditions require durable materials.
• Jewelry and Pens: Due to its rarity and durability, iridium finds use in fine jewelry and pen tips.

Oxidation States

Iridium exhibits oxidation states ranging from -3 to +9, with the most common states being +1, +3, and +4. The +3 and +4 oxidation states occur in most iridium compounds, such as iridium(III) chloride and iridium(IV) oxide. Examples of compounds displaying the +6 oxidation state include IrF6, Sr₂MgIrO₆, and Sr₂CaIrO₆. The +9 oxidation state is the highest for any element. It occurs in gaseous [IrO₄]⁺.

Biological Role, Health Effects, and Toxicity

Iridium has no known biological role in humans or other organisms. It is relatively non-toxic due to its inertness. However, finely divided iridium powder poses a respiratory and fire hazard. The radioisotope ¹⁹²Ir and other radioisotopes are hazardous due to its radioactivity. Risks include increased cancer risk, burns, and radiation poisoning.

Key Iridium Facts for Scientists

Interesting Facts About Iridium

• Meteoritic Origin: Iridium is more abundant in meteorites than in the Earth’s crust, suggesting an extraterrestrial source for some deposits.
• Asteroid Impact Marker: The unusually high levels of iridium in the K-T boundary layer worldwide support the theory of a massive asteroid impact, which contributed to the mass extinction event 66 million years ago.
• Volcanic Origin: Alternatively, the K-T boundary layer may relate to volcanic activity. Active volcanoes release iridium from deeper within the Earth.
• Space Applications: Due to its durability, iridium finds use in spacecraft and satellites, such as the Iridium communication satellites.
• Extreme Corrosion Resistance: Iridium’s corrosion resistance makes it one of the few materials that can withstand molten salts and hot, concentrated acids.
• High-Temperature Stability: Iridium maintains its structure and properties at extremely high temperatures. It is invaluable for high-precision crucibles and furnace parts.
• Fountain Pen Nibs: The first major use of iridium was in 1834 as an iridium-osmium alloy for fountain pen nibs. While modern pens often carry the “iridium” label, they often do not contain the element. Instead, ruthenium, tungsten, and osmium are common today.

References

• Arblaster, J. W. (1989). “Densities of Osmium and Iridium Recalculations Based upon a Review of the Latest Crystallographic Data”. Platinum Metals Rev. 33 (1): 14–16. doi:10.1595/003214089X3311416
• Kyte, Frank T.; Zhiming Zhou; John T. Wasson (1981). “High noble metal concentrations in a late Pliocene sediment”. Nature. 292 (5822): 417–420. doi:10.1038/292417a0
• Tennant, S. (1804). “On Two Metals, Found in the Black Powder Remaining after the Solution of Platina”. Philosophical Transactions of the Royal Society of London. 94: 411–418. doi:10.1098/rstl.1804.0018
• Wang, Guanjun; Zhou, Mingfei; Goettel, James T.; Schrobilgen, Gary G.; Su, Jing; Li, Jun; Schlöder, Tobias; Riedel, Sebastian (2014). “Identification of an iridium-containing compound with a formal oxidation state of IX”. Nature. 514 (7523): 475–477. doi:10.1038/nature13795
• Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.