Iridium (Ir): Properties, Reactions, and Uses
Introduction to Iridium (Ir)
Iridium is a remarkable chemical element, renowned for being one of the densest and most corrosion-resistant metals. Its extreme properties make it indispensable in high-performance applications where durability and chemical inertness are paramount. Often found in meteorites, its relative abundance in Earth’s crust is low, suggesting an extraterrestrial origin for some terrestrial deposits, famously linked to the K-Pg extinction event.
CBSE/JEE Quick Revision Notes
- Atomic Number (Z): 77
- Atomic Mass: 192.22 u
- Symbol: Ir
- Group: 9
- Period: 6
- Block: d-block (Transition Metal)
- Common Oxidation States: +3, +4 (most common); +1, +2, +5, +6 (also observed)
- Density: Approximately 22.56 g/cm³ (second densest element after osmium)
- Melting Point: 2466 °C
- Boiling Point: 4428 °C
- Physical State at STP: Solid
- Nature: Extremely hard, brittle, silvery-white transition metal. Highly resistant to chemical attack.
Electron Configuration & Bonding Behavior
Electron Configuration
The ground state electron configuration of Iridium is [Xe] 4f¹⁴ 5d⁷ 6s². This configuration, with partially filled d-orbitals, is characteristic of transition metals, enabling it to exhibit multiple oxidation states and form a variety of coordination compounds.
Bonding Behavior
Iridium predominantly forms metallic bonds in its elemental state. In compounds, it exhibits covalent character, particularly with electronegative elements like oxygen and halogens. Its d-orbitals participate extensively in bonding, leading to the formation of numerous stable coordination complexes, typical for a third-row transition metal. The most stable oxidation states are +3 and +4, but higher states up to +6 (e.g., in IrF₆) are also known.
Crucial Chemical Reactions
Iridium is exceptionally unreactive due to its high stability and electron configuration. It is known as the most corrosion-resistant metal.
Reaction with Oxygen
Iridium does not react with oxygen at ambient temperatures. It forms iridium dioxide (IrO₂) only at very high temperatures (above 1100 °C).
2 Ir(s) + 2 O₂(g) → 2 IrO₂(s) (at >1100 °C)
Reaction with Halogens
Iridium reacts with halogens, particularly fluorine, at elevated temperatures to form various halides.
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Reaction with Fluorine:
2 Ir(s) + 3 F₂(g) → 2 IrF₃(s)(at lower temperatures, or with limited F₂)Ir(s) + 3 F₂(g) → IrF₆(g)(at ~300 °C, with excess F₂) -
Reaction with Chlorine:
2 Ir(s) + 3 Cl₂(g) → 2 IrCl₃(s)(at ~340 °C)
Reaction with Acids and Bases
Iridium is remarkably resistant to attack by all common acids, including concentrated nitric acid, sulfuric acid, and hydrochloric acid. It is also unaffected by aqua regia (a mixture of concentrated nitric and hydrochloric acids), a property that distinguishes it from gold and platinum. It does not react with molten alkali metal hydroxides or other bases.
Formation of Complex Ions
Iridium forms a wide range of stable complex ions. A common example is the hexachloroiridate(III) ion:
IrCl₃(s) + 3 Cl⁻(aq) → [IrCl₆]³⁻(aq)
Industrial and Biological Importance
Industrial Importance
- Alloys: Iridium is primarily used as a hardening agent for platinum alloys. These alloys are employed in high-temperature electrical contacts, spark plug electrodes, crucibles for growing single crystals, and specialized scientific apparatus.
- Catalysis: Iridium compounds serve as catalysts in industrial processes, notably in the Cativa process for the large-scale production of acetic acid. They are also used in various hydrogenation reactions in organic chemistry.
- Scientific Standards: The international prototype meter bar and kilogram mass, established in 1889, were made from an alloy of 90% platinum and 10% iridium due to its exceptional hardness, corrosion resistance, and stability.
- Medical Applications: The radioactive isotope Iridium-192 (
¹⁹²Ir) is utilized in brachytherapy for cancer treatment and in industrial radiography. - Other Uses: Iridium is used for pen tips, compass bearings, and specialized laboratory equipment.
Biological Importance
Iridium is generally considered biologically inert and non-toxic in its elemental form due to its extreme lack of reactivity. While some iridium compounds can be toxic, its low bioavailability limits its interaction with biological systems. Research is ongoing into the potential use of certain iridium complexes as anticancer agents, leveraging their ability to interact with DNA or inhibit enzymes, though these are typically synthetic compounds not found naturally in biological systems.