The chemical symbol \(\text{Rh}\) represents Rhodium, a silvery-white, hard, and durable precious metal belonging to the Platinum Group Metals (PGMs). It is one of the rarest and most valuable precious metals on Earth. Its unique chemical and physical properties make it indispensable in modern technology, particularly for controlling environmental pollution.
Identity and Classification
Rhodium is identified by the chemical symbol \(\text{Rh}\) and possesses an atomic number of 45. This means every atom contains 45 protons. Its standard atomic weight is approximately \(102.906\) atomic mass units, placing it in Group 9 and Period 5 of the periodic table.
This classification confirms its status as a transition metal and a member of the Platinum Group Metals, alongside elements like Platinum, Palladium, and Iridium. The element’s name is derived from the Greek word rhodon, which means “rose,” a reference to the rose-red color of some of its compounds in solution.
Naturally occurring rhodium is composed entirely of a single stable isotope, \(\text{Rhodium-103}\) (\(\text{^{103}Rh}\)). As a noble metal, Rhodium is highly resistant to chemical reaction, which contributes significantly to its industrial utility.
Defining Physical and Chemical Traits
Rhodium has a remarkably high melting point of approximately \(1964^{\circ}\text{C}\) (\(3567^{\circ}\text{F}\)). This resistance to extreme heat is coupled with exceptional physical hardness and durability, making it highly resistant to mechanical wear and abrasion. Rhodium is also known for its superior reflectivity, producing a bright, mirror-like finish that is used extensively in high-precision optics.
Chemically, the metal is notably inert and highly stable, demonstrating an extreme resistance to corrosion and oxidation. Rhodium is not attacked by most common acids, including concentrated nitric or hydrochloric acids. It even resists dissolution by aqua regia, a mixture that readily dissolves Gold and Platinum.
Essential Roles in Industry
The primary application of Rhodium, consuming roughly 80% of the global supply, is as a catalyst in three-way catalytic converters in vehicles. In this role, Rhodium is highly effective at reducing harmful nitrogen oxides (\(\text{NO}_{\text{x}}\)) emissions into harmless nitrogen gas and oxygen. It works alongside Platinum and Palladium to simultaneously oxidize unburnt hydrocarbons and carbon monoxide, cleaning the exhaust before it enters the atmosphere.
Beyond emissions control, its chemical stability and high reflectance make it indispensable for electroplating. Very thin layers of Rhodium are applied to white gold and sterling silver jewelry to provide a durable, highly reflective, and tarnish-resistant finish. The metal’s low and stable electrical resistance also makes it an excellent choice for plating electrical contacts and electrodes in demanding applications, such as aircraft spark plugs. Furthermore, Rhodium is alloyed with Platinum to create thermocouples capable of accurately measuring temperatures up to \(1800^{\circ}\text{C}\) in industrial furnaces.
Origin and Rarity
Rhodium is one of the rarest non-radioactive elements in the Earth’s crust, occurring at estimated concentrations of less than one part per 200 million. Due to this scarcity, it is not mined independently. Instead, it is recovered as a trace byproduct during the extraction and refining of other metals. The majority of Rhodium comes from ores rich in Platinum and Palladium, or from copper and nickel sulfide deposits.
The world’s supply is geographically concentrated. South Africa’s Bushveld Complex is the dominant source, accounting for the vast majority of global production. Other notable sources include the Norilsk region in Russia and the copper-nickel sulfide areas of the Sudbury Basin in Canada.