- Rhodium (Rh) is a transition metal with atomic number 45, located in Group 9 of the periodic table along with cobalt, iridium, and meitnerium.
- It is a hard, silvery-white, and highly reflective metal with exceptional corrosion resistance and high melting point.
- Rhodium has nine valence electrons—one in the 5s subshell and eight in the 4d subshell—and it typically exhibits oxidation states of +1, +2, +3, and +4, with +3 being the most stable and commonly encountered in its compounds. Its atomic structure consists of forty-five protons, typically fifty-six neutrons, and forty-five electrons arranged in five shells. Naturally occurring rhodium consists entirely of one stable isotope, rhodium-103 (¹⁰³Rh).
- Rhodium is extremely rare in the Earth’s crust, with an abundance of approximately 0.0002 parts per million, making it one of the rarest and most valuable precious metals. It is found primarily in platinum ores and as a byproduct of nickel and copper refining. The main commercial sources of rhodium are South Africa, Russia, Canada, and Zimbabwe. Extraction involves dissolving platinum group ores in aqua regia, followed by precipitation and separation of rhodium compounds, which are then reduced to obtain the metallic form.
- The element was discovered in 1803 by English chemist William Hyde Wollaston while he was working with platinum ores. He named it “rhodium” from the Greek rhodon, meaning “rose,” due to the rose-red color of its salts, particularly rhodium chloride. Metallic rhodium was later isolated through reduction of its salts with hydrogen or other reducing agents.
- Rhodium is widely used in industrial applications, primarily as a catalyst. It is a key component in automotive catalytic converters, where it facilitates the conversion of harmful nitrogen oxides into nitrogen and oxygen, thereby reducing vehicle emissions. It is also employed in the chemical industry for hydrogenation, hydroformylation, and other catalytic processes. Additionally, rhodium’s bright, reflective finish makes it valuable for plating jewelry, mirrors, and optical instruments.
- Chemically, rhodium exhibits rich coordination chemistry, forming complexes with a variety of ligands including phosphines, carbonyls, and amines. It is resistant to corrosion and oxidation, and its compounds are generally stable. Rhodium(III) complexes, such as rhodium chloride and rhodium nitrate, are particularly important in catalysis.
- Biologically, rhodium has no known essential role in living organisms, and its compounds are generally considered to have low toxicity. However, fine powders or soluble rhodium salts should be handled with care to prevent inhalation or ingestion.
- From an environmental perspective, rhodium is largely inert and does not pose significant ecological risks under natural conditions. However, rhodium mining and refining, like that of other platinum group metals, must be carefully managed to minimize environmental impacts, including habitat disturbance and chemical waste.
