Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a rare, silvery-white, hard, corrosion-resistant, and chemically inert transition metal. It is a noble metal and a member of the platinum group. Naturally occurring Rh is usually found as free metal, as an alloy with similar metals, and rarely as a chemical compound in minerals such as bowieite and rhodplumsite.
It is one of the rarest and most valuable precious metals. Rh is found in platinum or nickel ores together with the other members of the platinum group metals. It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds.
It is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. White gold is often plated with a thin rhodium layer to improve its appearance while sterling silver is often rhodium-plated for tarnish resistance. It is sometimes used to cure silicones, a two part silicone where one part contains a silicon hydride and the other containing a vinyl terminated silicone are mixed. One of these liquids contains a rhodium complex. Rh detectors are used in nuclear reactors to measure the neutron flux level. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production of acetic acid.
Characteristics of Rhodium
It is a hard, silvery, durable metal that has a high reflectance. Rh metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by most acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.