(So named (
Greek rhodon, "rose") by its discoverer, after the color of a
dilute solution of its
salts) A hard, silvery-white
metallic
chemical element, one of the
platinum metals. Rhodium is the rarest of all
metals on Earth. It occurs native with other
platinum metals in river sands in
North and
South America, and as a byproduct of
nickel production in
copper-
nickel sulfide ores of
Ontario. The major use of rhodium is as a hardener for
platinum and
palladium, to produce
alloys used for
electrodes, furnace
windings,
crucibles and
thermocouples. It is often used as an electrical
contact material as it has a low
resistance and is highly resistant to
corrosion. Rhodium is used in unalloyed form to
electroplate optical instruments and
silverware,
jewelry, and other decorative articles. It is also used as a
catalyst. Rhodium is
inert to all
acids but is attacked by fused
alkali metals. It is
stable in
air up to 600°
C.
William Hyde Wollaston discovered rhodium in 1803 in London, England in crude platinum ore from South America. He dissolved the ore in aqua regia, neutralised the acid with sodium hydroxide, and precipitated the platinum by treatment with ammonium chloride, as ammonium chloroplatinate. Palladium was then removed as palladium cyanide by treatment with mercuric cyanide. What remained was a red material containing rhodium chloride salts from which rhodium metal was obtained by reduction with hydrogen gas.
Symbol: Rh
Atomic number: 45
Atomic weight: 102.90550
Density (at room temperature and pressure): 12.4 g/cc
Melting point: 1,966°C
Boiling point: 3,695°C
Main valences: +1, +3
Ground state electron configuration: [Kr]4d85s1