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Rhodium (Rh)

QUICK REFERENCE

  • Symbol: Rh
  • Atomic Number: 45
  • Atomic Weight: 102.90550
  • Element Classification: Transition Metal
  • Discovered By: William Hyde Wollaston
  • Discovery Date: 1803
  • Name Origin: Greek: ‘rhodon’ (rose), referring to the rose color of its salts
  • Density(g/cc): 12.41
  • Melting Point: 1964°C
  • Boiling Point: 3695°C
  • Appearance: Silvery-white, hard metal
  • Atomic Radius(pm): 173

Discovery

Rhodium was discovered in 1803 by English chemist William Hyde Wollaston shortly after his discovery of palladium. Wollaston discovered rhodium in platinum ore he obtained from South America, using crude platinum dissolved in aqua regia (a mixture of nitric and hydrochloric acids). He isolated rhodium as a distinct element by its rose-colored salts after the metal was precipitated by sodium chloride. The name “rhodium” comes from the Greek word ‘rhodon’, meaning rose, indicative of the color of its compounds.

Relation to Other Elements

Rhodium is a member of the platinum group metals (PGMs), which also includes platinum, palladium, ruthenium, osmium, and iridium. These metals are characterized by their excellent resistance to wear, corrosion, and oxidation, as well as their remarkable catalytic properties. Rhodium shares many physical and chemical properties with other PGMs but stands out due to its rarity, higher melting point, and exceptional reflectivity. It is most commonly found in the +3 oxidation state, though it can also exhibit other oxidation states under certain conditions.

Natural Occurrence

Rhodium is one of the rarest elements in the Earth’s crust and is found in very small quantities. It occurs naturally alloyed with other platinum group metals in alluvial deposits and within nickel, copper, and platinum ores. The major sources of rhodium are mines in South Africa, Russia, and Canada. Due to its scarcity, rhodium is often produced as a byproduct of nickel or platinum refining.

Uses

Rhodium has several key industrial and commercial applications:

  • Automotive Catalysts: The primary use of rhodium is in three-way catalytic converters for automobiles, where it reduces harmful emissions by converting nitrogen oxides into nitrogen and oxygen, carbon monoxide into carbon dioxide, and unburnt hydrocarbons into water and carbon dioxide.
  • Jewelry: Due to its excellent reflectivity and resistance to tarnishing, rhodium plating is used to give white gold and sterling silver a reflective white surface.
  • Chemical Industry: Rhodium is used as a catalyst in a variety of chemical reactions, including the production of acetic acid, hydrogenation reactions, and in processes for manufacturing certain silicone rubbers.
  • Electrical and Optical Applications: Rhodium is utilized in electrical contacts, optical instruments, and as a coating to prevent thermal radiation and corrosion.

The discovery of rhodium added a valuable element to the palette of materials available for reducing pollution, enhancing the beauty of jewelry, and facilitating various chemical processes. Its rarity and efficiency as a catalyst make it one of the most expensive precious metals in the world.

Technetium (Tc)

Ruthenium (Ru)