Metallic chemical element, one of the transition elements, chemical symbol Rh, atomic number 45. Chemically related to platinum, it is a precious, silver-white metal mainly used as an alloying agent for the latter element. Rhodium added to platinum in small amounts yields alloys that are harder and that lose mass at high temperatures more slowly than pure platinum. Such alloys are used for laboratory furnace crucibles, spark-plug electrodes, catalysts in very hot chemical environments (including automobile catalytic converters), and high-temperature thermocouples. Because rhodium resists tarnishing at ordinary temperatures and is highly reflective, it is in demand as a surface electroplating for jewelry and other decorative articles. The plated metal is also used for reflecting surfaces in optical instruments.
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