Rhodium is an element with atomic number 45 and the chemical sign Rh. The name comes from the Greek "Rhodon" which means rose.
Rhodium, which is a platinum metal, is the rarest metal on earth (apart from the radioactive metals) and is only a few (less than 10) tons a year are produced. The metal is silvery white and has a higher melting point and lower density than platinum. Rhodium has low electrical resistance, low and stable contact resistance and high resistance against corrosion.
Rhodium is mainly used in alloys with platinum and palladium. Rhodium can only be plated on nickel, silver, gold or platinum. Plated rhodium is extremely hard wearing. Rhodium is sometimes used in spark plugs for aircraft engines, the tip of fountain pens, telephone relays and in the reflectors of headlamps, mirrors and optical instruments. Rhodium is also used in jewellery, as decorations and as a catalyst.
Because of its low and stable contact resistance and its high resistance against corrosion and wear (for example contact surfaces grinding against each other) it is eminently suitable as material in different kinds of connectors. A surface plated with gold, which is a very soft metal, is worn off much faster than a surface plated with a hard metal like Rhodium.
Summing up: Gold is beautiful, but if you want the best (in sound as well) use Rhodium.
Atomic number: 45
Atomic symbol: Rh
Atomic weight: 102,9055
Electron config: [Kr]5s14d8
Atomic radius: 134,5 pm
Melting point: 1964 °C
Boiling point: 3695 °C
Oxidation states: 3
Rhodium occurs natively with other platinum metals in river sands of the Urals and in North and South America. It is also found with other platinum metals in the copper-nickel sulfide area of the Sudbury, Ontario (Canada) region. Although the quantity occurring there is very small, the large tonnages of nickel processed make the recovery commercially feasible. The annual world production of rhodium is only 7 or 8 tons.
The metal is silvery white and at red heat slowly changes in air to the resquioxide. At higher temperatures it converts back to the element. Rhodium has a higher melting point and lower density than platinum. It is highly reflective, hard and durable.
Rhodium's primary use is as an alloying agent to harden platinum and palladium. Such alloys are used for furnace windings, thermocouple elements, bushings for glass fiber production, electrodes for aircraft spark plugs, and laboratory crucibles. It is useful as an electrical contact material as it has a low electrica resistance, a low and stable contact resistance, and is highly resistant to corrosion. Plated rhodium, produced by electroplating or evaporation, is exceptionally hard and is used for optical instruments. Rhodium is also used for jewelry, for decoration, and as a catalyst.
Exposure to rhodium (metal fume and dust, as Rh) should not exceed 1 mg/m3 (8-hour time-weighted average, 40-hour week).
Rhodium costs about $ 1.000/troy oz.
Source: Los Alamos National Laboratories, Chemistry Division, USA