Rhodium

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 member of the platinum group. It has only one naturally occurring isotope, 103Rh. Naturally occurring rhodium is usually found as a 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.

Characteristics

Rhodium is a hard, silvery, durable metal that has a high reflectance. Rhodium 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.

Applications

The primary use of this element is in automobiles as a catalytic converter, changing harmful gases into less harmful substances. Rhodium is also used in jewelry and for decorations. It is electroplated on white gold and platinum to give it a reflective white surface.

History

Rhodium was discovered in 1803 by William Hyde Wollaston, who isolated it from platinum. He used crude platinum ore presumably obtained from South America. His procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide (NaOH). He then precipitated the platinum by adding ammonium chloride, NH4Cl, as ammonium chloroplatinate.

Occurrence

Rhodium is one of the rarest elements in the Earth's crust, comprising an estimated 0.0002 parts per million (2 × 10−10). Its rarity affects its price and its applications in commercial use.

See also

• Platinum group
• Catalytic converter
• William Hyde Wollaston

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