Science / Periodic Table of Elements / Rhodium (Rh): Atomic number: 45, Atomic mass: 102.91 g.mol -1, Electronegativity: 2.2, Density: 12.4 g.cm-3 at 20Â°C, Melting point: 1970 Â°C, Boiling point: 3727 Â°C, Isotopes: 9, Electronic shell: [ Kr ] 4d8 5s1, Energy of first ionisation: 742 kJ.mol -1, Standard Potential: 0.6 V, Discovered by: William Wollaston in 1803. Rhodium, together with ruthenium, palladium, osmium, iridium, and platinum form a group of elements referred to as the platinum group metals (PGM). Rhodium metal is lustrous and silvery white. Rhodium has a higher melting point and lower density than platinum. It has a high reflectance and is hard and durable. Upon heating it turns to the oxide when red and at higher temperatures turns back to the element. Rhodium it is unaffected by air and water up to 600 C. It is insoluble in most acids, including aqua regia, but is dissolved in hot concentrated sulfuric acid and it is attacked by molten alkalis. Applications: Most metal (85%) goes into catalytic converters for cars. The major use of the metal is in alloys with platinum and iridium, giving improved high-temperature strength and oxidation resistance. These alloys are used in furnace windings, pen nibs, phonograph needles, high-temperature thermocouple and resistance wires, electrodes for aircraft spark plugs, bearings and electrical contacts. The metal itself,is used to plate jewelry and the reflectors of searchlights, due to its brilliance and resistance to tarnish, It is also a highly useful catalyst in a number of industrial processes, such as the BP-Monsanto process. Rhodium in the environment: Rodhium occurs as rare deposits of the uncombined metal, for example in Montana, USA and in rare minerals. The metal, which is available commercialy, comes as a by product of the refining of certain copper and nickel ores which can contain up to 0.1% rhodium. Most rhodium comes from South Africa and Russia, and world production is around 16 toone per year. Estimated reserves are 3.000 tonnes.