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Rhodium (pronounced /'ro?di?m/) is a chemical element that is a rare, silvery-white hard transition metal and a member of the platinum group. Rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst. It is abbreviated in the periodic table to Rh and has atomic number 45. It is typically the most expensive precious metal[2][3] Rhodium is a hard silvery white and durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification the oxygen is released.[4] Rhodium has both a higher melting point and lower density than platinum. It is not attacked by acids it is completely insoluble in nitric acid and dissolves slightly in aqua regia. Common oxidation states of rhodium is +3, but oxidation states from +0 to +6 are observed.[5] Unlike ruthenium and osmium, rhodium forms no stable volatile oxygen compounds. Only the Rh2O3, RhO2·xH2O and the Na2RhO3 are stable. Halogen compounds are known in the nearly the full range of possible oxidation states. Rhodium(III) chloride, rhodium(IV) fluoride, rhodium(V) fluoride and rhodium(VI) fluoride are some examples. The lower oxidation states are only stable if ligands are present.
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