The platinum group is the six noble metals in group VIII of the periodic table, i.e., ruthenium (atomic number 44), rhodium (45), palladium (46), osmium (76), iridium (77), and platinum (78) (see also transition element). They are found together in pyroxene deposits in South Africa and in copper and nickel ores of Canada and Russia. All have a high melting point and density, and are very inert and corrosion-resistant, though palladium, osmium, and platinum dissolve in aqua regia (a mixture of one part nitric acid and three parts hydrochloric acid); the others can be dissolved by fused oxidizing alkalis. Palladium dissolves slowly in oxidizing acids. Ruthenium and osmium show chief oxidization states +3, 4, 6, and 8; the other metals seldom exceed +4. All six metals form numerous halides and complex halogen ions, and many other ligand complexes, including carbonyls resembling resembling those of iron, cobalt, and nickel.
The platinum group metals (PGM) are used, usually as alloys with each other, for jewelry, the tips of pen nibs, electric contacts, thermocouples, crucibles, surgical instruments, standard weights and measures, and (finely divided) as catalysts.