mol-1 Heat of Vaporization: 494 kJ.mol-1 Atomic: Oxidation States: 6, 5, 4, 3, 2, 1, -1 Electronegativity: 2.28 (Pauling scale) Atomic Radius: 134 picometre Covalent Radius: 142 7 picometre Ionization Energies: 719.7 kJ.mol-1 (first), 1740 kJ.mol-1 (second), 2997 kJ.mol-1 (third) The principal use of rhodium is as a catalytic converter in automotive vehicles, which reduces the toxicity of engine emissions by converting these into less harmful gases. Rhenium: Last Naturally Occurring Stable Precious Metal Discovered In the periodic table of elements, rhenium is found as a third-row transition metal in group 7. Known to be one of the rarest precious metals in the Earth's crust, rhenium has an average concentration of 1 part per billion. It is obtained mainly as a by-product of the refinement of two other chemical elements - copper and molybdenum. Palladium: Least Dense Of The Platinum Group Metals Palladium is a rare precious metal characterized by its lustrous silvery-white appearance. It was discovered by the English chemist William Hyde Wollaston in London, England in 1803, along with his discovery (together with Smithson Tennant, another English chemist) of the other metals in the platinum group. Gold: Gold, in its elemental form, does not cause irritation and is not toxic even when ingested. In fact, it is used as a component in some alcoholic drinks and as a food additive. However, ionic chemical compounds of gold (example, gold chloride) can be extremely harmful to the kidneys and liver. Platinum: Findings by the U. While some accounts point to the year 1848 when gold was discovered in California, history tells us that this precious metal was already being used extensively by the ancient Egyptians, Romans, Greeks, Chinese, and South Americans. Throughout history, gold has served as a measure of value and a symbol of wealth. Occurring in nature in metallic form, these two precious metals can be produced sans the use of extraction metallurgy. These other characteristics of gold and silver make them both well suited for coinage: They are not radioactive. They are more ductile or softer than most other elements. They are less reactive compared with other elements.