mol-1 Electron Configuration: [Xe] 4f14 5d7 6s2 Physical: Density (near room temperature): 22.56 g.cm-3 Liquid Density (at melting point): 19 g.cm-3 Melting Point: 2466 C, 4471 F, 2739 K Boiling Point: 4428 C, 8002 F, 4701 K Heat of Fusion: 41.12 kJ.mol-1 Heat of Vaporization: 563 kJ.mol-1 Atomic: Oxidation States: -3, -1, 0, 1, 2, 3, 4, 5, 6 Electronegativity: 2.20 (Pauling scale) Atomic Radius: 136 picometre Covalent Radius: 141 6 picometre Because of its characteristic of being very brittle, pure iridium is quite difficult - almost impossible, in fact - to machine. The hazardous property of rhenium halide, for example, may be attributed either to rhenium itself or to the other elements that make up the compound. Another rhenium compound - potassium perrhenate - is known to have a median lethal dose much like that of sodium chloride (commonly known as table salt). The Eight Precious Metals Considered Noble Metals Most metallic chemical elements are known to easily oxidize and corrode, especially at high temperatures. Metals that have excellent resistance to oxidation and corrosion, even at high temperatures, are called noble metals. While there is no strict definition for this group of metals, it usually includes those that are extremely rare. Also called fire assay, the method involves treating ores or alloyed metals under high temperatures and carefully controlled operations to separate gold from base metals (copper, zinc, or lead), which may be present in the ore. Once the base metals are heated at high temperatures, the gold (as well as the other precious metals that may be present) remains apart and the other non-precious metals react forming other compounds. Sluice Box: This method uses the same principle as that in gold panning, only on a larger scale. In this method, a short sluice box is used. The box is constructed with barriers along its bottom, so that the gold particles are trapped as all materials are washed by water. The sluice box method is best suited for excavation in which certain implements, such as shovels, are used to feed ore into the box. On the reverse of the coin is an image of the springbok, an antelope specie which is a South African national symbol. Atop the image is an inscription of the name of the coin, in capital letters, while below it is the coin's gold content, inscribed in Afrikaans and English. Special samples of the coin (proof Krugerrand) are minted and offered as collector's items.