In terms of mass abundance, expressed in parts per billion (ppb), here's how the nine precious metals are ranked (rare to rarest): 9th: silver (75 ppb); 8th: palladium (15 ppb); 7th: gold (4 ppb); 6th: osmium (1.5 ppb); 5th to 3rd: iridium, rhodium, and ruthenium (1 ppb); 2nd: rhenium (0.7 ppb); and 1st: platinum (0.003 ppb). 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. Rhenium was discovered as a trace element in the mineral columbite and in platinum ores. Cupellation: This technique is considered the most exact, elaborate, and destructive method of assay and is best suited for gold bullion or stock. 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. To put it another way, bullion is valued based on the mass and purity of the metal used, instead of its artificial currency value. New sources of ore have been discovered and there also have been improvements in the mining and refining processes. These two factors may cause the values of gold, silver, and the other precious metals to diminish. mol-1 Heat of Vaporization: 324 kJ.mol-1 Atomic: Oxidation States: -1, 1, 2, 3, 4, 5 Electronegativity: 2.54 (Pauling scale) Atomic Radius: 144 picometre Covalent Radius: 136 6 picometre Van der Waals Radius: 166 picometre Ionization Energies: 890.1 kJ.mol-1 (first), 1980 kJ.mol-1 (second) The term "gold" was derived from "geolu", an Old English Anglo-Saxon word which means "yellow". The extreme toxicity and volatility of osmium's oxide makes it nearly impossible for this element to be used in its pure state. For this reason, it is often necessary to alloy osmium with other elements for use in high-wear applications. For example, osmiridium (a natural alloy of osmium and iridium) is alloyed with the other metals in the platinum group and used in instrument pivots and phonograph needles (apart from electrical contacts and fountain pen tips as mentioned earlier).
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