Physics has an even more strict definition for noble metals. Here, it is required that the electronic structure's d-bands are filled. If this definition is followed, only gold and silver (among the precious metals) qualify as noble metals. Also note that the varying reactivity of the precious metals can readily be observed while preparing their surfaces in the vacuum regime called ultra high vacuum. 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). In another example, osmium tetroxide is used to detect fingerprints and to stain fatty tissue for optical and electron microscopy. A few also come in platinum though, including the American Platinum Eagle, the Canadian Platinum Maple Leaf, the Australian Platinum Koala, and the Manx Noble. Here's a quick rundown of each of these four platinum bullion coins: The American Platinum Eagle: The American Platinum Eagle bullion coins were first minted and released in 1997 by the United States Mint. They have higher melting points compared with other metals. The high-ductility property of gold and silver means they can be easily damaged as coins for circulation. Coins intended for circulation must be highly resistant to corrosion and wear. For this reason, gold or silver must be alloyed with other metals (example, manganese) so that the resulting coins will come out harder, more wear-resistant, and not easily damaged or deformed. mol-1 Atomic: Oxidation States: 1, 2, 3 Electronegativity: 1.93 (Pauling scale) Atomic Radius: 144 picometre Covalent Radius: 145 5 picometre Van der Waals Radius: 172 picometre Ionization Energies: 731 kJ.mol-1 (first), 2070 kJ.mol-1 (second), 3361 kJ.mol-1 (third) Despite having higher electrical conductivity than copper, silver isn't as much used for electrical purposes as copper is. cm-3 Liquid Density (at melting point): 10.38 g.cm-3 Melting Point: 1554.9 C, 2830.82 F, 1828.05 K Boiling Point: 2963 C, 5365 F, 3236 K Heat of Fusion: 16.74 kJ.mol-1 Heat of Vaporization: 362 kJ.mol-1 Atomic: Oxidation States: 0, +1, +2, +4, +6 Electronegativity: 2.2 (Pauling scale) Atomic Radius: 137 picometre Covalent Radius: 139 6 picometre Van der Waals Radius: 163 picometre Ionization Energies: 804.4 kJ.