Which Metals Are Considered to Be Noble Metals?
Noble metals, like noble gases, react with other substances less easily. Specifically, noble metals don't oxidize or corrode in moist air. Oxidization occurs when a substance, such as iron, has a chemical reaction with oxygen. Noble metals occur in the Earth's crust only rarely, as do precious metals. The scientific community acknowledges eight noble metals.
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Ruthenium
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Ruthenium appears as a silvery-white compound. At atomic number 44, it falls into group 8 of the periodic table, along with osmium and iron. Unlike most other elements in this group, ruthenium only has one electron in its valence shell, allowing it to form covalent or ionic bonds.
Rhodium
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Rhodium, an element similar in appearance to ruthenium, is a precious metal. According to Kitco Metals, rhodium costs over $2,000 per ounce as of March 4, 2010. Rhodium reflects most light that strikes it, creating a characteristic sheen. Rhodium serves as a converter in automobile engines in order to create less harmful exhaust fumes.
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Palladium
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Palladium belongs to the platinum group of metals, but has the lowest density and melting point. According to ChemicalElements.com, palladium melts at 1,552 degrees Celsius. Palladium can also neutralize automobile exhaust fumes.
Silver
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Silver, one of the most well-known precious and noble metals, serves as the most efficient conductor of electricity. High-end electrical conductors use silver as their main material, but silver's high cost offsets its benefits. It occurs naturally as a lustrous white metal. Most of it forms as a byproduct of gold, copper, lead and zinc refining. Silver causes mirrors to have their reflective property.
Platinum
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Platinum occurs naturally as a gray-white metal. Like other metals in its group, platinum forms the material for electrical contacts and conductors. It most commonly sees use as a chemical catalyst, first used to ignite hydrogen. Platinum also does not tarnish or wear out, which is why watchmakers often use platinum in watch components.
Gold
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Gold, due to its rarity, has seen use as currency, investment material and as an artistic material for almost all of humanity's history. Gold has high malleability and ductility, and as such is extremely soft. Gold has high electrical conductivity.
Osmium
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Osmium is a bluish-gray metal that occurs mostly in nature as a platinum alloy. Osmium has twice the density of lead, and is the densest of all naturally occurring--unsynthesized--elements. It has the fourth-highest melting point of all elements. Because of its hardness, brittleness and melting point of 3,033 degrees Celsius, osmium is difficult to process.
Iridium
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Iridium, like osmium, has an extremely high density. It also has the most resistance to corrosion of any known metal. According to Precious Metal Investment, iridium reacts with other metals and compounds rarely. When alloyed with platinum, iridium forms a high-performance electrical conductor, and sees use in spark plugs in automobiles and airplanes.
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References
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