Noble metal

A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, osmium, iridium) are most often so classified. Silver, copper and mercury are sometimes included as noble metals, however less often as each of these usually occurs in nature combined with sulfur.

Periodic table extract showing approximately how often each element tends to recognized as a noble metal:
 7  most often (Ru, Rh, Pd, Os, Ir, Pt, Au)[1]  1  often (Ag)[2]  2  sometimes (Cu, Hg)[3]  6  in a limited sense (Tc, Re, As, Sb, Bi, Po)
The thick black line encloses the seven to eight metals most often to often so recognized. Silver is sometimes not recognized as a noble metal on account of its greater reactivity.[4]
* may be tarnished in moist air or corrode in an acidic solution containing oxygen and an oxidant † attacked by sulfur or hydrogen sulfide
§ self-attacked by radiation-generated ozone

In more specialized fields of study and applications the number of elements counted as noble metals can be smaller or larger. In physics, there are only three noble metals: copper, silver and gold. In dentistry, silver is not always counted as a noble metal since it is subject to corrosion when present in the mouth. In chemistry, the term noble metal is sometimes applied more broadly to any metallic or semimetallic element that does not react with a weak acid and give off hydrogen gas in the process. This broader set includes copper, mercury, technetium, rhenium, arsenic, antimony, bismuth and polonium, as well as gold, the six platinum group metals, and silver.

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