Noble metal

Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. They tend to be precious metals, often due to rarity in the crust of the Earth. Examples include gold, silver, tantalum, platinum, palladium and rhodium.

Some of the noble metals can be dissolved in aqua regia, a highly concentrated mixture of acids.

The term can also be used in a relative sense. A "Galvanic series" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows designers to see at a glance how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver (even though it is alchemically more base) and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.

In physics the definition of a noble metal is even more strict. It is required that the d-bands of the electronic structure are filled. Taking this into account, only copper, silver and gold are noble metals, as all d-like band are filled and don't cross the Fermi level. For platinum two d-bands cross the Fermi level, changing its chemical behaviour; it is used (in contrast to gold, for example) as a catalyst. The different reactivity can easily be seen while preparing clean metal surfaces in ultra high vacuum; surfaces of noble metals (e.g., gold) are easy to clean and stay clean for a long time, while those of platinum or palladium, for example, are covered by carbon monoxide very quickly.