Noble metal


In chemistry, the noble metals are metals that are resistant to corrosion and oxidation in moist air. The short list of chemically noble metals comprises ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.
More inclusive lists include one or more of mercury, rhenium, and copper as noble metals. On the other hand, titanium, niobium, and tantalum are not included as noble metals although they are very resistant to corrosion.
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While the noble metals tend to be valuable – due to both their rarity in the Earth's crust and their applications in areas like metallurgy, high technology, and ornamentation – the terms noble metal and precious metal are not synonymous.
The term noble metal can be traced back to at least the late 14th century and has slightly different meanings in different fields of study and application. Only in atomic physics is there a strict definition, which includes only copper, silver, and gold, because they have completely filled d-subshells. For this reason, there are many quite different lists of "noble metals".
In addition to this term's function as a compound noun, there are circumstances where noble is used as an adjective for the noun metal. A galvanic series is a hierarchy of metals that runs from noble to active, and allows one to predict how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.

Properties

Platinum, gold and mercury can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, but iridium cannot. The solubility of silver is limited by the formation of silver chloride precipitate. Palladium and silver are, however, soluble in nitric acid. Ruthenium can be dissolved in aqua regia only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Niobium and tantalum are resistant to all acids, including aqua regia.

Physics

In physics, the definition of a noble metal is most strict. It requires that the d-bands of the electronic structure be filled. From this perspective, only copper, silver and gold are noble metals, as all d-like bands are filled and do not cross the Fermi level. However, d-hybridized bands do cross the Fermi level to a small extent. In the case of platinum, two d bands cross the Fermi level, changing its chemical behaviour such that it can function as a catalyst. The difference in reactivity can easily be seen during the preparation of clean metal surfaces in an ultra-high vacuum: surfaces of "physically defined" noble metals are easy to clean and keep clean for a long time, while those of platinum or palladium, for example, are covered by carbon monoxide very quickly.

Electrochemistry

Metallic elements, including metalloids :
ElementAtomic numberGroupPeriodReactionPotentialElectron configuration
Copernicium112127 + 2 e− → Cn2.1 V5f146d107s2
Roentgenium111117 + 3 e− → Rg1.9 V5f146d97s2
Darmstadtium110107 + 2 e− → Ds1.7 V5f146d87s2
Gold79116 + 3 e− → Au1.5 V4f145d106s1
Astatine85176 + e− → At1.0 V4f145d106s26p5
Platinum78106 + 2 + 2 e− → Pt + 0.98 V4f145d96s1
Palladium46105 + 2 e− → Pd0.915 V4d105s0
Flerovium114147 + 2 e− → Fl0.9 V5f146d107s27p2
Meitnerium10997 + 3 e− → Mt0.8 V5f146d77s2
Silver47115 + e− → Ag0.7993 V4d105s1
Mercury80126 + 2 e−→ 2 Hg0.7925 V4f145d106s2
Selenium34164 + 4 + 4 e− → Se + 3 0.739 V3d104s24p4
Iridium7796 + 4 + 4 e− → Ir + 2 0.73 V4f145d76s2
Osmium7686 + 4 + 4 e− → Os + 2 0.65 V4f145d66s2
Polonium84166 + 2 e− → Po0.6 V4f145d106s26p4
Nihonium113137 + e− → Nh0.6 V5f146d107s27p1
Rhodium4595 + 2 e− → Rh0.60 V4d85s1
Ruthenium4485 + 3 e− → Ru0.60 V4d75s1
Tellurium52165 + 4 + 4 e− → Te + 2 0.57 V4d105s25p4
Hassium10887 + 4 e− → Hs0.4 V5f146d67s2
Copper29114 + 2 e− → Cu0.339 V3d104s1
Bismuth83156 + 3 e− → Bi0.308 V4f145d106s26p3
Rhenium7576 + 4 + 4 e− → Re + 2 0.276 V4f145d56s2
Technetium4375 + 4 + 4 e− → Tc + 2 0.272 V4d55s2
Arsenic33154 + 12 + 12 e− → 4 As + 6 0.24 V3d104s24p3
Antimony51155 + 6 + 6 e− → 2 Sb + 3 0.147 V4d105s25p3
Livermorium116167 + 2 e− → Lv0.1 V5f146d107s27p4
Bohrium10777 + 5 e− → Bh0.1 V5f146d57s2

The columns group and period denote its position in the periodic table, hence electronic configuration. The simplified reactions, listed in the next column, can also be read in detail from the Pourbaix diagrams of the considered element in water. Finally the column potential indicates the electric potential of the element measured against a Standard hydrogen electrode. All missing elements in this table are either not metals or have a negative standard potential.
Arsenic, antimony and tellurium are considered to be metalloids and thus cannot be noble metals. Also, chemists and metallurgists consider copper and bismuth to not be noble metals because they easily oxidize due to the reaction + 2 + 4e− ⇄ 4 + 0.40 V which is possible in moist air.
The film of silver is due to its high sensitivity to hydrogen sulfide. Chemically, patina is caused by an attack of oxygen in wet air and by afterward. On the other hand, rhenium-coated mirrors are said to be very durable, although rhenium and technetium are said to tarnish slowly in moist atmosphere.
The superheavy elements from hassium to livermorium inclusive are expected to be "partially very noble metals"; chemical investigations of hassium has established that it behaves like its lighter congener osmium, and preliminary investigations of nihonium and flerovium have suggested but not definitively established noble behavior. Copernicium's behaviour seems to partly resemble both its lighter congener mercury and the noble gas radon.