In chemistry, noble metals are metallic elements that show outstanding resistance to chemical attack even at high temperatures. They are well known for their catalytic properties and associated capacity to facilitate or control the rates of chemical reactions. The short list of chemically noble metals (those elements upon which almost all chemists agree) comprises ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au). In periodic table terms the noble metals correspond to the noble gases.
More inclusive lists include one or more of copper (Cu), silver (Ag), rhenium (Re), and mercury (Hg) as noble metals. On the other hand, titanium (Ti), niobium (Nb), and tantalum (Ta) are not included as noble metals although they are very resistant to corrosion.
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 (jewelry, art, sacred objects, etc.) - 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. Although noble metal lists can differ, they tend to cluster around the six platinum group metals namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum; plus gold.
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 (or other electrically conductive materials, including composites and semimetals) 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.
The noble metals are siderophiles (iron-lovers). They tend to sink into the Earth's core because they dissolve readily in iron either as solid solutions or in the molten state. Most siderophile elements have practically no affinity whatsoever for oxygen: indeed, oxides of gold are thermodynamically unstable with respect to the elements.
|d = decomposes; if there are two figures, the 2nd is for|
the hydrated form; ? = not a noble metal; MD = metalloid
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.
According to Smith,
Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen. The table lists the melting points of the oxides of the noble metals, and for some of those of the non-noble metals, for the elements in their most stable oxidation states.
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 (e.g., gold) 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.
The following table lists standard reduction potential in volts;; electronegativity (revised Pauling); and electron affinity values (kJ/mol), for some metals and metalloids. Metals commonly recognised as noble metals are flagged with a ✣ symbol; elements marked ☢ are synthetic, radioactive, and short-lived; metalloids are denoted MD; values with a + are predicted; a blank = not available or applicable.
The simplified entries in the reaction column can be read in detail from the Pourbaix diagrams of the considered element in water. Noble metals have large positve potentials; elements not in this table have a negative standard potential or are not metals.
Electronegativity is included since it is reckoned to be, "a major driver of metal nobleness and reactivity". Predicted values for nihonium, flerovium, moscovium, and livermorium are given by Karol.
On account of their high electron affinity values, the incorporation of a noble metal in the electrochemical photolysis process, such as platinum and gold, among others, can increase photoactivity.
|Copernicium ☢||112||12||7||+ 2 e- -> Cn||2.1||+|
|Roentgenium ☢||111||11||7||+ 3 e- -> Rg||1.9||+|
|Darmstadtium ☢||110||10||7||+ 2 e- -> Ds||1.7||+|
|Gold ✣||79||11||6||+ 3 e- -> Au||1.5||2.54||223|
|Platinum ✣||78||10||6||+ 2 e- -> Pt||1.2||2.28||205|
|Iridium ✣||77||9||6||+ 3 e- -> Ir||1.16||2.2||151|
|Astatine ☢||85||17||6||+ e- -> At||1.0||2.2||233|
|Palladium ✣||46||10||5||+ 2 e- -> Pd||0.915||2.2||54|
|Flerovium ☢||114||14||7||+ 2 e- -> Fl||0.9||2.21||+|
|Osmium ✣||76||8||6||+ 4 + 4 e- -> Os + 2||0.85||2.2||104|
|Mercury||80||12||6||+ 2 e- -> Hg||0.85||2.0||-50|
|Rhodium ✣||45||9||5||+ 3 e- -> Rh||0.8||2.28||110|
|Meitnerium ☢||109||9||7||+ 3 e- -> Mt||0.8||+|
|Silver ✣||47||11||5||+ e- -> Ag||0.7993||1.93||126|
|Ruthenium ✣||44||8||5||+ 3 e- -> Ru||0.6||2.2||101|
|Polonium ☢||84||16||6||+ 2 e- -> Po||0.6||2.0||136|
|Nihonium ☢||113||13||7||+ e- -> Nh||0.6 +||2.09||+|
|Tellurium MD||52||16||5||+ 4 + 4 e- -> Te + 2||0.53||2.1||190|
|Rhenium||75||7||6||+ 3 e- -> Re||0.5||1.9||6|
|Water||75||7||6||+ 4 e- + -> 4 OH-||0.4|
|Hassium ☢||108||8||7||+ 4 e- -> Hs||0.4||+|
|Copper||29||11||4||+ 2 e- -> Cu||0.339||2.0||119|
|Bismuth||83||15||6||+ 3 e- -> Bi||0.308||2.02||91|
|Technetium ☢||43||7||5||3 e- -> Tc||0.3||1.9||53|
|Arsenic MD||33||15||4||+ 12 + 12 e- -> 4 As + 6||0.24||2.18||78|
|Antimony MD||51||15||5||+ 6 + 6 e- -> 2 Sb + 3||0.147||2.05||101|
|Bohrium ☢||107||7||7||+ 5 e- -> Bh||0.1||+|
|Livermorium ☢||116||16||7||+ 2 e- -> Lv||0.1||2.58||+|
Arsenic, antimony and tellurium are considered to be metalloids rather than noble metals.
The black tarnish commonly seen on silver arises from its sensitivity to hydrogen sulfide: 2Ag + H2S + ½O2 -> Ag2S + H2O. Rayner-Canham contends that, "silver is so much more chemically-reactive and has such a different chemistry, that it should not be considered as a 'noble metal'." In dentistry, silver is not regarded as a noble metal due to its tendency to corrode in the oral environment.
The relevance of the entry for water is addressed by Li et. al. in the context of galvanic corrosion. Such a process will only occur when:
The superheavy elements from hassium (element 108) to livermorium (116) 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.