Noble metal

In more specialized fields of study and applications the number of elements counted as noble metals can be smaller or larger.

It 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, polonium, gold, the six platinum group metals, and silver.

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.

[9] In 2010, US researchers discovered that an organic "aqua regia" in the form of a mixture of thionyl chloride SOCl2 and the organic solvent pyridine C5H5N achieved "high dissolution rates of noble metals under mild conditions, with the added benefit of being tunable to a specific metal" for example, gold but not palladium or platinum.

[citation needed] Platinum also exhibits similar properties with BaPt, BaPt2, Cs2Pt (Barium and Caesium Platinides, which are reddish salts).

[17] The elements to the left of gold and silver have incompletely filled d-bands, which is believed to play a role in their catalytic properties.

Noble metals, in contrast, are disinclined to react with oxygen and, for that reason (as well as their scarcity) have been valued for millennia, and used in jewellery and coins.

[22] The adjacent table lists standard reduction potential in volts;[23] electronegativity (revised Pauling); and electron affinity values (kJ/mol), for some metals and metalloids.

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.

[28] As long ago as 1890, Hiorns observed as follows: Smith, writing in 1946, continued the theme: Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen.

Periodic table extract showing approximately how often each element tends to be 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
Abundance of the chemical elements in the Earth's crust as a function of atomic number. The rarest elements (shown in yellow, including the noble metals) are not the heaviest, but are rather the siderophile (iron-loving) elements in the Goldschmidt classification of elements. These have been depleted by being relocated deeper into the Earth's core . Their abundance in meteoroid materials is relatively higher. Tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.