Noble metal

[[File:PT extract noble metalsN.png|thumb|upright=2.1|

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Periodic table extract showing approximately how often each element tends to be recognized as a noble metal:

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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.

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.

Prior to Mendeleev's publication in 1869 of the first (eventually) widely accepted periodic table, Odling published a table in 1864, in which the "noble metals" rhodium, ruthenium, palladium, platinum, iridium, and osmium were grouped together, adjacent to silver and gold.

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File:Chalcopyrite-199453.jpg|<div align="center">Chalcopyrite, which is copper iron sulfide (CuFeS₂), is the most abundant copper ore mineral</div>

File:Ruthenium a half bar.jpg|One half of a ruthenium bar.<br />Size ~ 40 × 15 × 10&nbsp;mm<br />Weight ~44 g

File:Rhodium powder pressed melted.jpg|<div align="center">Rhodium: 1 g powder, 1g pressed cylinder, 1 g pellet.</div>

File:Palladium (46 Pd).jpg|<div align="center">Palladium</div>

File:Acanthite - Imiter mine, Jbel Saghro, Tinghir, Drâa-Tafilalet, Morocco.jpg|<div align="center">Acanthite, or silver sulfide (Ag₂S)</div>

File:Osmium crystals.jpg|<div align="center">Osmium crystals, 2.2 g</div>

File:Iridium-2.jpg|<div align="center">Pieces of pure iridium, 1 g, size: 1–3&nbsp;mm each</div>

File:Platinum crystals.jpg|<div align="center">Crystals of pure platinum</div>

File:Gold nugget (Australia) 4 (16848647509).jpg|<div align="center">Gold nugget from Australia, nearly 9,000 g or 317 oz</div>

File:Cinnabarit 01.jpg|Cinnabar or mercury sulfide (HgS) is the most common source ore for refining elemental mercury

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Properties

thumb|350px|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.]]

Geochemical

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.

Copper, silver, gold, and the six platinum group metals are the only native metals that occur naturally in relatively large amounts.

Corrosion resistance

Noble metals tend to be resistant to oxidation and other forms of corrosion, and this corrosion resistance is often considered to be a defining characteristic. Some exceptions are described below.

Copper is dissolved by nitric acid and aqueous potassium cyanide.

Ruthenium can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Palladium and silver are soluble in nitric acid, while silver's solubility in aqua regia is limited by the formation of silver chloride precipitate.

Rhenium reacts with oxidizing acids, and hydrogen peroxide, and is said to be tarnished by moist air. Osmium and iridium are chemically inert in ambient conditions. Platinum and gold can be dissolved in aqua regia. Mercury reacts with oxidising acids.

However, gold can be dissolved in selenic acid (H₂SeO₄).

Anion (-ide) formation

The noble elements gold and platinum also have a comparatively high electronegativity for a metallic element, thus allowing them to exist as single-metallic anions.

For example:

:

Caesium auride, a yellow crystalline salt with the ion. Platinum also exhibits similar properties with

BaPt, BaPt₂, Cs₂Pt (barium and caesium platinides, which are reddish salts).

Electronic

The expression noble metal is sometimes confined to copper, silver, and gold since their full d-subshells can contribute to their noble character. There are also known to be significant contributions from how readily there is overlap of the d-electron states with the orbitals of other elements, particularly for gold. Relativistic contributions are also important, playing a role in the catalytic properties of gold.

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. A common explanation is the d-band filling model of Hammer and Jens Nørskov, where the total d-bands are considered, not just the unoccupied states.

The low-energy plasmon properties are also of some importance, particularly those of silver and gold nanoparticles for surface-enhanced Raman spectroscopy, localized surface plasmons and other plasmonic properties.

Electrochemical

Standard reduction potentials in aqueous solution are also a useful way of predicting the non-aqueous chemistry of the metals involved. Thus, metals with high negative potentials, such as sodium, or potassium, will ignite in air, forming the respective oxides. These fires cannot be extinguished with water, which also react with the metals involved to give hydrogen, which is itself explosive. 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.

{| class="wikitable sortable" style="font-size:90%; float:right; margin-left:20px"

|+ Electrochemical properties of some metals and metalloids

|-

!Element !! Z !! G !! P !! Reaction !! SRP(V) || EN||EA

|-

|| Gold ✣ || 79 || 11 || 6 || + 3&nbsp;e⁻ → Au || 1.5 ||2.54|| 223

|-

|| Platinum ✣ || 78 || 10 || 6 || + 2&nbsp;e⁻ → Pt || 1.2 ||2.28|| 205

|-

|| Iridium ✣ || 77 || 9 || 6 || + 3&nbsp;e⁻ → Ir || 1.16 ||2.2|| 151

|-

|| Palladium ✣ || 46 || 10 || 5 || + 2&nbsp;e⁻ → Pd || 0.915 ||2.2|| 54

|-

|| Osmium ✣ || 76 || 8 || 6 || + 4&nbsp; + 4&nbsp;e⁻ → Os + 2&nbsp; || 0.85 ||2.2|| 104

|-

|| Mercury || 80 || 12 || 6 || + 2&nbsp;e⁻ → Hg || 0.85 ||2.0|| −50

|-

|| Rhodium ✣ || 45 || 9 || 5 || + 3&nbsp;e⁻ → Rh || 0.8 ||2.28|| 110

|-

|| Silver ✣ || 47 || 11 || 5 || + e⁻ → Ag || 0.7993 ||1.93|| 126

|-

|| Ruthenium ✣ || 44 || 8 || 5 || + 3&nbsp;e⁻ → Ru || 0.6 ||2.2|| 101

|-

|| Polonium ☢ || 84 || 16 || 6 || + 2&nbsp;e⁻ → Po || 0.6 || 2.0 || 136

|- style="background:orange"

|| Water || || || || 2&nbsp; + 4&nbsp;e⁻ + → 4 OH⁻ || 0.4 || ||

|-

|| Copper || 29 || 11 || 4 || + 2&nbsp;e⁻ → Cu || 0.339 ||2.0 || 119

|-

|| Bismuth || 83 || 15 || 6 || + 3&nbsp;e⁻ → Bi || 0.308 ||2.02 || 91

|-

|| Technetium ☢ || 43 || 7 || 6 || + 4&nbsp; + 4&nbsp;e⁻ → Tc + 2&nbsp; || 0.28 ||1.9 || 53

|-

|| Rhenium || 75 || 7 || 6 || + 4&nbsp; + 4&nbsp;e⁻ → Re + 2&nbsp; || 0.251 ||1.9 || 6

|-

|| Arsenic^MD || 33 || 15 || 4 || + 12&nbsp; + 12&nbsp;e⁻ → 4 As + 6&nbsp; || 0.24 ||2.18 || 78

|-

|| Antimony^MD || 51 || 15 || 5 || + 6&nbsp; + 6&nbsp;e⁻ → 2 Sb + 3&nbsp; || 0.147 ||2.05 || 101

|-

|colspan=8|<small>Z atomic number; G group; P period; SRP standard reduction potential; EN electronegativity; EA electron affinity</small>

|-

|colspan=8|<small>✣ traditionally recognized as a noble metal; ^MD metalloid; ☢ radioactive</small>

|}

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

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 positive 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". 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:

:"(1) two metals which have different electrochemical potentials are...connected, (2) an aqueous phase with electrolyte exists, and (3) one of the two metals has...potential lower than the potential of the reaction ( + 4e + = 4 OH^{<big>•</big>}) which is 0.4 V...The...metal with...a potential less than 0.4 V acts as an anode...loses electrons...and dissolves in the aqueous medium. The noble metal (with higher electrochemical potential) acts as a cathode and, under many conditions, the reaction on this electrode is generally − 4&nbsp;e^{<big>•</big>} − = 4 OH^{<big>•</big>})."

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. Moscovium has been also investigated to behave similarly to its lighter congener bismuth.

Oxides

{| class="wikitable" style="font-size:90%; float:right; margin-left:20px"

|+ Oxide melting points, °C

|-

! Element !! I !! II !! III !! IV !! VI !! VII !! VIII

|-

| Copper || 1232 || 1326 || || || || ||

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| Ruthenium || || || || d1300 || || || 25

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| Rhodium || || || d1100 || d1050 || || ||

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| Palladium || || d750