Editing Noble metal (section)

== Properties ==
[[File:Elemental abundances.svg|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 [[structure of the Earth|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 [[siderophile element|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 metal]]s are the only [[native metal]]s that occur naturally in relatively large amounts.{{citation needed|date=October 2020}}

===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.<ref name="WM2017">W. Xing, M. Lee, Geosys. Eng. 20, 216, 2017</ref>

Rhenium reacts with [[oxidizing acid]]s, and [[hydrogen peroxide]], and is said to be tarnished by moist air. Osmium and iridium are chemically inert in ambient conditions.<ref name ="Parish">Parish RV 1977, ''The metallic elements,'' Longman, London, p. 53, 115</ref> Platinum and gold can be dissolved in aqua regia.<ref name="HW2001">A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001</ref> Mercury reacts with oxidising acids.<ref name ="Parish"/>

In 2010, US researchers discovered that an organic "aqua regia" in the form of a mixture of [[thionyl chloride]] SOCl<sub>2</sub> and the organic solvent [[pyridine]] C<sub>5</sub>H<sub>5</sub>N 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.<ref>Urquhart J 2010, "[https://www.chemistryworld.com/news/challenging-aqua-regias-throne/3000805.article Challenging aqua regia's throne]", ''Chemistry World,'' 24 September</ref>

However, Gold can be dissolved in [[selenic acid]] (H<sub>2</sub>SeO<sub>4</sub>).

=== 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|Cs]] + Au -> [[CsAu]]
([[Caesium auride]], a yellow crystalline salt with the {{chem|Au|-}} ion).{{Citation needed|date=December 2024}} Platinum also exhibits similar properties with
BaPt, BaPt<sub>2</sub>, Cs<sub>2</sub>Pt (Barium and Caesium Platinides, which are reddish salts).<ref>{{cite journal|doi=10.1002/anie.200352314|title=Cs2Pt: A Platinide(-II) Exhibiting Complete Charge Separation|date=2003|last1=Karpov|first1=Andrey|last2=Nuss|first2=Jürgen|last3=Wedig|first3=Ulrich|last4=Jansen|first4=Martin|journal=Angewandte Chemie International Edition|volume=42|issue=39|pages=4818–21|pmid=14562358}}</ref><ref>{{cite journal| doi = 10.1039/b514631c |title = An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides|first1=Andrey |last1= Karpov| first2=Mitsuharu| pmid = 16479284 |last2=Konuma|first3=Martin |last3=Jansen|journal = Chemical Communications|volume = 44|date = 2006| issue = 8|pages = 838–840}}</ref>

=== Electronic ===
The expression noble metal is sometimes confined to copper, silver, and gold since their full d-subshells can contribute to their noble character.<ref>{{Cite journal |last1=Ruban |first1=A |last2=Hammer |first2=B |last3=Stoltze |first3=P |last4=Skriver |first4=H.L |last5=Nørskov |first5=J.K |date=1997 |title=Surface electronic structure and reactivity of transition and noble metals1Communication presented at the First Francqui Colloquium, Brussels, 19–20 February 1996.1 |url=https://linkinghub.elsevier.com/retrieve/pii/S1381116996003482 |journal=Journal of Molecular Catalysis A: Chemical |language=en |volume=115 |issue=3 |pages=421–429 |doi=10.1016/S1381-1169(96)00348-2}}</ref> 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.<ref>{{Cite journal |last1=Hammer |first1=B. |last2=Norskov |first2=J. K. |date=1995 |title=Why gold is the noblest of all the metals |url=https://www.nature.com/articles/376238a0 |journal=Nature |language=en |volume=376 |issue=6537 |pages=238–240 |doi=10.1038/376238a0 |bibcode=1995Natur.376..238H |issn=0028-0836}}</ref> Relativistic contributions are also important,<ref>{{Cite journal |last=Bartlett |first=Neil |date=1998 |title=Relativistic effects and the chemistry of gold |url=https://link.springer.com/10.1007/BF03215471 |journal=Gold Bulletin |language=en |volume=31 |issue=1 |pages=22–25 |doi=10.1007/BF03215471 |issn=0017-1557}}</ref> playing a role in the catalytic properties of gold.<ref>{{Cite journal |last1=Gorin |first1=David J. |last2=Toste |first2=F. Dean |date=2007-03-22 |title=Relativistic effects in homogeneous gold catalysis |url=https://www.nature.com/articles/nature05592 |journal=Nature |language=en |volume=446 |issue=7134 |pages=395–403 |doi=10.1038/nature05592 |pmid=17377576 |bibcode=2007Natur.446..395G |issn=0028-0836}}</ref>

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]],<ref>{{Cite journal |last1=Hammer |first1=B. |last2=Nørskov |first2=J.K. |date=1995 |title=Electronic factors determining the reactivity of metal surfaces |url=https://linkinghub.elsevier.com/retrieve/pii/0039602896800070 |journal=Surface Science |language=en |volume=343 |issue=3 |pages=211–220 |doi=10.1016/0039-6028(96)80007-0|bibcode=1995SurSc.343..211H }}</ref><ref>{{Cite journal |last1=Greeley |first1=Jeff |last2=Nørskov |first2=Jens K. |last3=Mavrikakis |first3=Manos |date=2002 |title=Electronic Structure and Catalysis on Metal Surfaces |url=https://www.annualreviews.org/doi/10.1146/annurev.physchem.53.100301.131630 |journal=Annual Review of Physical Chemistry |language=en |volume=53 |issue=1 |pages=319–348 |doi=10.1146/annurev.physchem.53.100301.131630 |pmid=11972011 |bibcode=2002ARPC...53..319G |issn=0066-426X}}</ref> 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 nanoparticle|silver]] and [[Gold nanoparticle|gold]] nanoparticles for [[surface-enhanced Raman spectroscopy]], [[localized surface plasmon]]s and other [[Plasmonic nanoparticles|plasmonic]] properties.<ref>{{Cite journal |last=Garcia |first=M A |date=2011 |title=Surface plasmons in metallic nanoparticles: fundamentals and applications |url=https://hal.science/hal-00633991 |journal=Journal of Physics D: Applied Physics |volume=44 |issue=28 |pages=283001 |doi=10.1088/0022-3727/44/28/283001|bibcode=2011JPhD...44B3001G }}</ref><ref>{{Cite journal |last1=Zhang |first1=Junxi |last2=Zhang |first2=Lide |last3=Xu |first3=Wei |date=2012-03-21 |title=Surface plasmon polaritons: physics and applications |url=https://iopscience.iop.org/article/10.1088/0022-3727/45/11/113001 |journal=Journal of Physics D: Applied Physics |volume=45 |issue=11 |pages=113001 |doi=10.1088/0022-3727/45/11/113001 |bibcode=2012JPhD...45k3001Z |issn=0022-3727}}</ref>

=== Electrochemical ===
[[Standard reduction potential]]s 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.<ref>G. Wulfsberg 2000, "Inorganic Chemistry", University Science Books, Sausalito, CA, pp. 270, 937.</ref>
{| 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 || {{chem|Au|3+}} + 3&nbsp;e<sup>−</sup> → Au || 1.5 ||2.54|| 223
|-
|| [[Platinum]] ✣ || 78 || 10 || 6 || {{chem|Pt|2+}} + 2&nbsp;e<sup>−</sup> → Pt || 1.2 ||2.28|| 205
|-
|| [[Iridium]] ✣ || 77 || 9 || 6 || {{chem|Ir|3+}} +  3&nbsp;e<sup>−</sup> → Ir || 1.16 ||2.2|| 151
|-
|| [[Palladium]] ✣ || 46 || 10 || 5 || {{chem|Pd|2+}} + 2&nbsp;e<sup>−</sup> → Pd || 0.915 ||2.2|| 54
|-
|| [[Osmium]] ✣ || 76 || 8 || 6 || {{chem|OsO|2}} +  4&nbsp;{{chem|H|+}} + 4&nbsp;e<sup>−</sup> → Os + 2&nbsp;{{chem|H|2|O}} || 0.85 ||2.2|| 104
|-
|| [[Mercury (element)|Mercury]] || 80 || 12 || 6 || {{chem|Hg|2+}} + 2&nbsp;e<sup>−</sup> → Hg || 0.85 ||2.0|| −50
|-
|| [[Rhodium]] ✣ || 45 || 9 || 5 || {{chem|Rh|3+}} + 3&nbsp;e<sup>−</sup> → Rh || 0.8 ||2.28|| 110
|-
|| [[Silver]] ✣ || 47 || 11 || 5 ||  {{chem|Ag|+}} + e<sup>−</sup> → Ag || 0.7993 ||1.93|| 126
|-
|| [[Ruthenium]] ✣ || 44 || 8 || 5 || {{chem|Ru|3+}} + 3&nbsp;e<sup>−</sup> → Ru || 0.6 ||2.2|| 101
|-
|| [[Polonium]] ☢ || 84 || 16 || 6 || {{chem|Po|2+}} + 2&nbsp;e<sup>−</sup> → Po || 0.6 || 2.0 || 136 
|- style="background:orange"
|| Water || || || || 2&nbsp;{{chem|H|2|O}} + 4&nbsp;e<sup>−</sup> +{{chem|O|2}} → 4 OH<sup>−</sup> || 0.4 || || 
|-
|| [[Copper]] || 29 || 11 || 4 || {{chem|Cu|2+}} + 2&nbsp;e<sup>−</sup> → Cu || 0.339 ||2.0 || 119 
|-
|| [[Bismuth]] || 83 || 15 || 6 || {{chem|Bi|3+}} + 3&nbsp;e<sup>−</sup> → Bi || 0.308 ||2.02 || 91 
|-
|| [[Technetium]] ☢ || 43 || 7 || 6 || {{chem|Tc|O|2}} + 4&nbsp;{{chem|H|+}} + 4&nbsp;e<sup>−</sup> → Tc + 2&nbsp;{{chem|H|2|O}} || 0.28 ||1.9 || 53 
|-
|| [[Rhenium]] || 75 || 7 || 6 || {{chem|Re|O|2}} + 4&nbsp;{{chem|H|+}} + 4&nbsp;e<sup>−</sup> → Re + 2&nbsp;{{chem|H|2|O}} || 0.251 ||1.9 || 6 
|-
|| [[Arsenic]]<sup>MD</sup> || 33 || 15 || 4 || {{chem|As|4|O|6}} + 12&nbsp;{{chem|H|+}} + 12&nbsp;e<sup>−</sup> → 4 As + 6&nbsp;{{chem|H|2|O}} || 0.24 ||2.18 || 78 
|-
|| [[Antimony]]<sup>MD</sup> || 51 || 15 || 5 || {{chem|Sb|2|O|3}} + 6&nbsp;{{chem|H|+}} + 6&nbsp;e<sup>−</sup> → 2 Sb + 3&nbsp;{{chem|H|2|O}} || 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; <sup>MD</sup> metalloid; ☢ radioactive</small>
|}
The adjacent table lists [[standard reduction potential]] in volts;<ref>G. Wulfsberg, "Inorganic Chemistry", University Science Books, 2000, pp. 247–249 ✦ Bratsch S. G., "Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K", ''Journal of Physical Chemical Reference Data,'' vol. 18, no. 1, 1989, pp. 1–21 ✦ B. Douglas, D. McDaniel, J. Alexander, "Concepts and Models of Inorganic Chemistry", John Wiley & Sons, 1994, p. E-3</ref> 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 diagram]]s of the considered element in water. Noble metals have large positive potentials;<ref name="Ahmad 2006 40">{{cite book |last=Ahmad |first= Z|date=2006 |title= Principles of corrosion engineering and corrosion control|location= Amsterdam|publisher= Elsevier|page=40 |isbn= 9780080480336}}</ref> 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".<ref name="Kepp"/>

The black tarnish commonly seen on silver arises from its sensitivity to sulphur containing gases such as [[hydrogen sulfide]]:

:2 Ag + H<sub>2</sub>S + {{sfrac|1|2}}O<sub>2</sub> → Ag<sub>2</sub>S + H<sub>2</sub>O.

Rayner-Canham<ref name="RC">{{cite book |last=Rayner-Canham|first=G|editor-last1=Scerri |editor-first1=E |editor-last2=Restrepo |editor-first2=G |title=Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table |publisher=Oxford University |date=2018 |pages=195–205 |chapter=Organizing the transition metals|isbn=978-0-190-668532}}</ref> 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.<ref>
{{cite book |last1=Powers |first1= JM|last2=Wataha|first2=JE|date= 2013|title= Dental materials: Properties and manipulation|location=St Louis |publisher= Elsevier Health Sciences|page= 134|isbn= 9780323291507 |edition=10th}}</ref>

The relevance of the entry for water is addressed by Li et al.<ref>{{cite book |last1=Li |first1=Y |last2=Lu|first2=D|last3=Wong|first3=CP|date=2010 |title=Electrical conductive adhesives with nanotechnologies |location=New York |publisher=Springer |page=179 |isbn=978-0-387-88782-1}}</ref> 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 ({{chem|H|2|O}} + 4e + {{chem|O|2}} = 4 OH<sup><big>•</big></sup>) 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 {{chem|H|2|O}} − 4&nbsp;e<sup><big>•</big></sup> − {{chem|O|2}} = 4 OH<sup><big>•</big></sup>)."

The [[superheavy element]]s 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.<ref>{{cite journal |last1=Nagame |first1=Yuichiro |last2=Kratz |first2=Jens Volker |last3=Matthias |first3=Schädel |date=December 2015 |title=Chemical studies of elements with Z ≥ 104 in liquid phase |journal=Nuclear Physics A |volume=944 |pages=614–639 |doi=10.1016/j.nuclphysa.2015.07.013|bibcode=2015NuPhA.944..614N |url=https://jopss.jaea.go.jp/search/servlet/search?5050598 }}</ref> [[Copernicium]]'s behaviour seems to partly resemble both its lighter congener mercury and the noble gas [[radon]].<ref name=CRNL>{{cite journal |last1=Mewes |first1=J.-M. |last2=Smits |first2=O. R. |last3=Kresse |first3=G. |last4=Schwerdtfeger |first4=P. |title=Copernicium is a Relativistic Noble Liquid |journal=Angewandte Chemie International Edition  |date=2019 |volume=58 |issue=50 |pages=17964–17968 |doi=10.1002/anie.201906966 |pmid=31596013 |pmc=6916354 |url=https://www.researchgate.net/publication/336389017|doi-access=free }}</ref>

===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 ||  ||  ||  ||  ||
|-
| Ruthenium ||  ||  ||  || d1300 ||  ||  || 25
|-
| Rhodium ||  ||  || d1100 || d1050 ||  ||  ||
|-
| Palladium ||  || d750{{#tag:ref|Palladium oxide PdO can be reduced to palladium metal by exposing it to hydrogen in ambient conditions<ref name="HW2001"/>|group=n}} ||  ||  ||  ||  ||
|-
| Silver || d200 || d100{{#tag:ref|Ag<sub>4</sub>O<sub>4</sub> is a mixed oxidation state compound silver in the oxidation state of +1 and +3.|group=n}} ||  ||  ||  ||  ||
|-
| Rhenium ||  ||  ||  || d1000 || d400 || 327 ||
|-
| Osmium ||  ||  ||  || d500 ||  ||  || 40
|-
| Iridium ||  ||  ||  || d1100 ||  ||  || 
|-
| Platinum ||  ||  ||  || 450 ||  ||  ||
|-
| Gold ||  ||  || d150 ||   ||   ||   ||
|-
| Mercury ||  || d500 ||   ||   ||   ||   ||
|-
| Strontium‡ ||  || 2430 ||   ||   ||   ||   ||
|-
| Molybdenum‡ ||  || ||  ||  || 801 ||  ||
|-
| Antimony<sup>MD</sup> ||  || ||655   ||   ||    ||   ||
|-
| Lanthanum‡ ||  ||  ||2320   ||   ||   ||   ||
|-
| Bismuth‡ ||  || || 817  ||   ||   ||    ||
|-
|colspan="8"|d = decomposes; ‡ = not a noble metal; <sup>MD</sup> = metalloid
|}

As long ago as 1890, Hiorns observed as follows:

:"'''Noble Metals.''' Gold, Platinum, Silver, and a few  rare metals. The members of this class have little or no tendency to unite with oxygen in the free state, and when placed in water at a red heat do not alter its composition. The oxides are readily decomposed by heat in consequence of the feeble affinity between the metal and oxygen."<ref>Hiorns AH 1890, ''[https://archive.org/details/mixedmetalsormet00hiorrich Mixed metals or metallic alloys]'', p. 7</ref>

Smith, writing in 1946, continued the theme:
:"There is no sharp dividing line [between 'noble metals' and 'base metals'] but perhaps the best definition of a noble metal is a metal whose oxide is easily decomposed at a temperature below a red heat."{{#tag:ref|Incipient red heat corresponds to 525 °C<ref>Hiorns RH 1890, Mixed metals or metallic alloys, MacMillian, New York, p. 5</ref>|group=n}}<ref>{{cite book |last=Smith |first=JC |date=1946 |title= The chemistry and metallurgy of dental materials |location= Oxford|publisher= Blackwell|page=40}}</ref>

:"It follows from this that noble metals...have little attraction for oxygen and are consequently not oxidised or discoloured at moderate temperatures."
Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen.<ref name="Kepp"/> 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.

=== Catalytic properties ===
All the noble metals can act as catalysts. For example, platinum is used in [[catalytic converter]]s, devices which convert toxic gases produced in car engines, such as the oxides of nitrogen, into non-polluting substances.{{Citation needed|date=September 2024}}

Gold has many industrial applications; it is used as a catalyst in [[hydrogenation]] and the [[Water–gas shift reaction|water gas shift]] reaction.{{Citation needed|date=September 2024}}