Metallic elements that are nearly chemically inert
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 greaterreactivity.[4] * may betarnished in moist air orcorrode in anacidic solution containingoxygen and anoxidant † attacked bysulfur orhydrogen sulfide § self-attacked by radiation-generatedozone
In more specialized fields of study and applications the number of elements counted as noble metals can be smaller or larger. It is sometimes used for the three metalscopper, silver, and gold which have filledd-bands, while it is often used mainly for silver and gold when discussingsurface-enhanced Raman spectroscopy involving metalnanoparticles. It is sometimes applied more broadly to any metallic orsemimetallic 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 sixplatinum group metals, and silver.
Many of the noble metals are used in alloys for jewelry or coinage. Indentistry, silver is not always considered a noble metal because it is subject to corrosion when present in the mouth. All the metals are importantheterogeneous catalysts.
While lists of noble metals can differ, they tend to cluster around gold and the sixplatinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum.
In addition to this term's function as a compoundnoun, there are circumstances wherenoble is used as an adjective for the nounmetal. Agalvanic series is a hierarchy of metals (or other electrically conductive materials, including composites andsemimetals) 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 foraluminium andstainless steel in conditions of varyingpH.[5]
The termnoble metal can be traced back to at least the late 14th century[6] 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; and platinum, iridium, and osmium were grouped together,[7] and adjacent to silver and gold.
Chalcopyrite, which is copper iron sulfide (CuFeS2), is the most abundant copper ore mineral
One half of a ruthenium bar. Size ~ 40 × 15 × 10 mm Weight ~44 g
Rhodium: 1 g powder, 1g pressed cylinder, 1 g pellet.
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 theGoldschmidt classification of elements. These have been depleted by being relocated deeper into theEarth's core. Their abundance inmeteoroid materials is relatively higher. Tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.
The noble metals aresiderophiles (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.
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.
Ruthenium can be dissolved inaqua regia, a highly concentrated mixture ofhydrochloric acid andnitric acid, only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Palladium and silver are soluble innitric acid, while silver's solubility in aqua regia is limited by the formation ofsilver chloride precipitate.[8]
Rhenium reacts withoxidizing acids, andhydrogen peroxide, and is said to be tarnished by moist air. Osmium and iridium are chemically inert in ambient conditions.[9] Platinum and gold can be dissolved in aqua regia.[10] Mercury reacts with oxidising acids.[9]
In 2010, US researchers discovered that an organic "aqua regia" in the form of a mixture ofthionyl chloride SOCl2 and the organic solventpyridine 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.[11]
However, Gold can be dissolved inselenic acid (H2SeO4).
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:
Cs + Au ->CsAu(Caesium auride, a yellow crystalline salt with theAu− ion).[citation needed] Platinum also exhibits similar properties withBaPt, BaPt2, Cs2Pt (Barium and Caesium Platinides, which are reddish salts).[12][13]
The expression noble metal is sometimes confined to copper, silver, and gold since their full d-subshells can contribute to their noble character.[14] 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.[15] Relativistic contributions are also important,[16] playing a role in the catalytic properties of gold.[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. A common explanation is the d-band filling model of Hammer andJens Nørskov,[18][19] where the total d-bands are considered, not just the unoccupied states.
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.[22]
Electrochemical properties of some metals and metalloids
Z atomic number;G group;P period;SRP standard reduction potential;EN electronegativity;EA electron affinity
✣ traditionally recognized as a noble metal;MD metalloid; ☢ radioactive
The adjacent table listsstandard reduction potential in volts;[23] 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 thePourbaix diagrams of the considered element in water. Noble metals have large positive potentials;[24] 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".[3]
The black tarnish commonly seen on silver arises from its sensitivity to sulphur containing gases such ashydrogen sulfide:
2 Ag + H2S +1/2O2 → Ag2S + H2O.
Rayner-Canham[4] 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'." Indentistry, silver is not regarded as a noble metal due to its tendency to corrode in the oral environment.[25]
The relevance of the entry for water is addressed by Li et al.[26] 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 (H 2O + 4e +O 2 = 4 OH•) 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 generallyH 2O − 4 e• −O 2 = 4 OH•)."
Thesuperheavy elements fromhassium (element 108) tolivermorium (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 ofnihonium andflerovium have suggested but not definitively established noble behavior.[27]Copernicium's behaviour seems to partly resemble both its lighter congener mercury and the noble gasradon.[28]
d = decomposes; ‡ = not a noble metal;MD = 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."[29]
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."[n 3][31]
"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.[3] 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.
All the noble metals can act as catalysts. For example, platinum is used incatalytic converters, devices which convert toxic gases produced in car engines, such as the oxides of nitrogen, into non-polluting substances.[citation needed]
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^Schlamp, G (2018). "Noble metals and noble metal alloys". In Warlimont, H; Martienssen, W (eds.).Springer Handbook of Materials Data. Springer Handbooks. Cham: Springer. pp. 339–412.doi:10.1007/978-3-319-69743-7_14.ISBN978-3-319-69741-3.
^abRayner-Canham, G (2018). "Organizing the transition metals". In Scerri, E; Restrepo, G (eds.).Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table. Oxford University. pp. 195–205.ISBN978-0-190-668532.
^Everett Collier, "The Boatowner's Guide to Corrosion", International Marine Publishing, 2001, p. 21
^Constable EC 2019, "Evolution and understanding of the d-block elements in the periodic table",Dalton Transactions, vol. 48, no. 26, pp. 9408-9421doi:10.1039/C9DT00765B
^Karpov, Andrey; Nuss, Jürgen; Wedig, Ulrich; Jansen, Martin (2003). "Cs2Pt: A Platinide(-II) Exhibiting Complete Charge Separation".Angewandte Chemie International Edition.42 (39):4818–21.doi:10.1002/anie.200352314.PMID14562358.
^Karpov, Andrey; Konuma, Mitsuharu; Jansen, Martin (2006). "An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides".Chemical Communications.44 (8):838–840.doi:10.1039/b514631c.PMID16479284.
^G. Wulfsberg 2000, "Inorganic Chemistry", University Science Books, Sausalito, CA, pp. 270, 937.
^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
^Ahmad, Z (2006).Principles of corrosion engineering and corrosion control. Amsterdam: Elsevier. p. 40.ISBN9780080480336.
^Powers, JM; Wataha, JE (2013).Dental materials: Properties and manipulation (10th ed.). St Louis: Elsevier Health Sciences. p. 134.ISBN9780323291507.
^Li, Y; Lu, D; Wong, CP (2010).Electrical conductive adhesives with nanotechnologies. New York: Springer. p. 179.ISBN978-0-387-88782-1.
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