Silver

Silver,  ₄₇Ag

Electrolytically refined silver
General properties
Name, symbol	silver, Ag
Pronunciation	/ˈsɪlvər/ SIL-vər
Appearance	lustrous white metal
Silver in the periodic table
Cu ↑ Ag ↓ Au palladium ← silver → cadmium
Atomic number (Z)	47
Group, block	group 11, d-block
Period	period 5
Element category	transition metal
Standard atomic weight (±) (Ar)	107.8682(2)[1]
Electron configuration	[Kr] 4d^10 5s^1
per shell	2, 8, 18, 18, 1
Physical properties
Phase	solid
Melting point	1234.93 K ​(961.78 °C, ​1763.2 °F)
Boiling point	2435 K ​(2162 °C, ​3924 °F)
Density near r.t.	10.49 g/cm^3
when liquid, at m.p.	9.320 g/cm^3
Heat of fusion	11.28 kJ/mol
Heat of vaporization	254 kJ/mol
Molar heat capacity	25.350 J/(mol·K)
vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1283 1413 1575 1782 2055 2433
Atomic properties
Oxidation states	−2, −1, 1, 2, 3, 4 ​(an amphoteric oxide)
Electronegativity	Pauling scale: 1.93
Ionization energies	1st: 731.0 kJ/mol 2nd: 2070 kJ/mol 3rd: 3361 kJ/mol
Atomic radius	empirical: 144 pm
Covalent radius	145±5 pm
Van der Waals radius	172 pm
Miscellanea
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	2680 m/s (at r.t.)
Thermal expansion	18.9 µm/(m·K) (at 25 °C)
Thermal conductivity	429 W/(m·K)
Thermal diffusivity	174 mm^2/s (at 300 K)
Electrical resistivity	15.87 nΩ·m (at 20 °C)
Magnetic ordering	diamagnetic[2]
Young's modulus	83 GPa
Shear modulus	30 GPa
Bulk modulus	100 GPa
Poisson ratio	0.37
Mohs hardness	2.5
Vickers hardness	251 MPa
Brinell hardness	206–250 MPa
CAS Number	7440-22-4
History
Discovery	before 5000 BC
Most stable isotopes of silver
iso NA half-life DM DE (MeV) DP ^105Ag syn 41.2 d ε – ^105Pd γ 0.344, 0.280, 0.644, 0.443 – ^106mAg syn 8.28 d ε – ^106Pd γ 0.511, 0.717, 1.045, 0.450 – ^107Ag 51.839% is stable with 60 neutrons ^108mAg syn 418 y ε – ^108Pd IT 0.109 ^108Ag γ 0.433, 0.614, 0.722 – ^109Ag 48.161% is stable with 62 neutrons ^111Ag syn 7.45 d β^− 1.036, 0.694 ^111Cd γ 0.342 –

Silver is the metallic element with the atomic number 47. Its symbol is Ag, from the Latin argentum, derived from the Greek ὰργὀς (literally "shiny" or "white"), and ultimately from a Proto-Indo-European language root reconstructed as *h₂erǵ-, "grey" or "shining". A soft, white, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. The metal is found in the Earth's crust in the pure, free elemental form ("native silver"), as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a byproduct of copper, gold, lead, and zinc refining.

Silver has long been valued as a precious metal. Silver metal is used in many premodern monetary systems in bullion coins, sometimes alongside gold: while it is more abundant than gold, it is much less abundant as a native metal. Its purity is typically measured on a per-mille basis; a 94%-pure alloy is described as "0.940 fine". As one of the seven metals of antiquity, silver has had an enduring role in most human cultures.

Silver is used in numerous applications other than currency, such as solar panels, water filtration, jewelry, ornaments, high-value tableware and utensils (hence the term silverware), and as an investment medium (coins and bullion). Silver is used industrially in electrical contacts and conductors, in specialized mirrors, window coatings, and in catalysis of chemical reactions. Silver compounds are used in photographic film and X-rays. Dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides (oligodynamic effect), added to bandages and wound-dressings, catheters, and other medical instruments.

Characteristics

Silver is extremely ductile, and can be drawn into a monoatomic wire.^[3]

Silver is similar in its physical and chemical properties to its two vertical neighbours in group 11 of the periodic table, copper and gold. Its 47 electrons are arranged in the configuration [Kr]4d¹⁰5s¹, similarly to copper ([Ar]3d¹⁰4s¹) and gold ([Xe]4f¹⁴5d¹⁰6s¹); group 11 is one of the few groups in the d-block which has a completely consistent set of electron configurations.^[4] This distinctive electron configuration, with a single electron in the highest occupied s subshell over a filled d subshell, accounts for many of the singular properties of metallic silver.^[5]

Silver is an extremely soft, ductile and malleable transition metal, though it is slightly less malleable than gold. Silver crystallizes in a face-centered cubic lattice with bulk coordination number 12, where only the single 5s electron is delocalized, similarly to copper and gold.^[6] Unlike metals with incomplete d-shells, metallic bonds in silver are lacking a covalent character and are relatively weak. This observation explains the low hardness and high ductility of single crystals of silver.^[7]

Silver has a brilliant white metallic luster that can take a high polish,^[8] and which is so characteristic that the name of the metal itself has become a colour name.^[5] Unlike copper and gold, the energy required to excite an electron from the filled d band to the s-p conduction band in silver is large enough (around 385 kJ/mol) that it no longer corresponds to absorption in the visible region of the spectrum, but rather in the ultraviolet; hence silver is not a coloured metal.^[5] Protected silver has greater optical reflectivity than aluminium at all wavelengths longer than ~450 nm.^[9] At wavelengths shorter than 450 nm, silver's reflectivity is inferior to that of aluminium and drops to zero near 310 nm.^[10]

Very high electrical and thermal conductivity is common to the elements in group 11, because their single s electron is free and does not interact with the filled d subshell, as such interactions (which occur in the preceding transition metals) lower electron mobility.^[11] The electrical conductivity of silver is the greatest of all metals, greater even than copper, but it is not widely used for this property because of the higher cost. An exception is in radio-frequency engineering, particularly at VHF and higher frequencies where silver plating improves electrical conductivity because those currents tend to flow on the surface of conductors rather than through the interior. During World War II in the US, 13540 tons of silver were used in electromagnets for enriching uranium, mainly because of the wartime shortage of copper.^[12][13][14] Pure silver has the highest thermal conductivity of any metal, although the conductivity of carbon (in the diamond allotrope) and superfluid helium-4 are even higher.^[4] Silver also has the lowest contact resistance of any metal.^[4]

Silver readily forms alloys with copper and gold, as well as zinc. Zinc-silver alloys with low zinc concentration may be considered as face-centred cubic solid solutions of zinc in silver, as the structure of the silver is largely unchanged while the electron concentration rises as more zinc is added. Increasing the electron concentration further leads to body-centred cubic (electron concentration 1.5), complex cubic (1.615), and hexagonal close-packed phases (1.75).^[6]

Isotopes

Naturally occurring silver is composed of two stable isotopes, ¹⁰⁷Ag and ¹⁰⁹Ag, with ¹⁰⁷Ag being slightly more abundant (51.839% natural abundance). This almost equal abundance is rare in the periodic table. The atomic weight is 107.8682(2) u;^[15][16] this value is very important because of the importance of silver compounds, particularly halides, in gravimetric analysis.^[1] Both isotopes of silver are produced in stars via the s-process (slow neutron capture), as well as in supernovas via the r-process (rapid neutron capture).^[17]

Twenty-eight radioisotopes have been characterized, the most stable being ¹⁰⁵Ag with a half-life of 41.29 days, ¹¹¹Ag with a half-life of 7.45 days, and ¹¹²Ag with a half-life of 3.13 hours. Silver has numerous nuclear isomers, the most stable being ^108mAg (t_1/2 = 418 years), ^110mAg (t_1/2 = 249.79 days) and ^106mAg (t_1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives of less than an hour, and the majority of these have half-lives of less than three minutes.^[18]

Isotopes of silver range in relative atomic mass from 92.950 u (⁹³Ag) to 129.950 u (¹³⁰Ag);^[19] the primary decay mode before the most abundant stable isotope, ¹⁰⁷Ag, is electron capture and the primary mode after is beta decay. The primary decay products before ¹⁰⁷Ag are palladium (element 46) isotopes, and the primary products after are cadmium (element 48) isotopes.^[18]

The palladium isotope ¹⁰⁷Pd decays by beta emission to ¹⁰⁷Ag with a half-life of 6.5 million years. Iron meteorites are the only objects with a high-enough palladium-to-silver ratio to yield measurable variations in ¹⁰⁷Ag abundance. Radiogenic ¹⁰⁷Ag was first discovered in the Santa Clara meteorite in 1978.^[20] The discoverers suggest the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. ¹⁰⁷Pd–¹⁰⁷Ag correlations observed in bodies that have clearly been melted since the accretion of the solar system must reflect the presence of unstable nuclides in the early solar system.^[21]

Chemistry

Oxidation states and stereochemistries of silver^[22]

Oxidation state	Coordination number	Stereochemistry	Representative compound
0 (d^10s^1)	3	Planar	Ag(CO)3
1 (d^10)	2	Linear	[Ag(CN)2]^−
	3	Trigonal planar	AgI(PEt2Ar)2
	4	Tetrahedral	[Ag(diars)2]^+
	6	Octahedral	AgF, AgCl, AgBr
2 (d^9)	4	Square planar	[Ag(py)4]^2+
3 (d^8)	4	Square planar	[AgF4]^−
	6	Octahedral	[AgF6]^3−

Silver is a rather unreactive metal. This is because its filled 4d shell is not very effective in shielding the electrostatic forces of attraction from the nucleus to the outermost 5s electron, and hence silver is near the bottom of the electrochemical series (E⁰(Ag⁺/Ag) = +0.799 V).^[5] In group 11, silver has the lowest first ionization energy (showing the instability of the 5s orbital), but has higher second and third ionization energies than copper and gold (showing the stability of the 4d orbitals), so that the chemistry of silver is predominantly that of the +1 oxidation state, reflecting the increasingly limited range of oxidation states along the transition series as the d-orbitals fill and stabilize.^[23] Unlike copper, for which the larger hydration energy of Cu²⁺ as compared to Cu⁺ is the reason why the former is the more stable in aqueous solution and solids despite lacking the stable filled d-subshell of the latter, silver is large enough that this factor has a much smaller effect, and furthermore the second ionisation energy of silver is greater than that for copper. Hence, Ag⁺ is the stable species in aqueous solution and solids, with Ag²⁺ being much less stable as it oxidizes water.^[23]

It must be noted despite the above formulations that most silver compounds have significant covalent character due to the small size and high first ionization energy (730.8 kJ/mol) of silver.^[5] Furthermore, silver's Pauling electronegativity of 1.93 is higher than that of lead (1.87), and its electron affinity of 125.6 kJ/mol is much higher than that of hydrogen (72.8 kJ/mol) and not much less than that of oxygen (141.0 kJ/mol).^[24] Due to its full d-subshell, silver in its main +1 oxidation state exhibits relatively few properties of the transition metals proper from groups 4 to 10, forming rather unstable organometallic compounds, forming linear complexes showing very low coordination numbers like 2, and forming an amphoteric oxide^[25] as well as Zintl phases like the post-transition metals.^[26] Unlike the preceding transition metals, the +1 oxidation state of silver is stable even in the absence of π-acceptor ligands.^[23]

Silver does not react with air, even at red heat, and thus was considered by alchemists as a noble metal along with gold. Its reactivity is intermediate between that of copper (which forms copper(I) oxide when heated in air to red heat) and gold. Like copper, silver reacts with sulfur and its compounds; in their presence, silver tarnishes in air to form the black silver sulfide (copper forms the green sulfate instead, while gold does not react). Unlike copper, silver will not react with the halogens, with the exception of the notoriously reactive fluorine gas, with which it forms the difluoride. While silver is not attacked by non-oxidizing acids, the metal dissolves readily in hot concentrated sulfuric acid, as well as dilute or concentrated nitric acid. In the presence of air, and especially in the presence of hydrogen peroxide, silver dissolves readily in aqueous solutions of cyanide.^[22]

Silver metal is attacked by strong oxidizers such as potassium permanganate (KMnO
4) and potassium dichromate (K
2Cr
2O
7), and in the presence of potassium bromide (KBr). These compounds are used in photography to bleach silver images, converting them to silver bromide that can either be fixed with thiosulfate or redeveloped to intensify the original image. Silver forms cyanide complexes (silver cyanide) that are soluble in water in the presence of an excess of cyanide ions. Silver cyanide solutions are used in electroplating of silver.^[27]

Silver artifacts undergo three forms of deterioration, the most common of which is the formation of a black film of silver sulfide tarnish. Fresh silver chloride, formed when silver objects are immersed for long periods in salt water, is pale yellow colored, becoming purplish on exposure to light and projects slightly from the surface of the artifact or coin. The precipitation of copper in ancient silver can be used to date artifacts.^[28]

The common oxidation states of silver are (in order of commonness): +1 (for example, silver nitrate, AgNO₃); +2 (for example, silver(II) fluoride, AgF₂); +3 (for example, potassium tetrafluoroargentate(III), KAgF₄); and even occasionally +4 (for example, potassium hexafluoroargentate(IV), K₂AgF₆).^[29] The +1 state is by far the most common, followed by the reducing +2 state. The +3 state requires very strong oxidising agents to attain, such as fluorine or peroxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass.^[30] Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidizing agent, krypton difluoride.^[31]

Compounds

Oxides and chalcogenides

Silver(I) sulfide

Silver and gold have rather low chemical affinities for oxygen, lower than copper, and it is therefore expected that silver oxides are thermally quite unstable. Soluble silver(I) salts precipitate dark-brown silver(I) oxide, Ag₂O, upon the addition of alkali. (The hydroxide AgOH exists only in solution; otherwise it spontaneously decomposes to the oxide.) Silver(I) oxide is very easily reduced to metallic silver, and decomposes to silver and oxygen above 160 °C.^[32] This and other silver(I) compounds may be oxidized by the strong oxidizing agent peroxodisulfate to black AgO, a mixed silver(I,III) oxide of formula Ag^IAg^IIIO₂. Some other mixed oxides with silver in non-integral oxidation states, namely Ag₂O₃ and Ag₃O₄, are also known, as is Ag₃O which behaves as a metallic conductor.^[32]

Silver(I) sulfide, Ag₂S, is very readily formed from its constituent elements and is the cause of the black tarnish on some old silver objects. It may also be formed from the reaction of hydrogen sulfide with silver metal or aqueous Ag⁺ ions. Many non-stoichiometric selenides and tellurides are known; in particular, AgTe_~3 is a low-temperature superconductor.^[32]

Halides

The three common silver halide precipitates: from left to right, silver iodide, silver bromide, and silver chloride.

The only known dihalide of silver is the difluoride, AgF₂, which can be obtained from the elements under heat. A strong yet thermally stable fluorinating agent, silver(II) fluoride is often used to synthesize hydrofluorocarbons.^[33]

In stark contrast to this, all four silver(I) halides are known. The fluoride, chloride, and bromide have the sodium chloride structure, but the iodide has three known stable forms at different temperatures; that at room temperature is the cubic zinc blende structure. They can all be obtained from their elements.^[33] As the halogen group is descended, the silver halide gains more and more covalent character, solubility decreases, and the color changes from the white chloride to the yellow iodide as the energy required for ligand-metal charge transfer (X⁻Ag⁺ → XAg) decreases.^[33] The fluoride is anomalous, as the fluoride ion is so small that it has a considerable solvation energy and hence is highly water-soluble and forms di- and tetrahydrates.^[33] The other three silver halides are highly insoluble in aqueous solutions and are very commonly used in gravimetric analytical methods.^[1] All four are photosensitive (though the monofluoride is so only to ultraviolet light), especially the bromide and iodide which photodecompose to silver metal, and thus were used in traditional photography.^[33] The reaction involved is:^[34]

X⁻ + hν → X + e⁻ (excitation of the halide ion, which gives up its extra electron into the conduction band)

Ag⁺ + e⁻ → Ag (liberation of a silver ion, which gains an electron to become a silver atom)

The process is not reversible because the silver atom liberated is typically found at a crystal defect or an impurity site, so that the electron's energy is lowered enough that it is "trapped".^[34]

Other inorganic compounds

Crystals of silver nitrate

White silver nitrate, AgNO₃, is a versatile precursor to many other silver compounds, especially the halides, and is much less sensitive to light. It was once called lunar caustic because silver was called luna by the ancient alchemists, who believed that silver was associated with the moon.^[35] It is often used for gravimetric analysis, exploiting the insolubility of the silver halides which it is a common precursor to.^[1] Silver nitrate is used in many ways in organic synthesis, e.g. for deprotection and oxidations. Ag⁺ binds alkenes reversibly, and silver nitrate has been used to separate mixtures of alkenes by selective absorption. The resulting adduct can be decomposed with ammonia to release the free alkene.^[36]

Yellow silver carbonate, Ag₂CO₃ can be easily prepared by reacting aqueous solutions of sodium carbonate with a deficiency of silver nitrate.^[37] Its principal use is for the production of silver powder for use in microelectronics. It is reduced with formaldehyde, producing silver free of alkali metals:^[38]

Ag₂CO₃ + CH₂O → 2 Ag + 2 CO₂ + H₂

Silver carbonate is also used as a reagent in organic synthesis such as the Koenigs-Knorr reaction. In the Fétizon oxidation, silver carbonate on celite acts as an oxidising agent to form lactones from diols. It is also employed to convert alkyl bromides into alcohols.^[37]

Silver fulminate, AgCNO, a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol. Other dangerously explosive silver compounds are silver azide, AgN₃, formed by reaction of silver nitrate with sodium azide,^[39] and silver acetylide, Ag₂C₂, formed when silver reacts with acetylene gas in ammonia solution.^[23]

Coordination compounds

Structure of the diamminesilver(I) complex, [Ag(NH₃)₂]⁺

Silver complexes tend to be similar to those of its lighter homologue copper. Silver(III) complexes tend to be rare and very easily reduced to the more stable lower oxidation states, though they are slightly more stable than those of copper(III). For instance, the square planar periodate [Ag(IO₅OH)₂]⁵⁻ and tellurate [Ag{TeO₄(OH)₂}₂]⁵⁻ complexes may be prepared by oxidising silver(I) with alkaline peroxodisulfate. The yellow diamagnetic [AgF₄]⁻ is much less stable, fuming in moist air and reacting with glass.^[30]

Silver(II) complexes are more common. Like the valence isoelectronic copper(II) complexes, they are usually square planar and paramagnetic, which is increased by the greater field splitting for 4d electrons than for 3d electrons. Aqueous Ag²⁺, produced by oxidation of Ag⁺ by ozone, is a very strong oxidising agent, even in acidic solutions: it is stabilized in phosphoric acid due to complex formation. Peroxodisulfate oxidation is generally necessary to give the more stable complexes with heterocyclic amines, such as [Ag(py)₄]²⁺ and [Ag(bipy)₂]²⁺: these are stable provided the counterion cannot reduce the silver back to the +1 oxidation state. [AgF₄]²⁻ is also known in its violet barium salt, as are some silver(II) complexes with N- or O-donor ligands such as pyridine carboxylates.^[40]

However, the most important oxidation state for silver in complexes is +1. The Ag⁺ cation is diamagnetic, like its homologues Cu⁺ and Au⁺: its complexes are colourless provided the ligands are not too easily polarized such as I⁻. Ag⁺ forms salts with most anions, but it is reluctant to coordinate to oxygen and thus most of these salts are insoluble in water: the exceptions are the nitrate, perchlorate, and fluoride. The tetracoordinate tetrahedral aqueous ion [Ag(H₂O)₄]⁺ is known, but the characteristic geometry for the Ag⁺ cation is 2-coordinate linear. For example, silver chloride dissolves readily in excess aqueous ammonia to form [Ag(NH₃)₂]⁺; silver salts are dissolved in photography due to the formation of the thiosulfate complex [Ag(S₂O₃)₂]³⁻; and cyanide extraction for silver (and gold) works by the formation of the complex [Ag(CN)₂]⁻. Silver cyanide forms the linear polymer {Ag–C≡N→Ag–C≡N→}; silver thiocyanate has a similar structure, but forms a zigzag instead because of the sp³-hybridized sulfur atom. Chelating ligands are unable to form linear complexes and thus silver(I) complexes with them tend to form polymers; a few exceptions exist, such as the near-tetrahedral diphosphine and diarsine complexes [Ag(L–L)₂]⁺.^[41]

Organometallic

Under standard conditions, silver does not form simple carbonyls, due to the weakness of the Ag–C bond. A few are known at very low temperatures around 6–15 K, such as the green, planar paramagnetic Ag(CO)₃, which dimerizes at 25–30 K, probably by forming Ag–Ag bonds. Additionally, the silver carbonyl [Ag(CO)][B(OTeF₅)₄] is known. Polymeric AgLX complexes with alkenes and alkynes are known, but their bonds are thermodynamically weaker than even those of the platinum complexes (though they are formed more readily than those of the analogous gold complexes): they are also quite unsymmetrical, showing the weak π bonding in group 11. Ag–C σ bonds may also be formed by silver(I), like copper(I) and gold(I), but the simple alkyls and aryls of silver(I) are even less stable than those of copper(I) (which tend to explode under ambient conditions). For example, poor thermal stability is reflected in the relative decomposition temperatures of AgMe (−50 °C) and CuMe (−15 °C) as well as those of PhAg (74 °C) and PhCu (100 °C).^[42]

The C–Ag bond is stabilized by perfluoroalkyl ligands, for example in AgCF(CF₃)₂.^[43] Alkenylsilver compounds are also more stable than their alkylsilver counterparts.^[44] Silver-NHC complexes are easily prepared, and are commonly used to prepare other NHC complexes by displacing labile ligands. For example, the reaction of the bis(NHC)silver(I) complex with bis(acetonitrile)palladium dichloride or chlorido(dimethyl sulfide)gold(I):^[45]

Applications

Silver liquor goblet.

Silver is often used simply as a precious metal, including currency and decorative items. It has also long been used to confer high monetary value to objects (such as silver coins and investment bars) or make objects symbolic of high social or political rank.^[46]

The contrast between the bright white color of silver and other materials makes silver useful to the visual arts. By contrast, fine silver particles form the dense black in photographs and in silverpoint drawings. Silver salts have been used since the Middle Ages to produce a yellow or orange color in stained glass, and more complex decorative color reactions can be produced by incorporating silver metal in blown, kilnformed or torchworked glass.^[46]

Currency

Silver, in the form of electrum (a gold–silver alloy), was coined around 700 BC by the Lydians. Later, silver was refined and coined in its pure form. Many nations used silver as the basic unit of monetary value. In the modern world, silver bullion has the ISO currency code XAG. The name of the pound sterling (£) reflects the fact it originally represented the value of one pound Tower weight of sterling silver; the names of other historical currencies, such as the French livre, have similar origins. In some languages, including Sanskrit, Spanish, French, and Hebrew, the word for silver may be used to mean money.

During the 19th century, the bimetallism that prevailed in most countries was undermined by the discovery of large deposits of silver in the Americas; fearing a sharp decrease in the value of silver and inflation of the currency, most states moved to a gold standard by 1900.

The 20th century saw a gradual movement to fiat currency, with most of the world monetary system losing its link to precious metals after the United States dollar came off the gold standard in 1971; the last currency backed by gold was the Swiss franc, which became a pure fiat currency on 1 May 2000; the issues of 1967 and 1969 (for the 5 franc piece) and 1967 (for the others) were the last Swiss coins minted with silver.^[47] In the UK the silver standard was reduced from .925 to .500 in 1920. Coins that had been made of silver were changed to cupro-nickel in 1947; existing coins were not withdrawn, but ceased circulating as the silver content came to exceed the face value. In 1964 the United States stopped minting the silver dime and quarter; the last circulating silver coin was the 1970 40% half-dollar.^[48] In 1968, Canada minted its last circulating silver coins, the 50% dime and quarter.

For most of the century after the Civil War in the United States, the price of silver was less than the face value of circulating silver coins, reaching its nadir of about $.25 per ounce in 1932,^[49] and the silver coins of the United States were effectively fiat coins for much of that history. Not until 1963 did the price of silver rise above the threshold of $1.29 per ounce, at which time the silver content of pre-1965 United States coins was equal in value to the face value of the coins themselves.^[50]

Silver coins are still minted by several countries as commemorative or collectible items, not intended for general circulation.

Silver is used as a currency by many individuals, and is legal tender in the US state of Utah.^[51] Silver coin and bullion is an investment vehicle used by some people to guard against inflation and devaluation of the currency.

Jewelry and silverware

Jewelry and silverware are traditionally made from sterling silver (standard silver), an alloy of 92.5% silver with 7.5% copper. In the US, only alloys at least 0.900-fine silver can be sold as "silver" (frequently stamped 900). Sterling silver (stamped 925) is harder than pure silver and has a lower melting point (893 °C) than either pure silver or pure copper.^[4] Britannia silver is an alternative, hallmark-quality standard containing 95.8% silver, often used for silver tableware and wrought plate. The patented alloy Argentium sterling silver is formed by the addition of germanium, having improved properties including resistance to firescale.

Sterling silver jewelry is often plated with a thin coat of .999-fine silver to create a shiny finish. This process is called "flashing". Silver jewelry can also be plated with rhodium (for a bright shine) or gold (silver gilt).

Silver is a constituent of almost all colored carat gold alloys and carat gold solders, giving the alloys paler color and greater hardness.^[52] White 9-carat gold contains 62.5% silver and 37.5% gold, while 22-carat gold contains a minimum of 91.7% gold and 8.3% silver or copper or other metals.^[52]

Historically, the training and guild organization of goldsmiths included silversmiths, and the two crafts remain largely overlapping. Unlike blacksmiths, silversmiths do not shape the metal while it is softened with heat, but work it at room temperature with gentle and carefully placed hammer blows. The essence of silversmithing is to transform a piece of flat metal into a useful object with hammers, stakes, and other simple tools.^[53]

While silversmiths specialize and work principally in silver, they also work with other metals, such as gold, copper, steel, and brass, to make jewelry, silverware, armor, vases, and other artistic items. Because silver is so malleable, silversmiths have many choices for working the metal. Historically, silversmiths are usually called goldsmiths and are usually members of the same guild. The western Canadian silversmith tradition does not include guilds but mentoring through colleagues is a common method of professional advancement.^[54]

Traditionally, silversmiths mostly made "silverware" (cutlery, tableware, bowls, candlesticks and such). Handmade solid silver tableware is now much less common.

Solar energy

Solar modules mounted on solar trackers

Silver is used in the manufacture of crystalline solar photovoltaic panels.^[55] Silver is also used in plasmonic solar cells. 100 million ounces (685,714.3 pounds (311,034.8 kg)) of silver are projected for use by solar energy in 2015.^[56]

Silver is the reflective coating of choice for concentrated solar power reflectors.^[57] In 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that have the potential to be 30% less expensive than today's best collectors of concentrated solar power by replacing glass mirrors with a silver polymer sheet that has the same performance as the heavy glass, but at much less cost and weight, and much easier to deploy and install. The glossy film uses several layers of polymers, with an inner layer of pure silver.

Air conditioning

In 2014 researchers invented a mirror-like panel that, when mounted on a building, works as an air conditioner.^[58] The mirror is built from several layers of wafer-thin materials. The first layer is silver, the most reflective substance known. Above this are alternating layers of silicon dioxide and hafnium oxide. These layers improve the reflectivity, but also turn the mirror into a thermal radiator.

Water purification

Silver is used in water purifiers to prevent bacteria and algae from growing in the filters. The silver catalyzes oxygen and sanitizes the water, replacing chlorination. Silver ions are added to water purification systems in hospitals, community water systems, pools and spas, displacing chlorination.^[56]

Dentistry

Previously, silver was alloyed with mercury at room temperature to make amalgams widely used for dental fillings. To make dental amalgam, a mixture of powdered silver and other metals, such as tin and gold, was mixed with mercury to make a stiff paste that could be shaped to fill a drilled cavity. The dental amalgam achieves initial hardness within minutes and sets hard in a few hours.

Photography and electronics

The use of silver nitrate and silver halides in photography has rapidly declined with the advent of digital technology. From the peak global demand for photographic silver in 1999 (267,000,000 troy ounces or 8304.6 metric tonnes) the market contracted almost 70% by 2013.^[59]

Because even when tarnished, silver has superior electrical conductivity, it is used in some electrical and electronic products, notably high quality connectors for RF, VHF, and higher frequencies, particularly in tuned circuits such as cavity filters where conductors cannot be scaled by more than 6%. Printed circuits and RFID antennas are made with silver paints,^[4][60] and computer keyboards use silver electrical contacts. Silver cadmium oxide is used in high-voltage contacts because it withstands arcing.

Some manufacturers produce audio connector cables, speaker wires, and power cables with silver conductors, which have a 6% higher conductivity than those of copper with identical dimensions, despite increased cost. Though the issue is debated, many hi-fi enthusiasts believe silver wires improve sound quality.

Small devices, such as hearing aids and watches, commonly use silver oxide batteries because they have long life and a high energy-to-weight ratio. It is also used high-capacity silver-zinc and silver-cadmium batteries.

In World War II during a shortage of copper, silver was borrowed from the United States Treasury for electrical windings by several production facilities, including those of the Manhattan Project; see below under History, WWII.

Glass coatings

Telescopic mirrors

Mirrors in almost all reflective telescopes use vacuum aluminium coatings.^[61] However thermal or infrared telescopes use silver coated mirrors because it reflects some wavelengths of infrared radiation more effectively than aluminium, and because silver emits very little new thermal radiation (low thermal emissivity) from the mirror material.^[62]

Silver, in protected or enhanced coatings, is expected to be the next generation metal coating for reflective telescope mirrors.^[63]

Windows

Using a process called sputtering, silver, along with other optically transparent layers, is applied to glass, creating low emissivity coatings used in high-performance insulated glazing. The amount of silver used per window is small because the silver layer is only 10–15 nanometers thick.^[64] However, the amount of silver-coated glass worldwide is hundreds of millions of square meters per year, leading to silver consumption on the order of 10 cubic meters or 100 metric tons/year. Silver color seen in architectural glass and tinted windows on vehicles is produced by sputtered chrome, stainless steel or other alloys.

Silver-coated polyester sheets, used to retrofit windows, are another popular method for reducing window transparency.^[56]

Other industrial and commercial applications

This Yanagisawa A9932J alto saxophone has a solid silver bell and neck with a solid phosphor bronze body. The bell, neck, and key-cups are extensively engraved. It was manufactured in 2008.

Silver and silver alloys are used in some high-quality musical wind instruments.^[65] Flutes, in particular, are commonly constructed of silver alloy or silver-plated, both for appearance and for the surface friction properties of silver. Brass instruments, such as trumpets and baritone horns, are commonly plated in silver.^[66]

Silver is an ideal catalyst in oxidation reactions; for example, formaldehyde is produced from methanol and air using silver screens or crystallites of a minimum 99.95% silver. Silver (on some suitable support) is probably the only catalyst available today that converts ethylene to ethylene oxide (CH2-O-CH2) in the synthesis of ethylene glycol (used to produce polyesters) and polyethylene terephthalate. It is also used in the Oddy test to detect reduced sulfur compounds and carbonyl sulfides.

Because silver readily absorbs free neutrons, it is commonly added to control rods to regulate the fission chain reaction in pressurized water nuclear reactors, generally in the form of an alloy containing 80% silver, 15% indium, and 5% cadmium.

Silver is used in solder and brazing alloys, and as a thin layer on bearing surfaces, it provides a significant increase in galling resistance, reducing wear under heavy load, particularly against steel.

Biology

Silver stains are used in biology to increase the contrast and visibility of cells and organelles in microscopy. Camillo Golgi used silver stains to study cells of the nervous system and the Golgi apparatus.^[67] Silver stains are used to stain proteins in gel electrophoresis and polyacrylamide gels, either as primary stains or to enhance the visibility and contrast of colloidal gold stain.^[68]

Yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions. The fungus Aspergillus niger found growing in a gold mining solution was found to contain cyano metal complexes, such as gold, silver, copper, iron, and zinc. The fungus also plays a role in the solubilization of heavy metal sulfides.^[69]

Medicine

In medicine, silver is incorporated into wound dressings and used as an antibiotic coating in medical devices. Wound dressings containing silver sulfadiazine or silver nanomaterials are used to treat external infections. Silver is also used in some medical applications, such as urinary catheters (where tentative evidence indicates it reduces catheter-related urinary tract infections) and in endotracheal breathing tubes (where evidence suggests it reduces ventilator-associated pneumonia).^[70][71] The silver ion (Ag+
) is bioactive and in sufficient concentration readily kills bacteria in vitro. Silver and silver nanoparticles are used as an antimicrobial in a variety of industrial, healthcare, and domestic applications.^[72]

Investing

Silver coins and bullion are an investment vehicle. Silver investments of various types are available on stock markets, including mining, silver streaming, and silver-backed exchange-traded funds.^[73]

Clothing

Silver inhibits the growth of bacteria and fungi on clothing (such as socks) and is sometimes added to reduce odors and the risk of bacterial and fungal infections. It is incorporated into clothing or shoes either by integrating silver nanoparticles into the polymer from which yarns are made or by coating yarns with silver.^[74][75] The loss of silver during washing varies between textile technologies, and the effect on the environment is not yet fully known.^[76][77]

Gallery

• 

  Native silver

• 

  Silver 1000 oz t (~31 kg) bullion bar

• 

  Canada's Maple leaf 1 troy ounce silver bullion coin.

• 

  A Canadian 50 cent piece from 1951 made of 80% silver and 20% copper.

• 

  A silver cased so-called "mystery watch" with a transparent dial, c. 1890.

• 

  Shallow silver bowl, Persian, 6th century BC (Achaemenid).

• 

  Chain, Worn by a Woman. Silver, made in Syria. Brooklyn Museum.

• 

  Cessna 210 equipped with a silver iodide generator for cloud seeding

History

The crescent moon has been used since ancient times to represent silver.

Silver has been used for thousands of years for ornaments, utensils, and trade, and as the basis for many monetary systems. Its value as a precious metal was long considered second only to gold. The word "silver" appears in Anglo-Saxon in various spellings, such as seolfor and siolfor. A similar form is seen throughout the Germanic languages (compare Old High German silabar and silbir). The chemical symbol Ag is from the Latin word for "silver", argentum (compare Ancient Greek ἄργυρος, árgyros), from the Proto-Indo-European root *h₂erǵ- (formerly reconstructed as*arǵ-), meaning "white" or "shining". Silver is mentioned in the Book of Genesis. Slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate silver was being separated from lead as early as the 4th millennium BC.^[4] One of the earliest silver extraction centres in Europe was Sardinia in early Chalcolithic.^[78]

The stability of the Roman currency relied to a high degree on the supply of silver bullion, which Roman miners produced on a scale unparalleled before the discovery of the New World. Reaching a peak production of 200 t per year, an estimated silver stock of 10,000 t circulated in the Roman economy in the middle of the second century AD, five to ten times larger than the combined amount of silver available to medieval Europe and the Caliphate around 800 AD.^[79][80] Financial officials of the Roman Empire worried about the loss of silver to pay for silk from Sinica (China), which was in high demand.

Mines were worked in Laureion during 483 BC.^[81]

In the Gospels, Jesus' disciple Judas Iscariot is infamous for having taken a bribe of 30 coins of silver from religious leaders in Jerusalem to turn Jesus of Nazareth over to soldiers of the High Priest Caiaphas.^[82]

The Chinese Empire during most of its history used primarily silver as a means of exchange. In the 19th century, the threat to the balance of payments of the United Kingdom from Chinese merchants who required payment in silver for tea, silk, and porcelain led to the Opium War; Britain addressed the imbalance of payments by selling opium from British India to China.^[83]

Silver mining and processing in Kutná Hora, Central Europe, 1490s

Islam permits Muslim men to wear silver rings on the little finger of either hand.^[84] Muhammad himself wore a silver signet ring.^[85]

In the Americas, high temperature silver-lead cupellation technology was developed by pre-Inca civilizations as early as AD 60–120.^[86]

World War II

During World War II, the shortage of copper led to the substitution of silver in many industrial applications. The United States government loaned out silver from its massive reserve located in the West Point vaults to a wide range of industries. One important application was the bus bars in new aluminium plants for aircraft parts. During the war, many electrical connectors and switches were silver-plated. Silver was also used in aircraft master rod (and other) bearings. Since silver can replace tin in solder, but in a smaller proportion, substitution of government silver freed a large quantity of tin for other uses. Silver was also used for reflectors in searchlights and lights. Silver was used in nickels during the war to save that metal for use in steel alloys.^[87]

The Manhattan Project (to develop the atomic bomb) used about 14,700 tons of silver borrowed from the United States Treasury for calutron windings for the electromagnetic separation process in the Y-12 National Security Complex at the Oak Ridge National Laboratory. The oval "racetracks" had silver bus bars with a cross-section of one square foot. ^[88] After the war ended, the silver was returned to the government vaults.^[89]

Occurrence and extraction

Silver production in history

Silver is produced during certain types of supernova explosions by nucleosynthesis from lighter elements through the r-process, a form of nuclear fusion that produces many elements heavier than iron.^[90]

Silver is found in native form, as an alloy with gold (electrum), and in ores containing sulfur, arsenic, antimony, or chlorine. Ores include argentite (Ag₂S), chlorargyrite (AgCl, which includes horn silver), and pyrargyrite (Ag₃SbS₃). The principal sources of silver are the ores of copper, copper-nickel, lead, and lead-zinc obtained from Peru, Bolivia, Mexico, China, Australia, Chile, Poland and Serbia.^[4] Peru, Bolivia and Mexico have been mining silver since 1546, and are still major world producers. Top silver-producing mines are Cannington (Australia), Fresnillo (Mexico), San Cristóbal (Bolivia), Antamina (Peru), Rudna (Poland), and Penasquito (Mexico).^[91] Top near-term mine development projects through 2015 are Pascua Lama (Chile), Navidad (Argentina), Jaunicipio (Mexico), Malku Khota (Bolivia),^[92] and Hackett River (Canada).^[91] In Central Asia, Tajikistan is known to have some of the largest silver deposits in the world.^[93]

The metal is primarily produced as a byproduct of electrolytic copper refining, gold, nickel, and zinc refining, and by application of the Parkes process on lead bullion from ore that also contains silver. Commercial-grade fine silver is at least 99.9% pure, and purities greater than 99.999% are available. In 2014, Mexico was the top producer of silver (5,000 tonnes or 18.7% of the world's total of 26,800 t), followed by China (4,060 t) and Peru (3,780 t).^[94]

Price

Silver price history in 1960–2011

As of 4 April 2016, the price of silver was US$482.42 per kilogram (US$15.01 per troy ounce). This equates to approximately  ¹⁄₈₁ the price of gold at that time. The ratio has varied from  ¹⁄₁₅ to  ¹⁄₁₀₀ in the past 100 years. Physical silver bullion is higher priced than the paper certificates, with premiums increasing when demand is high and local shortages occur.^[95]

In 1980, the silver price rose to a peak for modern times of US$49.45 per troy ounce (ozt) due to market manipulation of Nelson Bunker Hunt and Herbert Hunt (equivalent to $142 in 2015). Some time after Silver Thursday, the price was back to $10/oz troy.^[96] From 2001 to 2010, the price moved from $4.37 to $20.19 (average London US$/oz).^[97] According to the Silver Institute, silver's recent gains have greatly stemmed from a rise in investor interest and an increase in fabrication demand.^[97] In late April 2011, silver reached an all-time high of $49.76/ozt.

In earlier times, silver has commanded much higher prices. In the early 15th century, the price of silver is estimated to have surpassed $1,200 per ounce, based on 2011 dollars.^[98] The discovery of massive silver deposits in the New World during the succeeding centuries has caused the price to diminish greatly.

The price of silver is important in Judaic law. The lowest fiscal amount over which a Jewish court, or Beth Din, can convene is a shova pruta (value of a Babylonian pruta coin). This is fixed at .025 grams (0.00088 oz) of pure, unrefined silver, at market price. In a Jewish tradition, still continuing today, on the first birthday of a first-born son, the parents pay the price of five pure-silver coins to a Kohen (priest). Today, the Israel mint fixes the coins at 117 grams (4.1 oz) of silver. The Kohen will often give those silver coins back as a gift for the child to inherit.^[99]

Human exposure and consumption

Silver has no known natural biological function in humans, and possible health effects of silver are a disputed subject.^[100] Silver itself is not toxic to humans, but most silver salts are. In large doses, silver and compounds containing it can be absorbed into the circulatory system and become deposited in various body tissues, leading to argyria, which results in a blue-grayish pigmentation of the skin, eyes, and mucous membranes. Argyria is rare, and so far as is known, does not otherwise harm a person's health, though it is disfiguring and usually permanent. Mild forms of argyria are sometimes mistaken for cyanosis.^[4]

Monitoring exposure

Overexposure to silver can occur in workers in the metallurgical industry, persons taking silver-containing dietary supplements, patients who have received silver sulfadiazine treatment, and individuals who accidentally or intentionally ingest silver salts. Silver concentrations in whole blood, plasma, serum, or urine may be monitored for safety in exposed workers, to confirm a diagnosis in suspected poisonings, or to assist the forensic investigation of a fatal overdose.^[101]

Use in food

Silver is used in food coloring; it has the E174 designation and is approved in the European Union.^[102]

Traditional Indian dishes sometimes include decorative silver foil known as vark,^[103] and in various other cultures, silver dragée are used to decorate cakes, cookies, and other dessert items.^[100]

Occupational safety and health

People can be exposed to silver in the workplace by inhalation, ingestion, skin contact, and eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (Permissible exposure limit) for silver exposure in the workplace at 0.01 mg/m³ over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 0.01 mg/m³ over an 8-hour workday. At levels of 10 mg/m³, silver is immediately dangerous to life and health.^[104]

See also

• Free silver
• List of countries by silver production
• List of silver compounds
• Silverpoint drawing

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Bibliography

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.

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