Silver

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Silver,  47Ag
Silver crystal.jpg
Silver
Appearancelustrous white metal
Standard atomic weight Ar, std(Ag)107.8682(2) [1]
Silver in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Cu

Ag

Au
palladiumsilvercadmium
Atomic number (Z)47
Group group 11
Period period 5
Block d-block
Element category   Transition metal
Electron configuration [ Kr ] 4d10 5s1
Electrons per shell
2, 8, 18, 18, 1
Physical properties
Phase at  STP 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/cm3
when liquid (at m.p.)9.320 g/cm3
Heat of fusion 11.28  kJ/mol
Heat of vaporisation 254 kJ/mol
Molar heat capacity 25.350 J/(mol·K)
Vapour pressure
P (Pa)1101001 k10 k100 k
at T (K)128314131575178220552433
Atomic properties
Oxidation states −2, −1, +1, +2, +3 (an  amphoteric oxide)
Electronegativity Pauling scale: 1.93
Ionisation 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
Color lines in a spectral range Silver spectrum visible.png
Color lines in a spectral range
Spectral lines of silver
Other properties
Natural occurrence primordial
Crystal structure face-centred cubic (fcc)
Cubic-face-centered.svg
Speed of sound thin rod2680 m/s(at r.t.)
Thermal expansion 18.9 µm/(m·K)(at 25 °C)
Thermal conductivity 429 W/(m·K)
Thermal diffusivity 174 mm2/s(at 300 K)
Electrical resistivity 15.87 nΩ·m(at 20 °C)
Magnetic ordering diamagnetic [2]
Magnetic susceptibility 19.5·10−6 cm3/mol(296 K) [3]
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
Main isotopes of silver
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
105Ag syn 41.2 d ε 105Pd
γ
106m Agsyn8.28 dε 106Pd
γ
107Ag51.839% stable
108mAgsyn418 yε 108Pd
IT 108Ag
γ
109Ag48.161%stable
110mAgsyn249.95 d β 110Cd
γ
111Agsyn7.45 dβ 111Cd
γ
| references

Silver is a chemical element with the symbol Ag (from the Latin argentum, derived from the Proto-Indo-European h₂erǵ: "shiny" or "white") and atomic number 47. 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.

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons.

Symbol (chemistry) an arbitrary or conventional sign used in chemical science to represent a chemical element

In chemistry, a symbol is an abbreviation for a chemical element. Symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the first letter capitalised.

Latin Indo-European language of the Italic family

Latin is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets and ultimately from the Phoenician alphabet.

Contents

Silver has long been valued as a precious metal. Silver metal is used in many bullion coins, sometimes alongside gold: [4] while it is more abundant than gold, it is much less abundant as a native metal. [5] 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.

Precious metal rare, naturally occurring metallic chemical element of high economic and cultural value

A precious metal is a rare, naturally occurring metallic chemical element of high economic value. Chemically, the precious metals tend to be less reactive than most elements. They are usually ductile and have a high lustre. Historically, precious metals were important as currency but are now regarded mainly as investment and industrial commodities. Gold, silver, platinum, and palladium each have an ISO 4217 currency code.

A bullion coin is a coin struck from precious metal and kept as a store of value or an investment rather than used in day-to-day commerce. A bullion coin is distinguished by an explicit statement of weight and fineness on the coin; this is because the weight and composition of coins intended for legal tender is specified in the coinage laws of the issuing nation, and therefore there is no need for an explicit statement on the coins themselves. The United Kingdom defines investment coins more specifically as coins that have been minted after 1800, have a purity of not less than 900 thousandths and are, or have been, legal tender in their country of origin. Under United States law, "coins" that fail the last of these requirements are not coins at all, and must be advertised as "rounds" instead. The American Eagle and Canadian Gold Maple Leaf series are the only coins available in gold, silver, platinum, and palladium.

Bimetallism monetary standard

Bimetallism is a monetary standard in which the value of the monetary unit is defined as equivalent to certain quantities of two metals, typically gold and silver, creating a fixed rate of exchange between them.

Other than in currency and as an investment medium (coins and bullion), silver is used in solar panels, water filtration, jewellery, ornaments, high-value tableware and utensils (hence the term silverware), in electrical contacts and conductors, in specialized mirrors, window coatings, in catalysis of chemical reactions, as a colorant in stained glass and in specialised confectionery. Its compounds are used in photographic and X-ray film. Dilute solutions of silver nitrate and other silver compounds are used as disinfectants and microbiocides (oligodynamic effect), added to bandages and wound-dressings, catheters, and other medical instruments.

A currency, in the most specific sense is money in any form when in use or circulation as a medium of exchange, especially circulating banknotes and coins. A more general definition is that a currency is a system of money in common use, especially for people in a nation. Under this definition, US dollars (US$), pounds sterling (£), Australian dollars (A$), European euros (€), Russian rubles (₽) and Indian Rupees (₹) are examples of currencies. These various currencies are recognized as stores of value and are traded between nations in foreign exchange markets, which determine the relative values of the different currencies. Currencies in this sense are defined by governments, and each type has limited boundaries of acceptance.

To invest is to allocate money in the expectation of some benefit in the future.

Jewellery Form of personal adornment

Jewellery or jewelry consists of small decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a western perspective, the term is restricted to durable ornaments, excluding flowers for example. For many centuries metal, often combined with gemstones, has been the normal material for jewellery, but other materials such as shells and other plant materials may be used. It is one of the oldest type of archaeological artefact – with 100,000-year-old beads made from Nassarius shells thought to be the oldest known jewellery. The basic forms of jewellery vary between cultures but are often extremely long-lived; in European cultures the most common forms of jewellery listed above have persisted since ancient times, while other forms such as adornments for the nose or ankle, important in other cultures, are much less common.

Characteristics

Silver is extremely ductile, and can be drawn into a wire one atom wide. Ag atomic wire.jpg
Silver is extremely ductile, and can be drawn into a wire one atom wide.

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. [7] 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. [8]

Group 11 element group of chemical elements

Group 11, by modern IUPAC numbering, is a group of chemical elements in the periodic table, consisting of copper (Cu), silver (Ag), and gold (Au). Roentgenium (Rg) is also placed in this group in the periodic table, although no chemical experiments have yet been carried out to confirm that it behaves like the heavier homologue to gold. Group 11 is also known as the coinage metals, due to their former usage. They were most likely the first three elements discovered. Copper, silver, and gold all occur naturally in elemental form.

Periodic table Tabular arrangement of the chemical elements ordered by atomic number

The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, which are arranged by atomic number, electron configuration, and recurring chemical properties. The structure of the table shows periodic trends. The seven rows of the table, called periods, generally have metals on the left and non-metals on the right. The columns, called groups, contain elements with similar chemical behaviours. Six groups have accepted names as well as assigned numbers: for example, group 17 elements are the halogens; and group 18 are the noble gases. Also displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals.

Copper Chemical element with atomic number 29

Copper is a chemical element with the symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

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. [9] 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. [10]

Ductility Material ability to undergo significant plastic deformation before rupture

Ductility is a measure of a material's ability to undergo significant plastic deformation before rupture, which may be expressed as percent elongation or percent area reduction from a tensile test. According to Shigley's Mechanical Engineering Design significant denotes about 5.0 percent elongation. See also Eq. 2–12, p. 50 for definitions of percent elongation and percent area reduction. Ductility is often characterized by a material's ability to be stretched into a wire.

In chemistry, the term transition metal has three possible meanings:

Covalent bond chemical bond that involves the sharing of electron pairs between atoms

A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration. In organic chemistry covalent bonds are much more common than ionic bonds.

Silver has a brilliant white metallic luster that can take a high polish, [11] and which is so characteristic that the name of the metal itself has become a colour name. [8] 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. [8] Protected silver has greater optical reflectivity than aluminium at all wavelengths longer than ~450 nm. [12] At wavelengths shorter than 450 nm, silver's reflectivity is inferior to that of aluminium and drops to zero near 310 nm. [13]

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials, polishing is also able to reduce diffuse reflection to minimal values. When an unpolished surface is magnified thousands of times, it usually looks like mountains and valleys. By repeated abrasion, those "mountains" are worn down until they are flat or just small "hills." The process of polishing with abrasives starts with coarse ones and graduates to fine ones.

Ultraviolet Electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays

Ultraviolet (UV) designates a band of the electromagnetic spectrum with wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, and contributes about 10% of the total electromagnetic radiation output from the Sun. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.

Aluminium Chemical element with atomic number 13

Aluminium is a chemical element with the symbol Al and atomic number 13. It is a silvery-white, soft, non-magnetic and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; it is the third most abundant element after oxygen and silicon and the most abundant metal in the crust, though it is less common in the mantle below. The chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals.

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. [14] 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. [15] [16] [17] 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. [7] Silver also has the lowest contact resistance of any metal. [7]

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). [9]

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; [18] [19] this value is very important because of the importance of silver compounds, particularly halides, in gravimetric analysis. [18] 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). [20]

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 (t1/2 = 418 years), 110mAg (t1/2 = 249.79 days) and 106mAg (t1/2 = 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. [21]

Isotopes of silver range in relative atomic mass from 92.950 u (93Ag) to 129.950 u (130Ag); [22] 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. [21]

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. [23] 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. [24]

Chemistry

Oxidation states and stereochemistries of silver [25]
Oxidation
state
Coordination
number
StereochemistryRepresentative
compound
0 (d10s1)3PlanarAg(CO)3
1 (d10)2Linear[Ag(CN)2]
3Trigonal planarAgI(PEt2Ar)2
4Tetrahedral[Ag(diars)2]+
6OctahedralAgF, AgCl, AgBr
2 (d9)4Square planar[Ag(py)4]2+
3 (d8)4Square planar[AgF4]
6Octahedral[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). [8] 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. [26] 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, with silver this effect is swamped by its larger second ionisation energy. Hence, Ag+ is the stable species in aqueous solution and solids, with Ag2+ being much less stable as it oxidizes water. [26]

Most silver compounds have significant covalent character due to the small size and high first ionization energy (730.8 kJ/mol) of silver. [8] 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). [27] 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 [28] as well as Zintl phases like the post-transition metals. [29] Unlike the preceding transition metals, the +1 oxidation state of silver is stable even in the absence of π-acceptor ligands. [26]

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 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. [25]

The three main forms of deterioration in historical silver artifacts are tarnishing, formation of silver chloride due to long-term immersion in salt water, as well as reaction with nitrate ions or oxygen. Fresh silver chloride is pale yellow, becoming purplish on exposure to light; it projects slightly from the surface of the artifact or coin. The precipitation of copper in ancient silver can be used to date artifacts, as copper is nearly always a constituent of silver alloys. [30]

Silver metal is attacked by strong oxidizers such as potassium permanganate (KMnO
4
) and potassium dichromate (K
2
Cr
2
O
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. [31]

The common oxidation states of silver are (in order of commonness): +1 (the most stable state; for example, silver nitrate, AgNO3); +2 (highly oxidising; for example, silver(II) fluoride, AgF2); and even very rarely +3 (extreme oxidising; for example, potassium tetrafluoroargentate(III), KAgF4). [32] The +1 state is by far the most common, followed by the easily reducible +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. [33] Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidizing agent, krypton difluoride. [34]

Compounds

Oxides and chalcogenides

Silver(I) sulfide Sulfid stribrny.PNG
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. [35] 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 AgIAgIIIO2. 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. [35]

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. [35]

Halides

The three common silver halide precipitates: from left to right, silver iodide, silver bromide, and silver chloride. Common Silver Halide Precipitates.jpg
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 and therefore safe fluorinating agent, silver(II) fluoride is often used to synthesize hydrofluorocarbons. [36]

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 by the direct reaction of their respective elements. [36] 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 (XAg+ → XAg) decreases. [36] 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. [36] The other three silver halides are highly insoluble in aqueous solutions and are very commonly used in gravimetric analytical methods. [18] 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. [36] The reaction involved is: [37]

X + → 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". [37]

Other inorganic compounds

Crystals of silver nitrate Silver nitrate crystals.jpg
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. [38] It is often used for gravimetric analysis, exploiting the insolubility of the heavier silver halides which it is a common precursor to. [18] 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. [39]

Yellow silver carbonate, Ag2CO3 can be easily prepared by reacting aqueous solutions of sodium carbonate with a deficiency of silver nitrate. [40] 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: [41]

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. [40]

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, [42] and silver acetylide, Ag2C2, formed when silver reacts with acetylene gas in ammonia solution. [26] In its most characteristic reaction, silver azide decomposes explosively, releasing nitrogen gas: given the photosensitivity of silver salts, this behaviour may be induced by shining a light on its crystals. [26]

2 AgN
3
(s) → 3 N
2
(g) + 2 Ag (s)

Coordination compounds

Structure of the diamminesilver(I) complex, [Ag(NH3)2] Diamminesilver(I)-3D-balls.png
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. [33]

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. [43]

By far the most important oxidation state for silver in complexes is +1. The Ag+ cation is diamagnetic, like its homologues Cu+ and Au+, as all three have closed-shell electron configurations with no unpaired electrons: 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]+. [44]

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). [45]

The C–Ag bond is stabilized by perfluoroalkyl ligands, for example in AgCF(CF3)2. [46] Alkenylsilver compounds are also more stable than their alkylsilver counterparts. [47] 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): [48]

Silver-NHC as carbene transmetallation agent.png

Intermetallic

Different colors of silver-copper-gold alloys Ag-Au-Cu-colours-english.svg
Different colors of silver–copper–gold alloys

Silver forms alloys with most other elements on the periodic table. The elements from groups 1–3, except for hydrogen, lithium, and beryllium, are very miscible with silver in the condensed phase and form intermetallic compounds; those from groups 4–9 are only poorly miscible; the elements in groups 10–14 (except boron and carbon) have very complex Ag–M phase diagrams and form the most commercially important alloys; and the remaining elements on the periodic table have no consistency in their Ag–M phase diagrams. By far the most important such alloys are those with copper: most silver used for coinage and jewellery is in reality a silver–copper alloy, and the eutectic mixture is used in vacuum brazing. The two metals are completely miscible as liquids but not as solids; their importance in industry comes from the fact that their properties tend to be suitable over a wide range of variation in silver and copper concentration, although most useful alloys tend to be richer in silver than the eutectic mixture (71.9% silver and 28.1% copper by weight, and 60.1% silver and 28.1% copper by atom). [49]

Most other binary alloys are of little use: for example, silver–gold alloys are too soft and silver–cadmium alloys too toxic. Ternary alloys have much greater importance: dental amalgams are usually silver–tin–mercury alloys, silver–copper–gold alloys are very important in jewellery (usually on the gold-rich side) and have a vast range of hardnesses and colours, silver–copper–zinc alloys are useful as low-melting brazing alloys, and silver–cadmium–indium (involving three adjacent elements on the periodic table) is useful in nuclear reactors because of its high thermal neutron capture cross-section, good conduction of heat, mechanical stability, and resistance to corrosion in hot water. [49]

Etymology

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": this was the usual Proto-Indo-European word for the metal, whose reflexes are missing in Germanic and Balto-Slavic. The Balto-Slavic words for silver are quite similar to the Germanic ones (e.g. Russian серебро [serebro], Polish srebro, Lithuanian sidabras) and they may have a common origin, although this is uncertain: some scholars have suggested the Akkadian sarpu "refined silver" as this origin, related to the word sarapu "to refine or smelt". [50] [51]

History

Silver plate from the 4th century Vadasztal (2).jpg
Silver plate from the 4th century

Silver was one of the seven metals of antiquity that were known to prehistoric humans and whose discovery is thus lost to history. [52] In particular, the three metals of group 11, copper, silver, and gold, occur in the elemental form in nature and were probably used as the first primitive forms of money as opposed to simple bartering. [53] However, unlike copper, silver did not lead to the growth of metallurgy on account of its low structural strength, and was more often used ornamentally or as money. [54] Since silver is more reactive than gold, supplies of native silver were much more limited than those of gold. [53] For example, silver was more expensive than gold in Egypt until around the fifteenth century BC: [55] the Egyptians are thought to have separated gold from silver by heating the metals with salt, and then reducing the silver chloride produced to the metal. [56]

The situation changed with the discovery of cupellation, a technique that allowed silver metal to be extracted from its ores. While slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate that silver was being separated from lead as early as the 4th millennium BC, [7] and one of the earliest silver extraction centres in Europe was Sardinia in the early Chalcolithic period, [57] these techniques did not spread widely until later, when it spread throughout the region and beyond. [55] The origins of silver production in India, China, and Japan were almost certainly equally ancient, but are not well-documented due to their great age. [56]

Silver mining and processing in Kutna Hora, Bohemia, 1490s Silver mining in Kutna Hora 1490s.jpg
Silver mining and processing in Kutná Hora, Bohemia, 1490s

When the Phoenicians first came to what is now Spain, they obtained so much silver that they could not fit it all on their ships, and as a result used silver to weight their anchors instead of lead. [55] By the time of the Greek and Roman civilizations, silver coins were a staple of the economy: [53] the Greeks were already extracting silver from galena by the 7th century BC, [55] and the rise of Athens was partly made possible by the nearby silver mines at Laurium, from which they extracted about 30 tonnes a year from 600 to 300 BC. [58] The stability of the Roman currency relied to a high degree on the supply of silver bullion, mostly from Spain, which Roman miners produced on a scale unparalleled before the discovery of the New World. Reaching a peak production of 200 tonnes per year, an estimated silver stock of 10000 tonnes 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 Abbasid Caliphate around AD 800. [59] [60] The Romans also recorded the extraction of silver in central and northern Europe in the same time period. This production came to a nearly complete halt with the fall of the Roman Empire, not to resume until the time of Charlemagne: by then, tens of thousands of tonnes of silver had already been extracted. [56]

Central Europe became the centre of silver production during the Middle Ages, as the Mediterranean deposits exploited by the ancient civilisations had been exhausted. Silver mines were opened in Bohemia, Saxony, Erzgebirge, Alsace, the Lahn region, Siegerland, Silesia, Hungary, Norway, Steiermark, Salzburg, and the southern Black Forest. Most of these ores were quite rich in silver and could simply be separated by hand from the remaining rock and then smelted; some deposits of native silver were also encountered. Many of these mines were soon exhausted, but a few of them remained active until the Industrial Revolution, before which the world production of silver was around a meagre 50 tonnes per year. [56] In the Americas, high temperature silver-lead cupellation technology was developed by pre-Inca civilizations as early as AD 60–120; silver deposits in India, China, Japan, and pre-Columbian America continued to be mined during this time. [56] [61]

With the discovery of America and the plundering of silver by the Spanish conquistadors, Central and South America became the dominant producers of silver until around the beginning of the 18th century, particularly Peru, Bolivia, Chile, and Argentina: [56] the last of these countries later took its name from that of the metal that composed so much of its mineral wealth. [58] The silver trade gave way to a global network of exchange. As one historian put it, silver "went round the world and made the world go round." [62] Much of this silver ended up in the hands of the Chinese. A Portuguese merchant in 1621 noted that silver "wanders throughout all the world... before flocking to China, where it remains as if at its natural center." [63] Still, much of it went to Spain, allowing Spanish rulers to pursue military and political ambitions in both Europe and the Americas. "New World mines," concluded several historians, "supported the Spanish empire." [64]

In the 19th century, primary production of silver moved to North America, particularly Canada, Mexico, and Nevada in the United States: some secondary production from lead and zinc ores also took place in Europe, and deposits in Siberia and the Russian Far East as well as in Australia were mined. [56] Poland emerged as an important producer during the 1970s after the discovery of copper deposits that were rich in silver, before the centre of production returned to the Americas the following decade. Today, Peru and Mexico are still among the primary silver producers, but the distribution of silver production around the world is quite balanced and about one-fifth of the silver supply comes from recycling instead of new production. [56]

Symbolic role

16th-century fresco painting of Judas being paid thirty pieces of silver for his betrayal of Jesus 6852 les deniers de judas.JPG
16th-century fresco painting of Judas being paid thirty pieces of silver for his betrayal of Jesus

Silver plays a certain role in mythology and has found various usage as a metaphor and in folklore. The Greek poet Hesiod's Works and Days (lines 109–201) lists different ages of man named after metals like gold, silver, bronze and iron to account for successive ages of humanity. [65] Ovid's Metamorphoses contains another retelling of the story, containing an illustration of silver's metaphorical use of signifying the second-best in a series, better than bronze but worse than gold:

But when good Saturn, banish'd from above,
Was driv'n to Hell, the world was under Jove.
Succeeding times a silver age behold,
Excelling brass, but more excell'd by gold.

Ovid, Metamorphoses, Book I, trans. John Dryden

In folklore, silver was commonly thought to have mystic powers: for example, a bullet cast from silver is often supposed in such folklore the only weapon that is effective against a werewolf, witch, or other monsters. [66] [67] From this the idiom of a silver bullet developed into figuratively referring to any simple solution with very high effectiveness or almost miraculous results, as in the widely discussed software engineering paper No Silver Bullet . [68]

Silver production has also inspired figurative language. Clear references to cupellation occur throughout the Old Testament of the Bible, such as in Jeremiah's rebuke to Judah: "The bellows are burned, the lead is consumed of the fire; the founder melteth in vain: for the wicked are not plucked away. Reprobate silver shall men call them, because the Lord hath rejected them." (Jeremiah 6:19–20) Jeremiah was also aware of sheet silver, exemplifying the malleability and ductility of the metal: "Silver spread into plates is brought from Tarshish, and gold from Uphaz, the work of the workman, and of the hands of the founder: blue and purple is their clothing: they are all the work of cunning men." (Jeremiah 10:9) [55]

Silver also has more negative cultural meanings: the idiom thirty pieces of silver, referring to a reward for betrayal, references the bribe Judas Iscariot is said in the New Testament to have taken from Jewish leaders in Jerusalem to turn Jesus of Nazareth over to soldiers of the high priest Caiaphas. [69] Ethically, silver also symbolizes greed and degradation of consciousness; this is the negative aspect, the perverting of its value. [70]

Occurrence and production

Acanthite sample from the Chispas Mine in Sonora, Mexico; scale at bottom of image as one inch with a rule at one centimetre Acanthite - Chispas Mine, Arizpe, Sonora, Mexico.jpg
Acanthite sample from the Chispas Mine in Sonora, Mexico; scale at bottom of image as one inch with a rule at one centimetre

The abundance of silver in the Earth's crust is 0.08  parts per million, almost exactly the same as that of mercury. It mostly occurs in sulfide ores, especially acanthite and argentite, Ag2S. Argentite deposits sometimes also contain native silver when they occur in reducing environments, and when in contact with salt water they are converted to chlorargyrite (including horn silver), AgCl, which is prevalent in Chile and New South Wales. [71] Most other silver minerals are silver pnictides or chalcogenides; they are generally lustrous semiconductors. Most true silver deposits, as opposed to argentiferous deposits of other metals, came from Tertiary period vulcanism. [72]

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. [7] 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). [73] Top near-term mine development projects through 2015 are Pascua Lama (Chile), Navidad (Argentina), Jaunicipio (Mexico), Malku Khota (Bolivia), [74] and Hackett River (Canada). [73] In Central Asia, Tajikistan is known to have some of the largest silver deposits in the world. [75]

Silver is usually found in nature combined with other metals, or in minerals that contain silver compounds, generally in the form of sulfides such as galena (lead sulfide) or cerussite (lead carbonate). So the primary production of silver requires the smelting and then cupellation of argentiferous lead ores, a historically important process. [76] Lead melts at 327 °C, lead oxide at 888 °C and silver melts at 960 °C. To separate the silver, the alloy is melted again at the high temperature of 960 °C to 1000 °C in an oxidizing environment. The lead oxidises to lead monoxide, then known as litharge, which captures the oxygen from the other metals present. The liquid lead oxide is removed or absorbed by capillary action into the hearth linings. [77] [78] [79]

Ag(s) + 2Pb(s) + O
2
(g) → 2PbO(absorbed) + Ag(l)

Today, silver metal is primarily produced instead as a secondary byproduct of electrolytic refining of copper, lead, and zinc, and by application of the Parkes process on lead bullion from ore that also contains silver. [80] In such processes, silver follows the non-ferrous metal in question through its concentration and smelting, and is later purified out. For example, in copper production, purified copper is electrolytically deposited on the cathode, while the less reactive precious metals such as silver and gold collect under the anode as the so-called "anode slime". This is then separated and purified of base metals by treatment with hot aerated dilute sulfuric acid and heating with lime or silica flux, before the silver is purified to over 99.9% purity via electrolysis in nitrate solution. [71]

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). [80]

Monetary use

1,000 oz silver bar 1000oz.silver.bullion.bar.top.jpg
1,000 oz silver bar

The earliest known coins were minted in the kingdom of Lydia in Asia Minor around 600 BC. [81] The coins of Lydia were made of electrum, which is a naturally occurring alloy of gold and silver, that was available within the territory of Lydia. [81] Since that time, silver standards, in which the standard economic unit of account is a fixed weight of silver, have been widespread throughout the world until the 20th century. Notable silver coins through the centuries include the Greek drachma, [82] the Roman denarius, [83] the Islamic dirham, [84] the karshapana from ancient India and rupee from the time of the Mughal Empire (grouped with copper and gold coins to create a trimetallic standard), [85] and the Spanish dollar. [86] [87]

The ratio between the amount of silver used for coinage and that used for other purposes has fluctuated greatly over time; for example, in wartime, more silver tends to have been used for coinage to finance the war. [88]

Today, silver bullion has the ISO 4217 currency code XAG, one of only four precious metals to have one (the others being palladium, platinum, and gold). [89] Silver coins are produced from cast rods or ingots, rolled to the correct thickness, heat-treated, and then used to cut blanks from. These blanks are then milled and minted in a coining press; modern coining presses can produce 8000 silver coins per hour. [88]

Price

As of July 2018, silver is valued at around $495 per kilogram, or about $15.5 per ounce. [90]

Silver prices are normally quoted in Troy ounces. One troy ounce is equal to 31.1034 grams. In 2015 China reverted to the metric system and currently prices silver (and gold) in grams. [91] The London silver fix is published once daily at noon London time. This price is determined by several major international banks and is used by London bullion market members for trading that day. Prices are most commonly shown as the United States dollar (USD), the Pound sterling (GBP), and the Euro (EUR).

Applications

Jewellery and silverware

Embossed silver sarcophagus of Saint Stanislaus in the Wawel Cathedral was created in main centers of the 17th century European silversmithery - Augsburg and Gdansk Rennen Silver sarcophagus of Saint Stanislaus.jpg
Embossed silver sarcophagus of Saint Stanislaus in the Wawel Cathedral was created in main centers of the 17th century European silversmithery - Augsburg and Gdańsk
17th century silver cutlery Lacko slott interior 42.jpg
17th century silver cutlery

The major use of silver besides coinage throughout most of history was in the manufacture of jewellery and other general-use items, and this continues to be a major use today. Examples include table silver for cutlery, for which silver is highly suited due to its antibacterial properties. Western concert flutes are usually plated with or made out of sterling silver; [93] in fact, most silverware is only silver-plated rather than made out of pure silver; the silver is normally put in place by electroplating. Silver-plated glass (as opposed to metal) is used for mirrors, vacuum flasks, and Christmas tree decorations. [94]

Because pure silver is very soft, most silver used for these purposes is alloyed with copper, with finenesses of 925/1000, 835/1000, and 800/1000 being common. One drawback is the easy tarnishing of silver in the presence of hydrogen sulfide and its derivatives. Including precious metals such as palladium, platinum, and gold gives resistance to tarnishing but is quite costly; base metals like zinc, cadmium, silicon, and germanium do not totally prevent corrosion and tend to affect the lustre and colour of the alloy. Electrolytically refined pure silver plating is effective at increasing resistance to tarnishing. The usual solutions for restoring the lustre of tarnished silver are dipping baths that reduce the silver sulfide surface to metallic silver, and cleaning off the layer of tarnish with a paste; the latter approach also has the welcome side effect of polishing the silver concurrently. [93] A simple chemical approach to removal of the sulfide tarnish is to bring silver items into contact with aluminium foil whilst immersed in water containing a conducting salt, such as sodium chloride.[ citation needed ]

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). [95] [96] The silver ion is bioactive and in sufficient concentration readily kills bacteria in vitro . Silver ions interfere with enzymes in the bacteria that transport nutrients, form structures, and synthesise cell walls; these ions also bond with the bacteria's genetic material. Silver and silver nanoparticles are used as an antimicrobial in a variety of industrial, healthcare, and domestic application: for example, infusing clothing with nanosilver particles thus allows them to stay odourless for longer. [97] [98] Bacteria can, however, develop resistance to the antimicrobial action of silver. [99] Silver compounds are taken up by the body like mercury compounds, but lack the toxicity of the latter. Silver and its alloys are used in cranial surgery to replace bone, and silver–tin–mercury amalgams are used in dentistry. [94] Silver diammine fluoride, the fluoride salt of a coordination complex with the formula [Ag(NH3)2]F, is a topical medicament (drug) used to treat and prevent dental caries (cavities) and relieve dentinal hypersensitivity. [100]

Electronics

Silver is very important in electronics for conductors and electrodes on account of its high electrical conductivity even when tarnished. Bulk silver and silver foils were used to make vacuum tubes, and continue to be used today in the manufacture of semiconductor devices, circuits, and their components. For example, silver is used in 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, [7] [101] Powdered silver and its alloys are used in paste preparations for conductor layers and electrodes, ceramic capacitors, and other ceramic components. [102]

Brazing alloys

Silver-containing brazing alloys are used for brazing metallic materials, mostly cobalt, nickel, and copper-based alloys, tool steels, and precious metals. The basic components are silver and copper, with other elements selected according to the specific application desired: examples include zinc, tin, cadmium, palladium, manganese, and phosphorus. Silver provides increased workability and corrosion resistance during usage. [103]

Chemical equipment

Silver is useful in the manufacture of chemical equipment on account of its low chemical reactivity, high thermal conductivity, and being easily workable. Silver crucibles (alloyed with 0.15% nickel to avoid recrystallisation of the metal at red heat) are used for carrying out alkaline fusion. Copper and silver are also used when doing chemistry with fluorine. Equipment made to work at high temperatures is often silver-plated. Silver and its alloys with gold are used as wire or ring seals for oxygen compressors and vacuum equipment. [104]

Catalysis

Silver metal is a good catalyst for oxidation reactions; in fact it is somewhat too good for most purposes, as finely divided silver tends to result in complete oxidation of organic substances to carbon dioxide and water, and hence coarser-grained silver tends to be used instead. For instance, 15% silver supported on α-Al2O3 or silicates is a catalyst for the oxidation of ethylene to ethylene oxide at 230–270 °C. Dehydrogenation of methanol to formaldehyde is conducted at 600–720 °C over silver gauze or crystals as the catalyst, as is dehydrogenation of isopropanol to acetone. In the gas phase, glycol yields glyoxal and ethanol yields acetaldehyde, while organic amines are dehydrated to nitriles. [104]

Photography

The photosensitivity of the silver halides allowed for their use in traditional photography, although digital photography, which does not use silver, is now dominant. The photosensitive emulsion used in black-and-white photography is a suspension of silver halide crystals in gelatin, possibly mixed in with some noble metal compounds for improved photosensitivity, developing, and tuning. Colour photography requires the addition of special dye components and sensitisers, so that the initial black-and-white silver image couples with a different dye component. The original silver images are bleached off and the silver is then recovered and recycled. Silver nitrate is the starting material in all cases. [105]

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. [106]

Nanoparticles

Nanosilver particles, between 10 and 100 nanometres in size, are used in many applications. They are used in conductive inks for printed electronics, and have a much lower melting point than larger silver particles of micrometre size. They are also used medicinally in antibacterials and antifungals in much the same way as larger silver particles. [98] In addition, according to the European Union Observatory for Nanomaterials (EUON), silver nanoparticles are used both in pigments, as well as cosmetics. [107] [108]

Miscellanea

A tray of South Asian sweets, with some pieces covered with shiny silver vark Diwali sweets India 2009.jpg
A tray of South Asian sweets, with some pieces covered with shiny silver vark

Pure silver metal is used as a food colouring. It has the E174 designation and is approved in the European Union. [109] Traditional Pakistani and Indian dishes sometimes include decorative silver foil known as vark , [110] and in various other cultures, silver dragée are used to decorate cakes, cookies, and other dessert items. [111]

Photochromic lenses include silver halides, so that ultraviolet light in natural daylight liberates metallic silver, darkening the lenses. The silver halides are reformed in lower light intensities. Colourless silver chloride films are used in radiation detectors. Zeolite sieves incorporating Ag+ ions are used to desalinate seawater during rescues, using silver ions to precipitate chloride as silver chloride. Silver is also used for its antibacterial properties for water sanitisation, but the application of this is limited by limits on silver consumption. Colloidal silver is similarly used to disinfect closed swimming pools; while it has the advantage of not giving off a smell like hypochlorite treatments do, colloidal silver is not effective enough for more contaminated open swimming pools. Small silver iodide crystals are used in cloud seeding to cause rain. [98]

Precautions

Silver
Hazards
GHS pictograms GHS-pictogram-pollu.svg
GHS signal word Warning
H410
P273, P391, P501 [112]
NFPA 704
Flammability code 0: Will not burn. E.g. waterHealth code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeSilver
0
0
0

Silver compounds have low toxicity compared to those of most other heavy metals, as they are poorly absorbed by the human body when digested, and that which does get absorbed is rapidly converted to insoluble silver compounds or complexed by metallothionein. However, silver fluoride and silver nitrate are caustic and can cause tissue damage, resulting in gastroenteritis, diarrhoea, falling blood pressure, cramps, paralysis, and respiratory arrest. Animals repeatedly dosed with silver salts have been observed to experience anaemia, slowed growth, necrosis of the liver, and fatty degeneration of the liver and kidneys; rats implanted with silver foil or injected with colloidal silver have been observed to develop localised tumours. Parenterally admistered colloidal silver causes acute silver poisoning. [113] Some waterborne species are particularly sensitive to silver salts and those of the other precious metals; in most situations, however, silver does not pose serious environmental hazards. [113]

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. [113] [7]

Metallic silver, like copper, is an antibacterial agent, which was known to the ancients and first scientifically investigated and named the oligodynamic effect by Carl Nägeli. Silver ions damage the metabolism of bacteria even at such low concentrations as 0.01–0.1 milligrams per litre; metallic silver has a similar effect due to the formation of silver oxide. This effect is lost in the presence of sulfur due to the extreme insolubility of silver sulfide. [113]

Some silver compounds are very explosive, such as the nitrogen compounds silver azide, silver amide, and silver fulminate, as well as silver acetylide, silver oxalate, and silver(II) oxide. They can explode on heating, force, drying, illumination, or sometimes spontaneously. To avoid the formation of such compounds, ammonia and acetylene should be kept away from silver equipment. Salts of silver with strongly oxidising acids such as silver chlorate and silver nitrate can explode on contact with materials that can be readily oxidised, such as organic compounds, sulfur and soot. [113]

See also

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Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Its hydroxide, known in pre-modern times as baryta, does not occur as a mineral, but can be prepared by heating barium carbonate.

Gallium Chemical element with atomic number 31

Gallium is a chemical element with the symbol Ga and atomic number 31. Elemental gallium is a soft, silvery blue metal at standard temperature and pressure; however in its liquid state it becomes silvery white. If too much force is applied, the gallium may fracture conchoidally. It is in group 13 of the periodic table, and thus has similarities to the other metals of the group, aluminium, indium, and thallium. Gallium does not occur as a free element in nature, but as gallium(III) compounds in trace amounts in zinc ores and in bauxite. Elemental gallium is a liquid at temperatures greater than 29.76 °C (85.57 °F), above room temperature, but below the normal human body temperature of 37 °C (99 °F). Hence, the metal will melt in a person's hands.

Lanthanum Chemical element with atomic number 57

Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air and is soft enough to be cut with a knife. It is the eponym of the lanthanide series, a group of 15 similar elements between lanthanum and lutetium in the periodic table, of which lanthanum is the first and the prototype. It is also sometimes considered the first element of the 6th-period transition metals, which would put it in group 3, although lutetium is sometimes placed in this position instead. Lanthanum is traditionally counted among the rare earth elements. The usual oxidation state is +3. Lanthanum has no biological role in humans but is essential to some bacteria. It is not particularly toxic to humans but does show some antimicrobial activity.

Oxide chemical compound with at least one oxygen atom

An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– atom. Metal oxides thus typically contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further corrosion. Individual elements can often form multiple oxides, each containing different amounts of the element and oxygen. In some cases these are distinguished by specifying the number of atoms as in carbon monoxide and carbon dioxide, and in other cases by specifying the element's oxidation number, as in iron(II) oxide and iron(III) oxide. Certain elements can form many different oxides, such as those of nitrogen. other examples are silicon, iron, titanium, and aluminium oxides.

Ruthenium Chemical element with atomic number 44

Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemicals. Russian-born scientist of Baltic-German ancestry Karl Ernst Claus discovered the element in 1844 at Kazan State University and named it after the Latin name of his homeland, Ruthenia. Ruthenium is usually found as a minor component of platinum ores; the annual production has risen from about 19 tonnes in 2009 to some 35.5 tonnes in 2017. Most ruthenium produced is used in wear-resistant electrical contacts and thick-film resistors. A minor application for ruthenium is in platinum alloys and as a chemistry catalyst. A new application of ruthenium is as the capping layer for extreme ultraviolet photomasks. Ruthenium is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario and in pyroxenite deposits in South Africa.

Thallium Chemical element with atomic number 81

Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861, in residues of sulfuric acid production. Both used the newly developed method of flame spectroscopy, in which thallium produces a notable green spectral line. Thallium, from Greek θαλλός, thallós, meaning "a green shoot or twig", was named by Crookes. It was isolated by both Lamy and Crookes in 1862; Lamy by electrolysis, and Crookes by precipitation and melting of the resultant powder. Crookes exhibited it as a powder precipitated by zinc at the International exhibition, which opened on 1 May that year.

Silver nitrate Chemical compound

Silver nitrate is an inorganic compound with chemical formula AgNO
3
. This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called lunar caustic because silver was called luna by the ancient alchemists, who believed that silver was associated with the moon.

Praseodymium Chemical element with atomic number 59

Praseodymium is a chemical element with the symbol Pr and atomic number 59. It is the third member of the lanthanide series and is traditionally considered to be one of the rare-earth metals. Praseodymium is a soft, silvery, malleable and ductile metal, valued for its magnetic, electrical, chemical, and optical properties. It is too reactive to be found in native form, and pure praseodymium metal slowly develops a green oxide coating when exposed to air.

Group 12 element group of chemical elements

Group 12, by modern IUPAC numbering, is a group of chemical elements in the periodic table. It includes zinc (Zn), cadmium (Cd) and mercury (Hg). The further inclusion of copernicium (Cn) in group 12 is supported by recent experiments on individual copernicium atoms. Formerly this group was named IIB by CAS and old IUPAC system.

A silver halide is one of the chemical compounds that can form between the element silver and one of the halogens. In particular, bromine, chlorine, iodine and fluorine may each combine with silver to produce silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), and three forms of silver fluoride, respectively.

Tetrafluoroborate Anion

Tetrafluoroborate is the anion BF
4
. This tetrahedral species is isoelectronic with tetrafluoroberyllate (BeF2−
4
), tetrafluoromethane (CF4), and tetrafluoroammonium (NF+
4
) and is valence isoelectronic with many stable and important species including the perchlorate anion, ClO
4
, which is used in similar ways in the laboratory. It arises by the reaction of fluoride salts with the Lewis acid BF3, treatment of tetrafluoroboric acid with base, or by treatment of boric acid with hydrofluoric acid.

Native metal Metal that is found in its metallic form, either pure or as an alloy, in nature

A native metal is any metal that is found pure in its metallic form in nature. Metals that can be found as native deposits singly or in alloys include aluminium, antimony, arsenic, bismuth, cadmium, chromium, cobalt, indium, iron, manganese, molybdenum, nickel, niobium, rhenium, selenium, tantalum, tellurium, tin, titanium, tungsten, vanadium, and zinc, as well as two groups of metals: the gold group, and the platinum group. The gold group consists of gold, copper, lead, aluminium, mercury, and silver. The platinum group consists of platinum, iridium, osmium, palladium, rhodium, and ruthenium. Amongst the alloys found in native state have been brass, bronze, pewter, German silver, osmiridium, electrum, white gold, and silver-mercury and gold-mercury amalgam.

Cerium Chemical element with atomic number 58

Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile and silvery-white metal that tarnishes when exposed to air, and it is soft enough to be cut with a knife. Cerium is the second element in the lanthanide series, and while it often shows the +3 oxidation state characteristic of the series, it also exceptionally has a stable +4 state that does not oxidize water. It is also considered one of the rare-earth elements. Cerium has no biological role in humans and is not very toxic.

Organosilver chemistry in chemistry is the study of organometallic compounds containing a carbon to silver chemical bond and the study of silver as catalyst in organic reactions. In the group 11 elements silver is the element below copper. The chemistries have much in common but organosilver catalysis is much less common (mostly academic study) than organocopper chemistry due both to the relatively high price of silver and to the poor thermal stability of organosilver compounds. The oxidation state for silver in organosilver compounds in exclusively +1 with the notable exception of Ag(III) in the trifluoromethyl silver anion Ag(CF3)4 because of the electron-withdrawing effect of the trifluoromethyl groups. Poor thermal stability is reflected in decomposition temperatures of AgMe (-50 °C) versus CuMe (-15 °C) and PhAg (74 °C) vs PhCu (100 °C).

Molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula AOmXn, X = F, Cl, Br, I. The element A may be a main group element, a transition element or an actinide. The term oxohalide, or oxyhalide, may also refer to minerals and other crystalline substances with the same overall chemical formula, but having an ionic structure.

Fluorine forms a great variety of chemical compounds, within which it always adopts an oxidation state of −1. With other atoms, fluorine forms either polar covalent bonds or ionic bonds. Most frequently, covalent bonds involving fluorine atoms are single bonds, although at least two examples of a higher order bond exist. Fluoride may act as a bridging ligand between two metals in some complex molecules. Molecules containing fluorine may also exhibit hydrogen bonding. Fluorine's chemistry includes inorganic compounds formed with hydrogen, metals, nonmetals, and even noble gases; as well as a diverse set of organic compounds. For many elements the highest known oxidation state can be achieved in a fluoride. For some elements this is achieved exclusively in a fluoride, for others exclusively in an oxide; and for still others the highest oxidation states of oxides and fluorides are always equal.

Compounds of thorium any chemical compound having at least one thorium atom

Many compounds of thorium are known: this is because thorium and uranium are the most stable and accessible actinides and are the only actinides that can be studied safely and legally in bulk in a normal laboratory. As such, they have the best-known chemistry of the actinides, along with that of plutonium, as the self-heating and radiation from them is not enough to cause radiolysis of chemical bonds as it is for the other actinides. While the later actinides from americium onwards are predominantly trivalent and behave more similarly to the corresponding lanthanides, as one would expect from periodic trends, the early actinides up to plutonium have relativistically destabilised and hence delocalised 5f and 6d electrons that participate in chemistry in a similar way to the early transition metals of group 3 through 8: thus, all their valence electrons can participate in chemical reactions, although this is not common for neptunium and plutonium.

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