47 Ag
Silver is a metallic chemical element with the chemical symbol Ag (Latin: argentum ) and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free 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 by-product of copper, gold, lead, and zinc refining.
Silver has long been valued as a precious metal, and it is used to make ornaments, jewelry, high-value tableware, utensils (hence the term silverware ), and currency coins. Today, silver metal is also used in electrical contacts and conductors, in mirrors and in catalysis of chemical reactions. Its compounds are used in photographic film and dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides. While many medical antimicrobial uses of silver have been supplanted by antibiotics, further research into clinical potential continues.
Characteristics
Silver is a very ductile and malleable (slightly harder than gold) monovalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost and tendency to tarnish have prevented it from being widely used in place of copper for electrical purposes. Despite this, 13,540 tons were used in the electromagnets used for enriching uranium during World War II (mainly because of the wartime shortage of copper). Another notable exception is in high-end audio cables.
Among metals, pure silver has the highest thermal conductivity (the non-metal diamond and superfluid helium II are higher) and one of the highest optical reflectivity. (Aluminium slightly outdoes silver in parts of the visible spectrum, and silver is a poor reflector of ultraviolet light). Silver also has the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for their ability to record a latent image that can later be developed chemically. Silver is stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulfide to form a black layer of silver sulfide which can be cleaned off with dilute hydrochloric acid. The most common oxidation state of silver is +1 (for example, silver nitrate: AgNO 3 ); in addition, +2 compounds (for example, silver(II) fluoride: AgF 2 ) and the less common +3 compounds (for example, potassium tetrafluoroargentate: K ) are known.
Isotopes
Main article: isotopes of silverNaturally occurring silver is composed of two stable isotopes, 107 Ag and 109 Ag, with 107 Ag being the most abundant (51.839% natural abundance). Silver's atomic weight is 107.8682(2) g/mol. Twenty-eight radioisotopes have been characterized, the most stable being 105 Ag with a half-life of 41.29 days, 111 Ag with a half-life of 7.45 days, and 112 Ag with a half-life of 3.13 hours. This element has numerous meta states, the most stable being 108m Ag ( t 1/2 = 418 years), 110m Ag ( t 1/2 = 249.79 days) and 106m Ag ( t 1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than 3 minutes.
Isotopes of silver range in relative atomic mass from 93.943 ( 94 Ag) to 126.936 ( 127 Ag). The primary decay mode before the most abundant stable isotope, 107 Ag, is electron capture and the primary mode after is beta decay. The primary decay products before 107 Ag are palladium (element 46) isotopes, and the primary products after are cadmium (element 48) isotopes.
The palladium isotope 107 Pd decays by beta emission to 107 Ag 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 107 Ag abundance. Radiogenic 107 Ag was first discovered in the Santa Clara meteorite in 1978. The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. 107 Pd– 107 Ag 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.
Compounds
Silver metal dissolves readily in nitric acid ( HNO 3 ) to produce silver nitrate ( AgNO 3 ), a transparent crystalline solid that is photosensitive and readily soluble in water. Silver nitrate is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass. Silver metal does not react with sulfuric acid, which is used in jewellery-making to clean and remove copper oxide firescale from silver articles after silver soldering or annealing. However, silver reacts readily with sulfur or hydrogen sulfide H 2 S to produce silver sulfide, a dark-colored compound familiar as the tarnish on silver coins and other objects. Silver sulfide also forms silver whiskers when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.
Silver chloride ( AgCl ) is precipitated from solutions of silver nitrate in the presence of chloride ions, and the other silver halides used in the manufacture of photographic emulsions are made in the same way using bromide or iodide salts. Silver chloride is used in glass electrodes for pH testing and potentiometric measurement, and as a transparent cement for glass. Silver iodide has been used in attempts to seed clouds to produce rain. Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods.
Silver oxide ( Ag 2 O ) produced when silver nitrate solutions are treated with a base, is used as a positive electrode (cathode) in watch (battery) batteries. Silver carbonate ( Ag 2 CO 3 ) is precipitated when silver nitrate is treated with sodium carbonate ( Na 2 CO 3 ).
Silver fulminate ( AgONC ), a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol ( C 2 H 5 OH ). Another dangerously explosive silver compound is silver azide ( AgN 3 ), formed by reaction of silver nitrate with sodium azide ( NaN 3 ).
Latent images formed in silver halide crystals are developed by treatment with alkaline solutions of reducing agents such as hydroquinone, metol (4-(methylamino)phenol sulfate) or ascorbate which reduce the exposed halide to silver metal. Alkaline solutions of silver nitrate can be reduced to silver metal by reducing sugars such as glucose, and this reaction is used to silver glass mirrors and the interior of glass Christmas ornaments. Silver halides are soluble in solutions of sodium thiosulfate ( Na 2 S 2 O 3 ) which is used as a photographic fixer, to remove excess silver halide from photographic emulsions after image development.
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 halides that can either be fixed with thiosulfate or re-developed 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.
Applications
Many well known uses of silver involve its precious metal properties, including currency, decorative items and mirrors. The contrast between the appearance of its bright white color in contrast with other media makes it very useful to the visual arts. It has also long been used to confer high monetary value as objects (such as silver coins and investment bars) or make objects symbolic of high social or political rank.
Currency
Main articles: Silver coin and Silver standardSilver, in the form of electrum (a gold-silv
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