Difference between revisions of "Silver" - New World Encyclopedia

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In the [[periodic table]], silver is a member of the group of [[transition metal]]s in period 5, situated between [[palladium]] and [[cadmium]]. In addition, it lies in group 11 (former group 1B), between [[copper]] and [[gold]].
 
In the [[periodic table]], silver is a member of the group of [[transition metal]]s in period 5, situated between [[palladium]] and [[cadmium]]. In addition, it lies in group 11 (former group 1B), between [[copper]] and [[gold]].
  
Silver is a very ductile and malleable (slightly harder than [[gold]]) univalent [[coinage metal]]. It has the highest [[electrical conductivity]] of all metals, even higher than [[copper]], but its greater cost and tarnishability has prevented it from being widely used in place of copper for electrical purposes.
+
Silver is a very ductile and malleable and is slightly harder than [[gold]]. It is stable in pure air and water, but it tarnishes when exposed to [[ozone]], [[hydrogen sulfide]]*, or air containing [[sulfur]].
  
Pure silver also has the highest [[thermal conductivity]], whitest color, the highest optical [[reflectivity]] (although it is a poor reflector of [[ultraviolet]] light), and the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for the effect of light upon them. This metal is stable in pure air and water, but does tarnish when it is exposed to [[ozone]], [[hydrogen sulfide]], or air containing [[sulfur]]. The most common [[oxidation state]] of silver is +1 (for example, [[silver nitrate]]; AgNO<sub>3</sub>); a few +2 (for example, [[silver(II) fluoride]]; AgF<sub>2</sub>) and +3 compounds (for example, [[silver(III) persulfate]]; Ag<sub>2</sub>(SO<sub>5</sub>)<sub>3</sub>) are also known.
+
It has the highest [[electrical conductivity]]* of all metals, even higher than that of [[copper]]. Yet, its higher cost and tarnishability have prevented it from replacing copper for electrical purposes. Pure silver also has the highest [[thermal conductivity]]*, whitest color, highest optical [[reflectivity]]* (except for being a poor reflector of [[ultraviolet]] light), and the lowest contact resistance of any metal.
 +
 
 +
In combining with other elements, its most common [[oxidation state]]* is +1, such as in [[silver nitrate]]* (AgNO<sub>3</sub>) and silver chloride (AgCl). In some compounds, such as [[silver(II) fluoride]]* (AgF<sub>2</sub>), its oxidation number is +2. It also forms a few compounds, such as [[silver(III) persulfate]] (Ag<sub>2</sub>(SO<sub>5</sub>)<sub>3</sub>), in which its oxidation state is +3.
 +
 
 +
Silver halides are photosensitive and remarkable for the effect of light upon them.
  
 
=== Isotopes ===
 
=== Isotopes ===
  
Naturally occurring silver is composed of the two stable [[isotope]]s <sup>107</sup>Ag and <sup>109</sup>Ag with <sup>107</sup>Ag being the more abundant (51.839% [[natural abundance]]). Twenty-eight [[radioisotope]]s have been characterised with the most stable being <sup>105</sup>Ag with a [[half-life]] of 41.29 days, <sup>111</sup>Ag with a half-life of 7.45 days, and <sup>112</sup>Ag with a half-life of 3.13 hours.
+
Naturally occurring silver is composed of the two stable [[isotope]]s, <sup>107</sup>Ag and <sup>109</sup>Ag, with <sup>107</sup>Ag being the more abundant (51.839% [[natural abundance]]*). In addition, 28 [[radioisotope]]*s have been characterised, of which <sup>105</sup>Ag (the most stable) has a [[half-life]]* of 41.29 days, <sup>111</sup>Ag has a half-life of 7.45 days, and <sup>112</sup>Ag has a half-life of 3.13 hours.
  
 
All of the remaining [[radioactive]] isotopes have half-lifes that are less than an hour and the majority of these have half lifes that are less than 3 minutes. This element also has numerous [[meta state]]s with the most stable being <sup>128m</sup>Ag (''t''<sub>*</sub> 418 years), <sup>110m</sup>Ag (''t''<sub>*</sub> 249.79 days) and <sup>107m</sup>Ag (''t''<sub>*</sub> 8.28 days).
 
All of the remaining [[radioactive]] isotopes have half-lifes that are less than an hour and the majority of these have half lifes that are less than 3 minutes. This element also has numerous [[meta state]]s with the most stable being <sup>128m</sup>Ag (''t''<sub>*</sub> 418 years), <sup>110m</sup>Ag (''t''<sub>*</sub> 249.79 days) and <sup>107m</sup>Ag (''t''<sub>*</sub> 8.28 days).

Revision as of 18:52, 23 September 2006

47 palladiumsilvercadmium
Cu

Ag

Au
Ag-TableImage.png
periodic table
General
Name, Symbol, Number silver, Ag, 47
Chemical series transition metals
Group, Period, Block 11, 5, d
Appearance lustrous white metal
Ag,47.jpg
Atomic mass 107.8682(2) g/mol
Electron configuration [Kr] 4d10 5s1
Electrons per shell 2, 8, 18, 18, 1
Physical properties
Color silver
Phase solid
Density (near r.t.) 10.49 g/cm³
Liquid density at m.p. 9.320 g/cm³
Melting point 1234.93 K
(961.78 °C, 1763.2 °F)
Boiling point 2435 K
(2162 °C, 3924 °F)
Heat of fusion 11.28 kJ/mol
Heat of vaporization 258 kJ/mol
Heat capacity (25 °C) 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
Crystal structure face-centered cubic
Oxidation states 1
(amphoteric oxide)
Electronegativity 1.93 (Pauling scale)
Ionization energies 1st: 731.0 kJ/mol
2nd: 2070 kJ/mol
3rd: 3361 kJ/mol
Atomic radius 160 pm
Atomic radius (calc.) 165 pm
Covalent radius 153 pm
Van der Waals radius 172 pm
Miscellaneous
Magnetic ordering diamagnetic
Electrical resistivity (20 °C) 15.87 nΩ·m
Thermal conductivity (300 K) 429 W/(m·K)
Thermal diffusivity (300 K) 174 mm²/s
Thermal expansion (25 °C) 18.9 µm/(m·K)
Speed of sound (thin rod) (r.t.) 2680 m/s
Speed of sound (thin rod) (r.t.) 83 m/s
Shear modulus 30 GPa
Bulk modulus 100 GPa
Poisson ratio 0.37
Mohs hardness 2.5
Vickers hardness 251 MPa
Brinell hardness 24.5 MPa
CAS registry number 7440-22-4
Notable isotopes
Main article: 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% Ag 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% Ag is stable with 62 neutrons
111Ag syn 7.45 d β- 1.036, 0.694 111Cd
γ 0.342 -
This page is about silver the chemical element. For the color, see silver (color). See also Silver (disambiguation)

Silver (chemical symbol Ag, atomic number ) is a soft metal with a brilliant white luster that can take a high degree of polish. Along with copper and gold, it is one of the coinage metals. It is also considered a noble metal—a metal that resists corrosion.

  • Silver has long been used for both decorative and practical purposes.

A transition metal, it has the highest electrical and thermal conductivity of any metal and occurs in minerals and in free form. This metal is used in coins, jewelry, tableware, photography, and in mirrors.

Occurrence and extraction

Silver ore (Lincoln cent is shown for scale)

Silver is found in native form and in various ores such as argentite (silver sulfide, Ag2S) and horn silver (silver chloride, AgCl). It may also be found combined with arsenic or antimony. The principal sources of silver are copper, copper-nickel, gold, lead, and lead-zinc ores obtained from Canada, Mexico, Peru, Australia, and the United States.

Mexico is the world's largest silver producer. According to the Secretary of Economics of Mexico, it produced 80,120,000 troy ounces (2492 metric tons) in 2000, about 15% of the annual worldwide production.

Silver is produced during the electrolytic refining of copper. Commercial grade fine silver is at least 99.9% pure silver, and purities higher than 99.999% are available.

Etymology and history

A silver nugget.

The name silver is derived from the Anglo-Saxon seolfor (compare with Old High German silabar). Its chemical symbol, Ag, is an abbreviation of the Latin word argentum.

Silver has been known since ancient times. Slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate that it was being separated from lead as early as the fourth millennium B.C.E. Historically, it has been used for making ornaments and utensils. In addition, it has been used for trade and as the basis for many monetary systems. Its value as a precious metal was long considered second only to gold. In ancient Egypt and medieval Europe, it was often more valuable than gold.

In the Bible, silver is mentioned as early as the book of Genesis. In the New Testament, Judas Iscariot is recorded as having taken a bribe of 30 pieces of silver from religious leaders in Jerusalem to turn Jesus Christ over to the Romans.

Alchemical symbol for silver.

Associated with the Moon, the sea, and various lunar goddesses, the metal was referred to by alchemists by the name luna. One of the alchemical symbols for silver is a crescent Moon with the concave part on the left (see diagram on the left).

The metal mercury was once thought of as a kind of silver, though the two elements are chemically unrelated. The Latin name for mercury was hydrargyrum, meaning "watery silver," and the English name was quicksilver.

Europeans found huge quantities of silver in the New World, such as in Zacatecas, Mexico and Potosí, Bolivia. The conquistador Pizarro was said to have resorted to having his horses shod with silver horseshoes because the metal was relatively abundant. By contrast, there was a relative lack of iron in Peru. Aside from that, silver was extremely valuable elsewhere, making it a global commodity and triggering a period of inflation in Europe. These developments contributed to the rise of the Spanish Empire. The rise and fall of the value of silver affected the world market.

The Rio de la Plata was named after silver, after the Spanish word plata. In addition, Argentina got its name from the Latin word for silver.

Silver mining was a driving force in the settlement of western North America, with major booms for silver and its associated minerals, particularly lead in the galena ore that silver is most commonly found in. Notable "silver rushes" took place in Colorado, Nevada, California, Cobalt, Ontario, and the Kootenay region of British Columbia. The largest silver ore deposits in the United States were discovered at the Comstock Lode in Virginia City, Nevada, in 1859.

Notable characteristics

In the periodic table, silver is a member of the group of transition metals in period 5, situated between palladium and cadmium. In addition, it lies in group 11 (former group 1B), between copper and gold.

Silver is a very ductile and malleable and is slightly harder than gold. It is stable in pure air and water, but it tarnishes when exposed to ozone, hydrogen sulfide, or air containing sulfur.

It has the highest electrical conductivity of all metals, even higher than that of copper. Yet, its higher cost and tarnishability have prevented it from replacing copper for electrical purposes. Pure silver also has the highest thermal conductivity, whitest color, highest optical reflectivity (except for being a poor reflector of ultraviolet light), and the lowest contact resistance of any metal.

In combining with other elements, its most common oxidation state is +1, such as in silver nitrate (AgNO3) and silver chloride (AgCl). In some compounds, such as silver(II) fluoride (AgF2), its oxidation number is +2. It also forms a few compounds, such as silver(III) persulfate (Ag2(SO5)3), in which its oxidation state is +3.

Silver halides are photosensitive and remarkable for the effect of light upon them.

Isotopes

Naturally occurring silver is composed of the two stable isotopes, 107Ag and 109Ag, with 107Ag being the more abundant (51.839% natural abundance). In addition, 28 radioisotopes have been characterised, of which 105Ag (the most stable) has a half-life of 41.29 days, 111Ag has a half-life of 7.45 days, and 112Ag has a half-life of 3.13 hours.

All of the remaining radioactive isotopes have half-lifes that are less than an hour and the majority of these have half lifes that are less than 3 minutes. This element also has numerous meta states with the most stable being 128mAg (t* 418 years), 110mAg (t* 249.79 days) and 107mAg (t* 8.28 days).

Isotopes of silver range in atomic weight from 93.943 u (94Ag) to 123.929 u (124Ag). 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.

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 Pd/Ag ratio to yield measurable variations in 107Ag abundance. Radiogenic 107Ag 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. 107Pd versus Ag correlations observed in bodies, which have clearly been melted since the accretion of the solar system, must reflect the presence of live short-lived nuclides in the early solar system.

Applications

Johnson Matthey silver bullion bar.

The principal use of silver is as a precious metal. Its salts, especially silver nitrate and silver halides, are also widely used in photography (which is the largest single end use of silver). Some other uses for silver are as follows:

  • Electrical and electronic products, which need silver's superior conductivity, even when tarnished. For example, printed circuits are made using silver paints, and computer keyboards use silver electrical contacts. Silver (silver cadmium oxide) is also used in high voltage contacts because it can minimise any arcing.
  • Mirrors which need superior reflectivity for visible light are made with silver as the reflecting material in a process called silvering. Common mirrors are backed with aluminium.
  • Silver has been coined to produce money since 700 B.C.E. by the Lydians, in the form of electrum. Later, silver was refined and coined in its pure form. The words for "silver" and "money" are the same in at least 14 languages.
  • The metal is chosen for its beauty in the manufacture of jewelry and silverware, which are traditionally made from the silver alloy known as Sterling silver, which is 92.5% silver and usually alloyed with copper. The name of United Kingdom monetary unit 'Pound' originally had the value of one troy pound of sterling silver.
  • Silver is also used in medals, denoting second place.
  • The malleability, non-toxicity and beauty of silver make it useful in dental alloys for fittings and fillings.
  • Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions; for example, the production of formaldehyde from methanol and air by means of silver screens or crystallites containing a minimum 99.95 weight-percent silver. Silver (upon some suitable support) is probably the only catalyst available today to convert ethylene to ethylene oxide (later hydrolyzed to ethylene glycol, used for making polyesters)—a very important industrial reaction.
  • Oxygen dissolves in silver relatively easily compared to other gases present in air. Attempts have been made to construct silver membranes of only a few monolayers thickness. Such a membrane could be used to filter pure oxygen from air.
  • Used to make solder and brazing alloys, electrical contacts, and high capacity silver-zinc and silver-cadmium batteries.
  • Silver sulfide, also known as Silver Whiskers, is formed when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.
  • Silver fulminate is a powerful explosive.
  • Silver chloride can be made transparent and is used as a cement for glass.
  • Silver chloride is also a widely used electrode for pH testing and potentiometric measurement.
  • Silver iodide has been used in attempts to seed clouds to produce rain.
  • Silver oxide is used as a positive electrode (cathode) in watch batteries.
  • Colloidal silver is a possible antibacterial / antibiotic treatment espoused in alternative medicine, also notorious for causing argyria.
  • Silver nitrate (liquid) was a standard of care but was largely replaced by silver sulfadiazine cream (SSD Cream) which was generally the "standard of care" for the antibacterial/antibiotic treatment of serious burns until the late 1990's. Now, other options such as silver coated dressings (activated silver dressings) are used in addition to SSD cream, and may present advantages such as pain reduction and capacity for treatment at home.
  • Silver is often used in modern horror fiction as a weapon against certain supernatural entities, especially werewolves, who are sometimes described as being burned by silver.


Silver in medicine

Hippocrates, the father of modern medicine, wrote that silver had beneficial healing and anti-disease properties, and the Phoenicians used to store water, wine, and vinegar in silver bottles to prevent spoiling. In the early 1900s people would put silver dollars in milk bottles to prolong the milk's freshness. Silver compounds were used successfully to prevent infection in World War I before the advent of antibiotics.

The widespread use of silver went out of fashion with the development of modern antibiotics. However, recently there has been renewed interest in silver as a broad spectrum antimicrobial. In particular, it is being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of silver alginate products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims. In addition, Samsung has introduced washing machines with a final rinse containing silver ions to provide several days of antibacterial protection in the clothes. [1]

Health precautions

Silver plays no known natural biological role in humans, and possible health effects of silver are a subject of dispute. Silver itself is not toxic but most silver salts are, and some may be carcinogenic.

Silver and compounds containing silver (like colloidal silver) can be absorbed into the circulatory system and become deposited in various body tissues leading to a condition called argyria which results in a blue-grayish pigmentation of the skin, eyes, and mucous membranes. Although this condition does not harm a person's health, it is disfiguring and usually permanent. Argyria is rare and mild forms are sometimes mistaken for cyanosis.

Silver-ions and silver compounds show a toxic effect on some bacteria, viruses, algae and fungi typical for heavy metals like lead or mercury, but without the high toxicity to humans that is normally associated with them. Its germicidal effects kills many microbial organisms in vitro (i.e. in a test tube or a petri dish). The exact process by which this is done is still not well understood, although several different theories exist. One of these is a process generally known for heavy metals called the oligodynamic effect, which goes a long way explaining the effect on microbial lifeforms but does not explain certain antiviral functions.

Alternative medicine

Today, various kinds of silver compounds, or devices to make solutions or colloids containing silver, are sold as remedies for a wide variety of diseases. Although mostly harmless, some people using these home-made solutions use far too much and develop argyria over a period of months or years, and several have been documented in the last few years in the medical literature, including one possible case of coma associated with a high intake of silver (see medical references). It is strongly advised to notify a doctor when taking silver as a form of self-medication.

In India, foods can be found decorated with a thin layer of silver, known as Varak.

Superstition

Because of the mysticism surrounding silver's lunar associations, as well as the aesthetic qualities of the white, reflective metal that cause it to be associated with purity, silver in European Folklore has long been traditionally believed to be an antidote to various maladies and fictional monsters. Notably, silver was believed to be a repellant against vampires and the undead and it was believed that a werewolf, in his bestial form, could only be killed by a weapon or bullet made of silver, and was equally effective against vampires, as described in Eastern European folklore. This has given rise to the term "silver bullet," which is used to describe things designed to cure or fix a wide array of maladies.

Notes

  1. Samsung laundry featuring SilverCare Technology. Samsung. Retrieved 2006-05-20.

References
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See also

  • Precious metal
  • Silver as an investment
  • Silver coin
  • Silver standard
  • Silver compounds
  • Colloidal silver

External links

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Publications on health effects

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