Silver

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

47 palladium ← silver → cadmium Cu ↑ Ag ↓ Au Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	silver, Ag, 47
Chemical series	transition metals
Group, Period, Block	11, 5, d
Appearance	lustrous white metal
Atomic mass	107.8682 (2) g/mol
Electron configuration	[Kr] 4d10 5s1
Electrons per shell	2, 8, 18, 18, 1
Physical properties
Colour	silver
Phase	solid
Density (near r.t.)	10.49 g·cm−3
Liquid density at m.p.	9.320 g·cm−3
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−1
Heat of vaporization	258 kJ·mol−1
Heat capacity	(25 °C) 25.350 J·mol−1·K−1
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−1·K−1
Thermal diffusivity	(300 K) 174 mm²/ s
Thermal expansion	(25 °C) 18.9 µm·m−1·K−1
Speed of sound (thin rod)	( r.t.) 2680   m·s−1
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	24.5 MPa
CAS registry number	7440-22-4
Selected 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 - 106 mAg syn 8.28 d ε - 106Pd γ 0.511, 0.717, 1.045, 0.450 - 107Ag 51.839% Ag is stable with 60 neutrons 108 mAg 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 -
References

Silver ( IPA: /ˈsɪlvə/) is a chemical element with the symbol Ag ( Latin: argentum). A soft white lustrous 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.

Notable characteristics

A nugget of silver

Silver is a very ductile and malleable (slightly harder than gold) univalent 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 tarnishability has prevented it from being widely used in place of copper for electrical purposes.

Pure silver also has the highest thermal conductivity, whitest colour, 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; AgNO₃); a few +2 (for example, silver(II) fluoride; AgF₂) and +3 compounds (for example, silver(III) persulfate; Ag₂(SO₅)₃) are also known.

Applications

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 minimize 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.
• Many high end musical instruments are made with silver, which benefit from a higher tone quality.
• Silver has been coined to produce money since 700 BC 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.

History

Alchemical symbol for silver

Silver (from Anglo-Saxon seolfor, compare Old High German silabar; Ag is from the Latin argentum) has been known since ancient times. It is mentioned in the book of Genesis, and 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.

Silver has been used for thousands of years for ornaments and utensils, 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.

Judas Iscariot is infamous for having, according to the New Testament, taken a bribe of thirty pieces of silver from religious leaders in Jerusalem to turn Jesus Christ over to the Romans.

Associated with the moon, as well as with 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 open part on the left (see picture, left).

The metal mercury was thought of as a kind of silver, though the two elements are chemically unrelated; its Latin and English names, hydrargyrum ("watery silver") and quicksilver, respectively, reflect this history.

In heraldry, the argent, in addition to being shown as silver (this has been shown at times with real silver in official representations), can also be shown as white. Occasionally, the word "silver" is used rather than argent; sometimes this is done across-the-board, sometimes to avoid repetition of the word "argent" in blazon.

Europeans found a huge amount of silver in the New World in Zacatecas and Potosí, which triggered a period of inflation in Europe. The conquistador Pizarro was said to have resorted to having his horses shod with silver horseshoes due to the metal's abundance, in contrast to the relative lack of iron in Peru. Silver, which was extremely valuable in China, became a global commodity, contributing to the rise of the Spanish Empire. The rise and fall of its value affected the world market.

The Rio de la Plata was named after silver (in Spanish, plata), and in turn lent the meaning of its name to Argentina.

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

Occurrence and extraction

Silver ore (Lincoln cent is shown for scale)

Silver is found in native form, combined with sulfur, arsenic, antimony, or chlorine and in various ores such as argentite (Ag₂S) and horn silver (AgCl). The principal sources of silver are copper, copper-nickel, gold, lead and lead-zinc ores obtained from Canada, Cobalt, Ontario, Mexico (historically Batopilas), Peru, Australia and the United States.

This metal is also produced during the electrolytic refining of copper. Commercial grade fine silver is at least 99.9% pure silver and purities greater than 99.999% are available. 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 production of the world.

Price

Johnson Matthey silver bullion bar

Silver is currently about 1/50th the price of gold by mass, and approximately 70 times more valuable than copper. Silver did once trade at 1/6th to 1/12th the price of gold, prior to the Age of Discovery and the discovery of great silver deposits in the Americas, most notably the vast Comstock Lode in Virginia City, Nevada, USA. This then resulted in the debate over cheap Free Silver to benefit the agricultural sector was among the most prolongued and difficult in that country's history and dominated public discourse during the latter decades of the nineteenth century.

Over the last 100 years the price of silver and the gold/silver price ratio has fluctuated greatly due to competing industrial and store of value demands. In 1980 the silver price rose to an all-time high of US$49.45 per troy ounce. By December 2001 the price had fallen to US$4.15 per ounce, and in May 2006 it had risen back as high as US$15.21 per ounce. As of 2006, current silver prices (and most other metal prices) have been rather volatile, for example quickly dropping from the May high of US$15.21 per ounce to a June low of US$9.60 per ounce before rising back above US$12 per ounce by August.

Isotopes

Naturally occurring silver is composed of the two stable isotopes ¹⁰⁷Ag and ¹⁰⁹Ag with ¹⁰⁷Ag being the more abundant (51.839% natural abundance). Twenty-eight radioisotopes have been characterised with 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.

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 ^108mAg (t_* 418 years), ^110mAg (t_* 249.79 days) and ^106mAg (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.

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. Additionally, Kohler has introduced a line of toilets that have silver ions embedded in the porcelain to kill germs.

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 food

In India, foods can be found decorated with a thin layer of silver, known as Varak. Silver as a food additive is given the E number E174 and classed as a food coloring. It is used solely for external decoration, such as on chocolate confectionery, in the covering of dragées and the decoration of sugar-coated flour confectionery. In Australia, it is banned as a food additive.

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 (this primarily originates from its holy connotations; also, mirrors were originally polished silver, and as such, vampires allegedly cannot be seen in them because they are wicked) 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 that very effectively deal with one specific problem.