Silver

Silver is a chemical element in the periodic table that has the symbol Ag and atomic number 47. A soft white lustrous transition metal, silver 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 and photography.

Notable characteristics

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 has prevented it from being widely used in place of copper for electrical purposes.

Pure silver also has the highest thermal conductivity, whitest color, the highest optical reflectivity (although it is a poor reflector of ultraviolet), 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 with sulfur in it. The most common oxidation states of silver are +1 and +2.

Applications

The principal use of silver is as a precious metal and its halide salts, especially silver nitrate, 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.
• Mirrors which need silver's superior reflectivity for visible light are made with silver as the reflecting material. Common mirrors are backed with aluminium.
• Silver has been coined to produce money since 700 BC with 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 beauty of the metal is chosen for the manufacture of jewelry and silverware, and are traditionally made from the silver alloy known as Sterling silver, which is 92.5% silver.
• 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.
• Used to make solder and brazing alloys, electrical contacts, and high capacity silver-zinc and silver-cadmium batteries.
• Silver sulphide, also known as Silver Whiskers, is formed when silver electrical contacts are used in an atmosphere rich in Hydrogen Sulphide.
• Silver fulminate is a powerful explosive.
• Silver chloride can be made transparent and is used as a cement for glass.
• Silver iodide has been used in attempts to seed clouds to produce rain.
• In legend, silver is traditionally seen as harmful to supernatural creatures like werewolves and vampires. The use of silver fashioned into bullets for firearms is a popular appllication.

History

Silver (Anglo-Saxon, Seolfor siolfur; 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 as ornaments and utensils, for trade, and as the basis for many monetary systems. It was long considered the second most precious metal, second only to gold.

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. The alchemical symbol for silver is a half-moon with the open part on the left.

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

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 been 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 huge amount of silver in the New World in Zacatecas, Mexico and Potosí, which triggered a period of inflation in Europe.

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

Occurrence

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, Mexico, Peru, 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 largest silver producer. According to the Secretary of Economics of Mexico, it produced 2747 tons in 2000, about 15% of the annual production of the world.

Isotopes

Naturally occurring silver is composed of the two stable isotopes Ag-107 and Ag-109 with Ag-107 being the most abundant (51.839% natural abundance). Twenty-eight radioisotopes have been characterized with the most stable being Ag-105 with a half-life of 41.29 days, Ag-111 with a half-life of 7.45 days, and Ag-112 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 Agm-128 (t_* 418 years), Agm-110 (t_* 249.79 days) and Agm-107 (t_* 8.28 days).

Isotopes of silver range in atomic weight from 93.943 amu (Ag-94) to 123.929 amu (Ag-124). The primary decay mode before the most abundant stable isotope, Ag-107, is electron capture and the primary mode after is beta decay. The primary decay products before Ag-107 are palladium (element 46) isotopes and the primary products after are cadmium (element 48) isotopes.

The palladium isotope Pd-107 decays by beta emission to Ag-107 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 Ag-107 abundance. Radiogenic Ag-107 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. Pd-107 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.

Precautions and health effects

Silver itself is not toxic but most of its salts are poisonous and may be carcinogenic.

Compounds containing silver can be absorbed into the circulatory system and become deposited in various body tissues leading to the condition called argyria which results in a permanent grayish pigmentation of the skin and mucous membranes. Although this condition does not harm a person's health, it is disfiguring.

This metal plays no natural biological role in humans.

The possible health effects of silver are a subject of dispute. Silver has germicidal effects and kills many microbial organisms in vitro without causing noticeable harm to more complex life-forms. Hippocrates, the father of modern medicine, wrote that silver had beneficial healing and anti-disease properties. Various kinds of silver compounds are sold as remedies for a variety of diseases. However, no clinical study has yet demonstrated a therapeutic use for silver as an antibiotic in vivo. Ingestion of silver compounds can lead to argyria.

Silver compounds are used to accelerate healing in burn victims. According to Dr. Robert O. Becker, they can promote the healing of bones as well.

Silver is used in along with copper as an agent to remove algae in swimming pools in the United States by use of electrolysis. Copper is active against algae while silver is active against bacteria primarily due to silver's activity in absorption of oxygen causing bacteria to oxidize on contact. Some swimming pools do not use silver due to staining problems so they instead used copper - zinc colloids instead.

Reference

• Los Alamos National Laboratory - Silver

External links

• WebElements.com - Silver
• EnvironmentalChemistry.com - Silver