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Chemistry 100-05

December 2, 2007

Silver

Silver is a chemical element with the symbol Ag. The Latin word is argentum, and its atomic number is 47. Silver is a soft white lustrous transition metal, it has the highest electrical and thermal conductivity for a metal. It occurs as free metal native silver, as well as various minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc mining. Silver has been known since antiquity. It has long been valued as a precious metal and used in ornaments and jewellery, as well as utensils. Silver bullion has the ISO currency code of XAG. Today, it is also used in photographic film, electrical contacts, and mirrors. Elemental silver is also used to catalyze chemical reactions. Silver has certain antimicrobial activity. In the past, dilute solutions of silver nitrate were used as disinfectants, though this has been supplanted by other treatments.

Notable 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 tarnishability has prevented it from being widely used in place of copper for electrical purposes, though it was 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, although the actual benefits of its use in this application are questionable. Among metals, pure silver has the highest thermal conductivity, only the non-metal diamond's is higher, whitest color, the highest optical reflectivity although aluminium slightly outdoes it in parts of the visible spectrum, and 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 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, potassium tetrafluoroargentate: K[AgF4] are also known.

( A nugget of silver )

Occurrence and extraction: Silver is found in native form, combined with sulfur, arsenic, antimony, or chlorine and in various ores such as argentite (Ag2S), horn silver (AgCl), and pyrargyrite (Ag3SbS3). The principal sources of silver are copper, copper-nickel, gold, lead and lead-zinc ores obtained from Peru, Mexico, China and Australia. Peru and Mexico both countries mining silver from 1546 and still major world producers. This metal can also be produced during the electrolytic refining of copper and by application of the Parkes process on lead metal obtained from lead ores that contain small amounts of silver. Commercial grade fine silver is at least 99.9% pure silver and purities greater than 99.999% are available. In 2005, Peru was the top producer of silver with almost one-seventh world share closely followed by Mexico, reports the British Geological Survey.

( Time trend of silver production )

Isotopes: Naturally occurring silver is composed of the two stable isotopes 107Ag and 109Ag with 107Ag being the more abundant, 51.839% natural abundance. Standard atomic mass is 107.8682(2) u. 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-lives that are less than an hour and the majority of these have half-lives that are less than 3 minutes. This element 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 palladium/silver 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: A major use of silver is as a precious metal. Jewellery and silverware are traditionally made from Sterling silver an alloy of 92.5% silver with 7.5% copper. Sterling silver is harder than pure silver, and has a lower melting point 893 oC than either pure silver or pure copper. Britannia silver is an alternative hallmark-quality standard containing 95.8% silver, often used to make silver tableware and wrought plate. With the addition of Germanium, a modified alloy Argentium sterling silver is formed, with improved properties including resistance to firescale. Silver is used in medals, denoting second place. Some high end musical instruments are made from sterling silver, such as the flute. The malleability and non-toxicity of silver make it useful in amalgams for fittings and fillings. Photography used 24% of silver consumed in 2001, in the form of silver nitrate and silver halides, while 33% was used in jewellery, 40% for industrial uses and only 3% for coins and medals. 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 kill many microbial organisms in vitro. Its germicidal effects make silver utensils valued, and increase its value as jewelry. The exact process of silver's germicidal effect is still not well understood, although different theories exist. One of these

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