Silver - chemistry.

Silver - chemistry. I INTRODUCTION Silver, symbol Ag, white, lustrous metallic element that conducts heat and electricity better than any other metal. Silver is one of the transition elements of the periodic table (see Periodic Law). The atomic number of silver is 47. Silver has been known and valued as an ornamental and coinage metal since ancient times. Silver mines in Asia Minor were probably worked before 2500 BC. The alchemists called the metal Luna or Diana after the goddess of the moon and ascribed to it the symbol of a crescent moon. The element's name comes from the AngloSaxon word for the metal. II PROPERTIES With the exception of gold, silver is the most malleable and ductile of all metals. Its hardness ranges between 2.5 and 2.7; it is harder than gold but softer than copper. Silver melts at about 962°C (about 1764°F), boils at about 2162°C (about 3924°F), and has a specific gravity of 10.5. The atomic weight of silver is 107.868. Chemically silver is not very active. It is insoluble in dilute acids and in alkalies but dissolves in concentrated nitric or sulfuric acid, and it does not react with oxygen or water at ordinary temperatures. Sulfur and sulfides attack silver, and tarnishing is caused by the formation of silver sulfide on the surface of the metal. Eggs, which contain a considerable quantity of sulfur as a constituent of protein, tarnish silver extremely quickly. Small amounts of sulfide, which occurs naturally in the atmosphere and which, as hydrogen sulfide (H2S), is added to natural gas used domestically, tarnish silver. The black silver sulfide (AG2S) is among the most insoluble salts in aqueous solution, a property that is exploited for separating silver ions from other positive ions. III OCCURRENCE Silver ranks about 66th among elements in natural abundance in crustal rocks. It occurs in the pure state to a small extent; the most notable deposits of native silver are in Peru and Norway, where the mines have been worked for centuries. Pure silver is also found associated with pure gold in the form of an alloy known as electrum, and considerable amounts are recovered in the processing of gold. Silver is usually found combined with other elements (of which sulfur is the most predominant) in minerals and ores. Some of the important silver minerals are cerargyrite (or horn silver), pyrargyrite, sylvanite, and argentite. Silver also occurs as a constituent of lead, copper, and zinc ores, and half the world production of silver is obtained as a by-product in the processing of such ores. Practically all the silver produced in Europe is obtained from the lead sulfide ore, galena. In the United States relatively few mines are worked for their silver alone--the silver is mined in conjunction with lead, copper, and zinc. In 2004, U.S. mines produced 1,250 metric tons of silver, about one-eighth of the production worldwide. Most of the silver mined in the world comes from Mexico, the United States, Peru, Australia, and Canada. The leading silver-producing states in the United States are Nevada, Idaho, Alaska, and Arizona. IV METALLURGY Silver is usually recovered from silver ores by roasting the ore in a furnace to convert the sulfides to sulfates and then chemically precipitating metallic silver. Several metallurgical processes are used to extract silver from ores of other metals. In the amalgamation process, liquid mercury, which forms an amalgam with the silver, is added to the crushed ore. After the amalgam is washed out of the ore the mercury is removed by distillation, leaving metallic silver. In lixiviation methods the silver is dissolved in a solution of a salt, usually sodium cyanide, after which metallic silver is precipitated by bringing the solution in contact with metallic zinc or aluminum. For the Parkes process, which is used extensively in separating silver from copper and lead ores, see Lead. The impure silver obtained in the metallurgical processes is usually refined by electrolytic methods (see Electrochemistry) or by cupellation, a process that involves removing impurities by vaporization or absorption. V USES The use of silver in jewelry, tableware (see Flatware; Metalwork), and as coinage is well known. The metal is usually alloyed with small amounts of other metals to make it harder and more durable. In the United States, coin silver was an alloy of 90 percent silver and 10 percent copper until 1965, when the silver content was reduced to 40 percent for half dollars; silver was eliminated from dimes and quarters after 1964. In 1970 the U.S. government sold the last of its marketable silver, which in earlier periods of U.S. economic history had been used to support a monetary system of bimetallism (see Money). Sterling silver for tableware and other solid-silver objects is 92.5 percent silver and 7.5 percent copper. Silver is used to coat smooth glass surfaces for mirrors by vaporization of the metal or by precipitation from a solution; however, aluminum has largely replaced silver in this application. Silver is also widely used in the circuitry of electrical and electronic components. Colloidal silver, dilute solutions of silver nitrate (AgNO3), and some insoluble compounds, such as potassium, are used in medicine as antiseptics and bactericides. Argyrol, a silver-protein compound, is a local antiseptic for the eyes, ears, nose, and throat. The silver-halide salts--silver bromide, silver chloride, and silver iodide--which darken on exposure to light, are used in emulsions for photographic plates, film, and paper. The salts are soluble in sodium thiosulfate, which is the compound used in the photographic fixing process (see Photography). Contributed By: Seymour Z. Lewin Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.