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CHEMISTRY : Metal Cation Identification


Information on TIN




  1. General Information

  2. Occurence Uses and Properties

  3. History of the Metal

  4. Compounds
  5. Back to Main Metal List




General Information


Tin (Sn), a chemical element belonging to the carbon family, Group IVa of the periodic table. It is a soft, silvery-white metal with a bluish tinge, known to the ancients in bronze, an alloy with copper. Tin is widely used for plating steel cans used as food containers, in metals used for bearings, and in solder.



Occurrence, uses, and properties.


The element is present in the igneous rocks of the Earth's crust to the extent of about 0.001 percent, occurring in grains of the native metal but chiefly as the oxide in the mineral cassiterite, from which the metal is obtained by reduction (removal of the oxygen) with coal or coke in smelting furnaces. The oldest tin mines were those in England and Spain, but those in Indonesia, Thailand, Congo (Kinshasa), Nigeria, and China have surpassed them in modern production. Several processes have been devised for reclaiming the metal from scrap tin or tin-plated articles. For a full treatment of tin mining, refining, and recovery, see tin processing.

Tin is nontoxic, ductile, malleable, and adapted to all kinds of cold-working, such as rolling, spinning, and extrusion. The colour of pure tin is retained during exposure because a thin, invisible, protective film of tin(IV) oxide is formed spontaneously by reaction with the oxygen of the air. The low melting point of tin and its firm adhesion to clean surfaces of iron, steel, copper, and copper alloys facilitate its use as an oxidation-resistant coating material. Tin exists in two different forms, or allotropes: the familiar form, white (or beta) tin, and gray (or alpha) tin, which is powdery and of little use. The gray form changes to the white above 13.2 C (55.8 F), rapidly at temperatures above 100 C (212 F); the reverse transformation, called tin pest, occurs at low temperatures but is prevented by small amounts of antimony, bismuth, copper, lead, silver, or gold normally present in commercial grades of tin.

On a weight basis, tin is a scarce but not rare element in the crust of the Earth, its abundance being of the same order of magnitude as such technically useful elements as cobalt, nickel, copper, cerium, and lead, and it is essentially equal to the abundance of nitrogen. In the Cosmos, there are 1.33 atoms of tin per 1 106 atoms of silicon, an abundance roughly equal to that of niobium, ruthenium, neodymium, or platinum. Cosmically, tin is a product of neutron absorption. Its richness in stable isotopes is noteworthy. The only mineral of commercial significance is cassiterite (SnO2). No high-grade deposits are known. The major sources are alluvial deposits, averaging about 0.01 percent tin. Lode deposits, containing up to 4 percent, are found in Bolivia and Cornwall. The latter were worked at least as early as Phoenician times but are no longer of major consequence. Some 90 percent of world production comes from Malaysia, Thailand, Indonesia, Bolivia, Congo (Brazzaville), Nigeria, and China. The United States has no significant deposits. (For commercial production see tin processing.) White tin has a body-centred tetragonal crystal structure, and gray tin has a face-centred cubic structure. When bent, tin makes an eerie, crackling "cry" as its crystals crush each other. Tin is attacked by strong acids and alkalies, but nearly neutral solutions do not affect it appreciably. Chlorine, bromine, and iodine react with tin, but fluorine reacts with it only slowly at room temperature.

Because pure tin is relatively weak, it is not put to structural uses unless alloyed with other metals. Numerous alloys of tin are used, including soft solder, type metal, pewter, bronze, bell metal, babbitt metal, and low-temperature casting alloys. A crystalline alloy with niobium is a superconductor at temperatures as high as 18 K (-427 F) and retains this property in very strong magnetic fields.

Tin has 10 stable isotopes, occurring in the following percentages in natural tin: tin-112, 0.97; tin-114, 0.65; tin-115, 0.36; tin-116, 14.53; tin-117, 7.68; tin-118, 24.22; tin-119, 8.58; tin-120, 32.59; tin-122, 4.63; and tin-124, 5.79.



History


There is evidence from both archaeology and literature that tin was one of the earliest metals to be known and used. Its earliest appliCATion was as an alloy with copper to form bronze, which was fashioned into tools and weapons. Bronze articles (typically containing about 10 percent tin) have been found in the Middle East dating from about 3500 BC and in Egypt from 3000 BC. Other ancient civilizations also used bronze articles; for example, Chinese bronzes have been dated to about 2250 BC.

Tin was obviously an important item of trade from early times, as it is mentioned in at least three books of the Bible (Numbers, Isaiah, and Ezekiel) dating from as long ago as 700 BC.

Pewter is a tin alloy that also has a long history. Probably the oldest known piece, dating from about 1500 BC, was found in Egypt, but it was the Roman civilization that developed pewter ware for household vessels and ornamental use. These appliCATions have continued to the present day, although the alloy compositions have changed markedly.

The use of tin as a coating for other metals also has ancient historical roots, with tinned copper vessels for cooking tracing back to Roman times. Most important was the development of tinning iron sheet in order to form tinplate. This began in central Europe in the 14th and 15th centuries and gradually spread throughout the continent. The original uses of tinplate were for household articles, including lanterns, plates, and drinking vessels; however, with the introduction of food canning in 1812, packaging became the major use of tinplate.

An important date from more recent history is 1839, when the American metalsmith Isaac Babbitt first used tin-based alloys in bearings for machinery. Babbitt metal considerably aided the development of the industrial society. Further developments in tin alloys, coatings, and chemicals have contributed to advances in transport, telecommuniCATion, aerospace, packaging, agriculture, and environmental protection.



Chemical compounds


Tin forms two series of compounds: stannous, or tin(II), compounds and stannic, or tin(IV), compounds. Some of the more commercially important compounds of tin(II) are stannous chloride, SnCl2, used in tin galvanizing and as a reducing agent in the manufacture of polymers and dyes; stannous oxide, SnO, employed in making tin salts for chemical reagents and for plating; and stannous fluoride, SnF2, an active ingredient in toothpastes. Tin(IV) compounds of significance include stannic chloride, SnCl4, widely used as a stabilizer for perfumes and as a starting material for other tin salts; and stannic oxide, SnO2, a useful CATalyst in certain industrial processes and a polishing powder for steel.

Tin can form a bond with carbon, as in the more than 500 known organotin compounds. Organotin stabilizers are used to prevent changes in polyvinyl chloride upon exposure to light and heat. A number of organotin compounds are major ingredients in biocides and fungicides.

atomic number 50 atomic weight 118.69 melting point 231.97 C (449.54 F) boiling point 2,270 C (4,100 F) density (white) 7.28 (gray) 5.75 valence 2, 4 electron config. 2-8-18-18-4 or 4d105s25p2

    Reference: Encyclopędia Britannica, Inc. 1994-2000 ©


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