Tin

March 19, 2009 by  


tufail

Tin is a chemical element with the symbol Sn and atomic number 50. Tin is obtained chiefly from the mineral cassiterite, where it occurs as tin dioxide, SnO2. This silvery, malleable poor metal is not easily oxidized in air, and is used to coat other metals to prevent corrosion. It is used in many alloys, most notably bronze, pewter alloys (from 85% to 99% tin), and tin/lead soft solders typically containing 60% or more of tin.

Tin is a malleable, ductile, and highly crystalline silvery-white metal. Tin is malleable at ordinary temperatures but is brittle when it is cooled, due to the properties of two different allotropes. When a bar of tin is bent, a crackling sound known as the tin cry can be heard due to the twinning of the crystals.

Tin’s chemical properties fall between those of metals and non-metals, just as the semiconductors silicon and germanium do. Tin has two allotropes at normal pressure and temperature: gray tin and white tin. A third allotrope, called brittle tin, exists at temperatures above 161 °C.

Below 13.2 °C, it exists as gray or alpha tin, which has a cubic crystal structure similar to silicon and germanium. Gray tin has no metallic properties at all, is a dull-gray powdery material, and has few uses, other than a few specialized semiconductor applications.

Although the transformation temperature is 13.2 °C, the change does not take place unless the metal is of high purity, and only when the exposure temperature is well below 0 °C. This process is known as tin disease or tin pest. Tin pest was a particular problem in northern Europe in the 18th century as organ pipes made of tin alloy would sometimes be affected during long cold winters. Some sources also say that during Napoleon’s Russian campaign of 1812, the temperatures became so cold that the tin buttons on the soldiers’ uniforms disintegrated, contributing to the defeat of the Grande Armée. The veracity of this story is debatable, because the transformation to gray tin often takes a reasonably long time.

Commercial grades of tin (99.8%) resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony, lead, and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase its hardness. Tin tends rather easily to form hard, brittle intermetallic phases, which are often undesirable. It does not form wide solid solution ranges in other metals in general, and there are few elements that have appreciable solid solubility in tin.

Tin is the 49th most abundant element in the Earth’s crust, representing 2 ppm compared with 75 ppm for zinc, 50 ppm for copper, and 14 ppm for lead.

Tin does not occur naturally by itself, and must be extracted from a base compound, usually cassiterite (SnO2), the only commercially important source of tin, although small quantities of tin are recovered from complexsulfides such as stannite, cylindrite, franckeite, canfieldite, and teallite. Minerals with tin are almost always in association with granite rock, which when contain the mineral, have a 1% tin oxide content. Due to the higher specific gravity of tin and its resistance to corrosion, about 80% of mined tin is from secondary deposits found downstream from the primary lodes.

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