Allotropes of tin

Tin

Tin has two allotropic forms: grey tin and white tin. On warming, grey tin or a tin, changes at 13.2oC into white or ß tin, the ordinary form of the metal.

Classification: Metallic

Colour: White tin- Silvery- white metal. Grey tin- Grey metal

Natural occurrence: The principal tin bearing ore is casserite (SnO2). Tin makes up only about 0.001% of the earth's crust and is chiefly mined in Malaysia. Ores are also found abundantly in Cornwall, Germany, Bolivia, Brazil and Australia.

Structure:

White tin

White tin has an irregular, close packed, highly crystalline structure. Due to the breaking of these crystals a "tin cry" is heard when a bar is bent. White tin has a tetragonal structure and a co-ordination number of 6.

Grey tin

Grey tin has a cubic diamond structure.

The strusture of grey tin

Uses and industrial applications: All uses of tin are of the white form:

Isolation: The ore, cassiterite, is first ground and washed to remove impurities, then roasted to oxidise the sulphides of iron and copper. It is washed again and is reduced by carbon in a reverberatory furnace. The molten tin that collects on the bottom is drawn off and moulded into blocks. It is remelted at low temperatures so that the impurities form an insoluble mass.

SnO2 + 2C Arrow Sn + 2C0

Tin may also be purified by electrolysis.

Interconversions: Grey tin can be converted to white tin by heating it past 13.2oC. White tin returns to grey tin as it cools below 13.2oC. The metallic surface becomes covered with a grey powder, which is easily rubbed off. The grey patches gradually spread and eventually the object covered may totally lose its structural integrity and fall to pieces. Due to the spreading nature of the condition it is often called tin disease or tin pest. In the cold cathedrals of northern Europe, tin disease was a serious problem in the last century when organ pipes were commonly made of tin. Tin disease was, on occasion, responsible for the complete disintegration of organ pipes in some of these cathedrals in long, cold winters. This transformation can be prevented by alloying tin with antimony or bismuth.

Allotropes of carbon

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Author: Anna Coffey (document modification date: 24th May 2002)