Hydrogen, carbon, and lead are examples of elements. One might think that for any particular element, the chemical form would always be the same. This is not always the case. Allotropy, that is, the property of existing as allotropes, does not apply merely to the substance existing in different physical states as, for example, when lead melts and changes from solid lead to liquid lead. In this article we examine some of the best known allotropes, those of carbon, oxygen, and tin.
Carbon forms two well-known allotropes, graphite and diamond.
The different properties of the allotropes arise from their chemical structures. In graphite, the carbon atoms are arranged in sheets. Graphite is used as a lubricant, because the sheets can easily slip over each other. Recently it has been shown that the lubricant properties of graphite disappear under ultra high vacuum, and hence that molecules of gases, such as oxygen and nitrogen, most probably act as a kind of molecular grease to help the sheets slide past each other. Diamond is very hard and finds important industrial applications in cutting tools.