Chemical compound that has the same molecular formula – the same number and kinds of atoms – as another compound, but a different structural arrangement of the atoms in space, and, therefore, different properties. For example, graphite (pencil lead) and diamond are isomers of carbon. Both are composed of pure carbon, but have very different physical properties. As the number of carbon atoms in a molecule increases, the number of possible combinations, or isomers, increases sharply. For example, octane, an 8-carbon-atom molecule, has 18 isomers; decane, a 10-carbon-atom molecule, has 75 isomers.