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The Three-Dimensional Structure of Molecules - Isomerism

The Lack of Information of Empirical Formulas

After Justus von Liebig evolved the elementary analysis, it was realized that some compounds have the same empirical formula but totally different chemical and physical properties. Dimethyl ether and ethyl alcohol, for example, have the same empirical formula C2H6O but behave very differently. Therefore, identical empirical formulas are no evidence of describing the same compounds.

The appearance of different compounds with the same empirical formula is called isomerism.

Isomers differ in the arrangement of their atoms in a molecule, e.g., which atom is connected with which other atom (connectivity). To describe a compound precisely, it is therefore necessary to show not only the empirical but also the structural formula. However, even the information of the connectivity is not enough. Molecules are three-dimensional entities, which have a spatial structure. The molecular structure of a compound is not described completely until one knows:

  • the constitution, which means the connectivity of the atoms and the
  • configuration, which means the geometrical arrangement of the atoms.
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