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Isomers are molecules that have the same formulas (chemical or structural) but differ in their structures and properties. Notice that when talking about isomers, we are always refering to two or more molecules. One molecule is not an isomer by itself, but can be an isomer of one or more other molecules.
We also have to define what we means by the terms chemical formula, structural formula, and three-dimensional chemical structure.
| chemical formula | the number of each type of atom, C2H6O, for example |
| structural formula | the connectivity of the atoms, often given in the form of a condensed formula, CH3CH2OH. This may also be represented in the form of shorthand "stick figures". |
| chemical structure | the three-dimensional arrangement of atoms. This information cannot be shown in a condensed formula. Stick figures must be drawn specially to explicitly show certain arrangements of atoms. |
Now, using the above definitions, we can define two types of iosmers.
- Structural isomers are two molecules that have the same chemical formula but different structural formulas.
- Geometrical isomers are two molecules that have the same structural formula but different chemical structures.
The next two pages give examples of structural and geometrical isomers.
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