Heptane, which has a chemical formula of C7H16, has nine structural isomers: heptane, 2-methylhexane, 3-methylhexane, 2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 3-ethylpentane and 2,2,3-trimethylbutane. These are also referred to as constitutional isomers.
To be considered a structural isomer, each different compound must have the same molecular formula but with a different name. In addition, the isomers present different physical properties, although the chemical properties may be the same or different. The numbers in front of the names represent the carbon atoms that contain methyl groups on the molecular chain. For example, 2-methylhexane contains a methyl group on the number 2 carbon atom.