Structural or constitutional isomers have the carbon skeleton arranged differently. For example, pentane (a), 2-methyl-butane (b) and 2,2-dimethyl-propane (c), all have five carbons. In the first case, the longest chain is five atoms; in the second four; and in the last, only three.

Moving a representation in space or rotating one or more single bonds cannot produce a structural isomer; this can only be done by making and breaking bonds.