Editing Structural isomer (section)

===Symmetry and positional isomerism===
Structural equivalences between atoms of a parent molecule reduce the number of positional isomers that can be obtained by replacing those atoms for a different element or group. Thus, for example, the structural equivalence between the six hydrogens of [[ethane]] {{chem|C|2|H|6}} means that there is just one structural isomer of [[ethanol]] {{chem|C|2|H|5|OH}}, not 6. The eight hydrogens of [[propane]] {{chem|C|3|H|8}} are partitioned into two structural equivalence classes (the six on the methyl groups, and the two on the central carbon); therefore there are only two positional isomers of propanol ([[1-propanol]] and [[2-propanol]]). Likewise there are only two positional isomers of [[butanol]], and three of [[pentanol]] or [[hexanol]].