When more than one molecular structure correspond to the same molecular formula, the two or more structures are called isomeric structures, or isomers. For example, there are two structures corresponding to the formula CHN. They are H–C≡N: (hydrogen cyanide) and H–N≡C: (hydrogen isocyanide). At room temperature and especially at lower temperatures, each structure exists as a separate compound. That is, the substance represented by one structure can be purified and separated from the other, and each substance has different properties.
When two molecular formulas are different, they must be describing substances with different properties. This is because the formula conveys how many atoms of each kind are in a molecule, and chemical bonds would have to be broken to change the number of atoms or type of atoms. Breaking covalent bonds requires energy, and at room temperature, very few molecules have enough energy for bond breaking to occur.
Similarly, isomers occur when different molecular structures with the same molecular formula do not have sufficient energy to change from one structure to another. In the CHN example, changing H–N≡C: into H–C≡N: requires breaking a N–H bond. At room temperature, almost none of the H–N≡C: molecules has enough energy to break the N–H bond; that is, there are essentially no molecules with energy as large as the bond energy of the N–H bond. Therefore the H–N≡C: molecules do not change and they can be purified and separated from H–C≡N:.
Typically, isomers are defined by whether they can interchange at room temperature. At room temperature molecules do not have sufficient energy to break most covalent bonds. Thus, if interchanging two structures requires breaking a covalent bond, the two structures are isomers. If the temperature is raised, the molecules have greater average energy and it becomes more likely that one structure can change into another. If the temperature is lowered, it becomes harder to interchange structures; that is, the average energy of the molecules is lower and structures that could interchange at room temperature no longer have sufficient energy.