Isomer

In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. Many isomers share similar if not identical properties in most chemical contexts.
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and the isomers
Propan-1-ol (n-propyl alcohol; left) Propan-2-ol (isopropyl alcohol; right)

Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first isomer, and to the center carbon in the second. It can be readily shown that the number of possible isomers increases rapidly as the number of atoms increase; for example the next largest alcohol, named butanol (C4H10O), has five different isomers.
In the example above it should also be noted that in both isomers all the bonds are single bonds; there is no type of bond that appears in one isomer and not in the other. Also the number of bonds is the same. From the structures of the two molecules it could be deduced that their chemical stabilities are liable to be identical or nearly so.
There is, however, another isomer of C3H8O which has significantly different properties: methyl ethyl ether:

Notice that unlike the top two examples, the oxygen is connected to two carbons rather than to one carbon and one hydrogen. As it lacks a hydroxl group the above molecule is no longer considered an alcohol but is classified as an ether, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.