In chemistry,<unk> s are compounds with the same molecular formula but different structural formulae<unk> s do not necessarily share similar properties unless they also have the same functional groups.  This should not be confused with a nuclear<unk> , which involves a nucleus at different states of excitement.  There are many different classes of<unk> s, like stereoisomers, enantiomers, geometrical<unk> s, et cetera (see graph below).
A simple example of<unk> ism is given by propanol: it has the formula C3H8O (or C3H7OH) and occurs as two<unk> s: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)
Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first<unk> , and to the center carbon in the second.
There is, however, another<unk>  of C3H8O which has significantly different properties: methoxyethane (methyl-ethyl-ether; III). Unlike the<unk> s of propanol, methoxyethane has an oxygen atom that is connected to two carbons rather than to one carbon and one hydrogen.  This makes it an ether, not an alcohol, as it lacks a hydroxyl group, and has chemical properties more similar to other ethers than to either of the above alcohol<unk> s.
An example of<unk> s having more subtly different properties can be found in certain xanthines. Theobromine, found in chocolate, is a vasodilator with some effects in common with caffeine, but if one of the two methyl groups is moved to a different position on the two-ring core, the<unk>  is theophylline, which has a variety of effects, including bronchodilation and anti-inflammatory action.
Allene and propyne are examples of<unk> s containing different bond types. Allene contains two double bonds, while propyne contains one triple bond.
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Isotopomers
Nuclear isomer
Oligomer
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