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Isomers are molecules that have the same molecular formula but are arranged differently.
The functional group us attached to the main chain at different points.
E.g. C3H7Cl could represent:
1-chloropropane or 2-chloropropane
Functional Group Isomerism
There are different functional groups.
E.g. C2H6O could represent:
ethanol (an alcohol) or methoxymethane
The hydrocarbon chain is arranged differently.
E.g. C4H9OH could represent:
butan-1-ol or 2-methlypropan-1-ol