Isomers
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Isomers are compounds with the same molecular formula.
eg. 1: Butane and isobutane have the same molecular formula: C4H10. Therefore, they are isomers.
eg. 2: Maleic acid and fumaric acid have the same molecular formula: C4H4O4. Therefore, they are isomers.
eg. 3: (R)-Lactic acid and (S)-Lactic acid have the same molecular formula: C3H6O3. Therefore, they are isomers.
see also constitutional isomers, stereoisomers