Isomer
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
One of two or more compounds with the same molecular formula but a different arrangement of atoms.
Isomer in plain English
Think of the same set of Lego bricks: you can build a car or a boat from the exact same pieces. Isomers are molecules made of the identical count and type of atoms (same "molecular formula"), but those atoms are snapped together in a different arrangement. That different shape or connection pattern can give them totally different properties, which is why the arrangement matters as much as the ingredient list.
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