Give two kids the same bag of Lego and they build different things — same pieces, different arrangement. Isomers are molecules with identical formulas but rearranged atoms, which can make one a medicine and its twin useless.
One formula, many arrangements
Isomers are molecules with the same molecular formula but different arrangements of atoms. Structural (constitutional) isomers differ in how atoms are connected; stereoisomers share the same connectivity but differ in spatial arrangement. Because shape governs chemistry, isomers can have very different properties.
Butane and isobutane share C₄H₁₀ but differ in connectivity — one is a straight chain, the other branched — giving them different boiling points.
Glucose and fructose are both C₆H₁₂O₆ structural isomers, yet one is an aldehyde sugar and the other a ketone sugar with distinct chemistry.
- Two molecules share a formula and the same bonds between atoms.
- They differ only in how groups are oriented in space.
- Classify the relationship.
What you should see: Same connectivity but different spatial arrangement means they are stereoisomers, not structural isomers.