Isomer – IGCSE Chemistry Definition
IGCSE Chemistry definition of isomer: compounds with the same molecular formula but different structural arrangements. Covers examples and exam tips for 0620.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Isomerism is a Supplement topic that tests structural understanding. The examiner usually provides a molecular formula and asks candidates to draw two different structural formulae, or provides two structures and asks candidates to identify them as isomers. Drawing skills are essential.
The 0620 definition
Isomers are compounds that have the same molecular formula but different structural formulae (different arrangements of atoms).
Example: isomers of C₄H₁₀
| Isomer | Name | Structure |
|---|---|---|
| Straight chain | Butane | CH₃−CH₂−CH₂−CH₃ |
| Branched | Methylpropane | CH₃−CH(CH₃)−CH₃ |
Both have the formula C₄H₁₀ but different structures. Butane has a higher boiling point than methylpropane because the straight chain has stronger intermolecular forces.
Example: isomers of C₃H₈O
| Isomer | Name | Functional group |
|---|---|---|
| Propan-1-ol | CH₃CH₂CH₂OH | Alcohol (−OH) |
| Propan-2-ol | CH₃CH(OH)CH₃ | Alcohol (−OH) |
| Methoxyethane | CH₃OCH₂CH₃ | Ether (−O−) |
The first two are positional isomers (same functional group, different position). The third is a functional group isomer (different functional group entirely).
How to find isomers
- Write out the longest possible carbon chain
- Shorten the chain by one carbon and attach it as a branch
- Move the branch to different positions
- Check that each structure is genuinely different (not just the same molecule drawn differently)
- Check each structure has the correct molecular formula
Worked exam question
Draw two structural isomers of C₃H₆O. [2]
Mark scheme
Two valid and distinct structural formulae with the formula C₃H₆O [1 each]. Possible answers include: propanal (CH₃CH₂CHO), propanone (CH₃COCH₃), or prop-2-en-1-ol (CH₂=CHCH₂OH).
Common exam mistakes
- Drawing the same structure in a different orientation and claiming they are isomers — rotating or flipping a molecule does not create an isomer.
- Writing structures that do not have the correct molecular formula — always count all C, H, and O atoms.
- Confusing isomers with allotropes — isomers are different compounds with the same formula; allotropes are different forms of the same element (e.g. diamond and graphite).
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