Structural Formula
Summary: A structural formula is an unambiguous description of the way atoms in a molecule are arranged. It shows which atoms are bonded to which, without drawing every bond. Examples at IGCSE include CH₂=CH₂ (ethene), CH₃CH₂OH (ethanol), and CH₃COOCH₃ (methyl ethanoate). Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-23
A structural formula is a way of representing a molecule that shows which atoms are bonded to which, without necessarily drawing all the individual bonds. It provides an unambiguous description of how the atoms in the molecule are arranged.
Formula Types at IGCSE
| Formula Type | What It Shows | Example (Ethanol) |
|---|---|---|
| Empirical formula | Simplest ratio of atoms | C₂H₆O |
| Molecular formula | Number of each type of atom | C₂H₆O |
| Structural formula | How atoms are arranged/bonded | CH₃CH₂OH |
| Displayed formula | Every atom and every bond drawn out | (full structural drawing) |
Structural Formula Notation
At IGCSE, structural formulae are written by grouping atoms that are bonded to each carbon:
Alkanes:
- Methane: CH₄
- Ethane: CH₃CH₃
- Propane: CH₃CH₂CH₃
Alkenes:
- Ethene: CH₂=CH₂ (the double bond is shown)
- Propene: CH₃CH=CH₂
- But-1-ene: CH₃CH₂CH=CH₂ or CH₂=CHCH₂CH₃
Alcohols:
- Methanol: CH₃OH
- Ethanol: CH₃CH₂OH
- Propan-1-ol: CH₃CH₂CH₂OH
Carboxylic Acids:
- Methanoic acid: HCOOH
- Ethanoic acid: CH₃COOH
Esters:
- Methyl ethanoate: CH₃COOCH₃
- Ethyl ethanoate: CH₃COOC₂H₅
Structural Formula vs Displayed Formula
| Structural Formula | Displayed Formula | |
|---|---|---|
| Shows bonds | Shows double bonds (=) but NOT all single bonds | Shows ALL bonds as lines |
| Compactness | More compact | Takes up more space |
| Example (ethane) | CH₃CH₃ | H H | | H—C—C—H | | H H |
Structural Isomers
Structural isomers have the same molecular formula but different structural formulae. At IGCSE:
C₄H₁₀:
- CH₃CH₂CH₂CH₃ (butane — straight chain)
- CH₃CH(CH₃)CH₃ (methylpropane — branched)
C₄H₈:
- CH₃CH₂CH=CH₂ (but-1-ene)
- CH₃CH=CHCH₃ (but-2-ene)
Key Facts
- Structural formula = shows how atoms are arranged in a molecule
- More detail than a molecular formula, less than a displayed formula
- Double bonds are shown explicitly: CH₂=CH₂
- Used to show isomers — same molecular formula, different arrangement
- Functional groups shown: —OH, —COOH, —COO—
- For IGCSE: limited to unbranched chains containing up to 4 carbon atoms
See Also
- Displayed Formula
- Isomer
- Homologous Series
- Functional Group
- Organic Chemistry Fundamentals
- IGCSE-Chem-Index
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.