Interpret organic formula representations
Translate general, structural, displayed and skeletal formulas while preserving atom connectivity.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does each vertex in a skeletal formula normally represent?
What you will learn
- Distinguish four formula representations
- Read skeletal vertices and omitted carbon-bound hydrogens
- Convert between condensed and displayed representations
Each representation carries different detail
A general formula describes a homologous series, such as CₙH₂ₙ₊₂ for open-chain alkanes. A structural (condensed) formula groups atoms, for example CH₃CH₂OH.
A displayed formula shows every atom and every bond. A skeletal formula shows the carbon framework as lines: each end and vertex is a carbon, carbon-bound hydrogens are omitted, and heteroatoms plus hydrogens attached to them are shown.
Connectivity must remain unchanged
When translating a formula, count carbon vertices and preserve which atoms are joined. Fill each carbon to valency four, then count implied hydrogen atoms.
A line crossing or branching point is a carbon only when it is an actual vertex in the drawing. Do not infer a carbon at the crossing of two unconnected lines.
Pause and explain
Which formula explicitly shows every atom and every bond?
Worked example
Convert CH₃CH₂CH₃ into a skeletal description and molecular formula.
Show the worked solution
- The condensed formula has three connected carbon atoms in a straight chain.
- A skeletal formula is a two-segment zigzag with three carbon positions.
- Completing carbon valencies gives C₃H₈.
Answer Propane is a three-carbon zigzag in skeletal form and has molecular formula C₃H₈.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State what CₙH₂ₙ₊₂ tells you about an open-chain alkane series.
Give me a hint
Interpret n as the number of carbon atoms.
Compare my reasoning
- The formula applies to an acyclic saturated alkane series.
- For n = 3 it gives C₃H₈.
- It is a general formula, not a displayed structure.
CₙH₂ₙ₊₂ is the general formula for open-chain alkanes; for n = 3 it gives C₃H₈.
Look for these in your work
- Series identified
- Example evaluated
- Representation named
Write a condensed structural formula for a straight-chain three-carbon alcohol with the –OH on an end carbon.
Give me a hint
Build a three-carbon chain and satisfy carbon valencies.
Compare my reasoning
- Use three connected carbon atoms.
- Place –OH on an end carbon.
- The condensed formula is CH₃CH₂CH₂OH.
CH₃CH₂CH₂OH.
Look for these in your work
- Three carbons shown
- Hydroxyl position correct
- Valencies satisfied
A skeletal drawing has four line ends and two internal vertices in one unbranched chain. How many carbon atoms and how many C–C links are shown?
Give me a hint
Count both ends and every vertex.
Compare my reasoning
- Four ends plus two vertices give six carbon atoms.
- A single unbranched chain of six carbons has five C–C links.
- Carbon-bound hydrogens are not written in skeletal form.
It shows six carbon atoms joined by five C–C links.
Look for these in your work
- Endpoints included
- Vertices counted
- Links counted
Common mistakes
- Counting the line ends but missing a vertex
- Showing carbon-bound H atoms in a skeletal formula
- Changing connectivity while redrawing a structure
What can you explain now?
What does each vertex in a skeletal formula normally represent?
Compare with the explanation
A carbon atom
Skeletal line ends and vertices represent carbon atoms.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which atoms and bonds are omitted by convention in a skeletal formula?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.