Displayed formulas: show every atom and every bond
Read and draw complete connectivity, distinguishing single and double bonds from a molecular formula's atom total.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Explain shared-pair covalent bonding and read atom subscripts.
- Distinguish carbon, hydrogen and oxygen element symbols.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many bonds does ordinary hydrogen make in these examples?
What you will learn
- Draw and interpret a displayed formula showing every atom and bond.
- Check ordinary carbon, oxygen and hydrogen bond counts in the supplied molecules.
A displayed formula supplies connectivity
A displayed formula shows every atom and every bond, using one line for a single covalent bond and two parallel lines for a double bond. CH₄ gives methane's molecular atom totals, while its displayed drawing puts one carbon bonded to four separately shown hydrogens. The formula CH₄ alone does not draw those connections.
In these ordinary neutral molecules, carbon makes four bonds, oxygen two and hydrogen one; a double bond counts twice towards the bond count. Ethene has C=C with two hydrogens bonded to each carbon. Ethane has C–C with three hydrogens on each carbon. Verify both the atoms and the bond orders rather than adding hydrogens blindly to every line.
Read the oxygen connections rather than just its colour
The ethanol diagram has a two-carbon chain ending C–O–H. The oxygen is bonded to a carbon and a hydrogen. Ethanoic acid has a carbonyl C=O and a separate C–O–H on the same end carbon. Every H and every bond appears in the displayed drawings; writing a CH₃ group in place of three individually bonded H atoms is a condensed structural representation rather than a fully displayed one.
Diagram positions organise the connections; they do not establish a molecule's three-dimensional shape, scale or bond angle. You can rotate a whole drawing without changing which atoms are bonded. A good drawing is complete, respects the given bond orders and has the correct totals; it need not reproduce the exact pixel positions of this example.
Sources for this lesson
Pause and explain
In ethene, why does each carbon need only two hydrogens?
Worked example
Compare the displayed drawings of ethane and ethene. Count atoms and explain why each carbon still has four bonds.
Show the worked solution
- Ethane is C₂H₆: each C has one C–C bond and three C–H bonds.
- Ethene is C₂H₄: each C has the double C=C bond plus two C–H bonds.
- The double counts as two, so every carbon reaches four without using the same hydrogen count in both molecules.
Answer Ethane C₂H₆ has C–C; ethene C₂H₄ has C=C. Carbon has four bonds in both.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Draw methane with every atom and bond, then check its counts.
Give me a hint
Put C at the centre and separately show each H.
Compare my reasoning
- Draw one carbon joined by four separate single lines to four hydrogens.
- Each H has one bond, and C has four.
- The totals are one C and four H, matching CH₄ without hiding hydrogens inside a group label.
One C with four individually bonded H atoms; CH₄, C four bonds and each H one.
Look for these in your work
- All five atoms appear separately.
- Every C–H bond is shown.
Draw ethene from its displayed example, count atoms and show how each carbon reaches four bonds.
Give me a hint
Use the double carbon–carbon bond and do not copy ethane's hydrogen count.
Compare my reasoning
- Join two C atoms with two parallel bond lines.
- Attach two separately shown H atoms to each carbon.
- The formula is C₂H₄ and each C has double-bond count two plus two single bonds.
Displayed C=C with two individually bonded H on each C; C₂H₄ and four bonds at each carbon.
Look for these in your work
- The double bond and all H atoms are explicit.
- Both atom and bond totals are correct.
A proposed ethanol drawing uses C–C–O–H but omits all carbon-bound hydrogens. Explain how to complete it and check the molecular formula without interpreting the line arrangement as a shape measurement.
Give me a hint
The first carbon needs three H, the second two, and the O already connects to C and H.
Compare my reasoning
- Add three separate C–H bonds on the first carbon and two on the second.
- Together with the O–H hydrogen, the totals are C₂H₆O; C has four bonds, O two and H one.
- The diagram supplies connectivity, while its flat layout is not a scale or three-dimensional shape measurement.
Add three H to the first C and two to the second; C₂H₆O with every atom/bond shown, without a shape claim.
Look for these in your work
- All missing hydrogens are restored.
- Connectivity and measured shape remain distinct.
Common mistakes
- Showing a CH₃ group and calling all its individual C–H bonds displayed.
- Counting a double bond as only one towards carbon's bond total.
- Putting two bonds on a hydrogen in these examples.
- Treating a flat connectivity drawing as a measured molecular shape.
What can you explain now?
What must a displayed formula show?
Compare with the explanation
Every atom and every bond.
Atom totals alone or grouped CH₃ notation do not fully display the individual connections.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw the five supplied molecules with every atom and bond. Check totals and carbon/oxygen/hydrogen bond counts before comparing with the examples.
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.