Multiple covalent bonds and molecular properties
Account for all outer electrons in double and triple bonds and explain simple molecular properties.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Draw single covalent bonds and lone pairs.
- Count the outer electrons supplied by each element.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A double bond contains how many shared electrons?
What you will learn
- Represent bonding in CH₃OH, C₂H₄, O₂, CO₂ and N₂.
- Explain low melting points and poor conductivity using structure and bonding.
Share enough pairs to complete the shells
A double covalent bond has two shared pairs; a triple bond has three. In O₂, the oxygen atoms share two pairs and each oxygen has two lone pairs. In N₂, three pairs are shared and each nitrogen has one lone pair. CO₂ has structure O=C=O: two double bonds, two lone pairs on each oxygen and none on carbon.
Ethene, C₂H₄, has H₂C=CH₂. Each carbon has two single C–H bonds and one double C=C bond, so it counts eight outer electrons. Methanol, CH₃OH, has three C–H bonds, one C–O bond and one O–H bond; oxygen has two lone pairs. All five bonds in methanol are single. A diagram must include the correct atoms and unshared electrons, not just enough lines to look symmetrical.
Match the mechanism to the property
Simple molecules have strong covalent bonds internally but weak attractions between molecules. A low melting or boiling point reflects the relatively small energy needed to overcome these intermolecular attractions. The covalent bonds inside intact molecules do not need to be broken during an ordinary state change.
These molecular substances conduct poorly because there are no mobile ions or delocalised electrons throughout the pure sample. A comparison between molecular compounds should use supplied evidence: molecule size and attractions can affect boiling point, but this syllabus does not require naming specific types of intermolecular force.
Pause and explain
Which statement explains the low boiling point of a simple molecular substance?
Worked example
Count shared and lone pairs in CO₂ and verify each atom's outer-shell count.
Show the worked solution
- Use O=C=O, with two shared pairs in each C=O bond.
- Add two lone pairs to each oxygen; carbon has no lone pairs.
- Carbon counts eight shared electrons; each oxygen counts four shared and four unshared electrons.
Answer Four shared pairs in total; two lone pairs on each oxygen, none on carbon.
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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.
Draw outer-shell diagrams for O₂ and N₂ and compare shared and lone pairs.
Give me a hint
Oxygen begins with six outer electrons; nitrogen with five.
Compare my reasoning
- O₂ shares two pairs and has two lone pairs on each oxygen.
- N₂ shares three pairs and has one lone pair on each nitrogen.
- Check that each atom counts eight outer electrons.
O₂: double bond, two lone pairs per atom. N₂: triple bond, one lone pair per atom.
Look for these in your work
- The correct bond multiplicity is shown.
- All lone pairs and octets are checked.
Describe displayed and outer-shell models for C₂H₄ and CH₃OH.
Give me a hint
Make four bonds around each carbon; include oxygen's unshared electrons.
Compare my reasoning
- Ethene is H₂C=CH₂, with a double bond between the carbons.
- Methanol is CH₃–O–H, with only single bonds.
- Methanol's oxygen has two lone pairs; both carbons and oxygen count eight electrons.
Ethene has C=C; methanol has C–O and O–H single bonds with two oxygen lone pairs.
Look for these in your work
- Every atom in the formulas is represented.
- Bond order and oxygen lone pairs are correct.
A molecular liquid boils but does not conduct electricity. Explain both observations and say why these properties do not prove its identity.
Give me a hint
Use intermolecular attractions for boiling and charge mobility for conduction.
Compare my reasoning
- Boiling overcomes attractions between its molecules.
- Lack of mobile ions or delocalised electrons explains poor conductivity.
- Many molecular substances share these properties, so further identifying evidence is needed.
Intermolecular attractions are overcome; there are no mobile charge carriers; the exact substance is not identified.
Look for these in your work
- Different properties receive different mechanisms.
- The limits of the structural inference are stated.
Common mistakes
- Drawing O₂ with only one shared pair.
- Explaining low boiling points by weak covalent bonds inside the molecules.
What can you explain now?
How many shared pairs are in the N≡N bond?
Compare with the explanation
Three
A triple bond is three shared pairs; each nitrogen also retains one lone pair.
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Make it stick
Draw all five Extended molecules from memory. For one example, count the total supplied outer electrons and check that every electron appears in your diagram.
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.