Describe sigma bonds, pi bonds and planar ethene
Relate sigma and pi bonding to single, double and triple bonds, and explain planar structure around C=C.
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
How many π bonds are in a carbon–carbon triple bond?
What you will learn
- Describe sigma and pi overlap
- Count sigma and pi bonds in multiple bonds
- Explain planarity and restricted rotation at C=C
Sigma and pi bonds differ in overlap
A sigma (σ) bond forms by end-on overlap along the line joining the nuclei. Every single covalent bond is one σ bond.
A pi (π) bond forms by sideways overlap of parallel p orbitals, with electron density above and below the internuclear axis. A double bond contains one σ and one π bond; a triple bond contains one σ and two π bonds.
The π bond fixes the alkene arrangement
The sp² carbon atoms and attached atoms around a C=C lie in a plane in examples such as ethene. The π bond results from parallel p orbitals above and below this plane.
Rotation around C=C would disrupt sideways p-orbital overlap, so rotation is restricted. This planar geometry and restricted rotation help explain geometrical isomerism.
Pause and explain
Why is rotation restricted around C=C?
Worked example
Count the σ and π bonds in ethene's C=C and explain why the atoms around it are planar.
Show the worked solution
- The C=C double bond has one σ bond and one π bond.
- The two carbons are sp² and trigonal planar.
- Parallel p orbitals overlap sideways to make the π bond above and below the plane.
Answer Ethene's C=C contains one σ and one π bond. The sp² atoms are trigonal planar; sideways p overlap creates the π bond above and below the plane.
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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 the sigma and pi bond count for C=C and C≡C.
Give me a hint
Use one sigma bond in each multiple bond, then count remaining pi bonds.
Compare my reasoning
- C=C has two total bond components.
- C≡C has three total bond components.
- Their counts are 1σ+1π and 1σ+2π.
C=C: one σ and one π; C≡C: one σ and two π.
Look for these in your work
- Double bond count
- Triple bond count
- Bond types named
Describe the location of π electron density in a carbon–carbon double bond.
Give me a hint
Think about sideways overlap of p orbitals.
Compare my reasoning
- Parallel p orbitals overlap sideways.
- The π density lies above and below the internuclear axis.
- It is distinct from the σ density along the axis.
π electron density is above and below the internuclear axis from sideways p-orbital overlap.
Look for these in your work
- Overlap type
- Location
- Sigma distinction
Explain how a pi bond contributes to the planarity and restricted rotation of ethene.
Give me a hint
Connect orbital alignment, geometry and rotation.
Compare my reasoning
- The alkene carbons are sp² and trigonal planar.
- Parallel p orbitals form a π bond above and below the plane.
- Rotation would break this alignment, so it is restricted.
Ethene is planar around C=C because its sp² atoms and p orbitals align; rotation is restricted because it would disrupt π overlap.
Look for these in your work
- sp² geometry
- Pi overlap
- Rotation link
Common mistakes
- Counting a double bond as two π bonds
- Saying rotation about C=C is unrestricted
- Placing the π electron density along the internuclear axis
What can you explain now?
How many π bonds are in a carbon–carbon triple bond?
Compare with the explanation
Two
A triple bond is one σ plus two π bonds.
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Make it stick
What would be disrupted if one carbon rotated freely around a C=C bond?
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