Explain sigma and pi bonding with hybridisation
Relate orbital overlap to sigma and pi bonds, and use sp, sp² and sp³ hybrid orbitals in specified molecules.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the supplied Pauling values when given, show particle and bond reasoning, and distinguish intramolecular bonds from intermolecular forces. Written practice is self-checked
Before you begin
- Read element symbols and simple molecular formulae; use the Periodic Table.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How do orbitals overlap to make a sigma bond?
What you will learn
- Describe sigma and pi bonds by orbital overlap
- Explain their formation in H₂, C₂H₆, C₂H₄, HCN and N₂
- Use sp, sp² and sp³ hybridisation
Sigma overlap lies along the internuclear axis
A σ bond forms by direct overlap of orbitals between the bonding atoms along the internuclear axis. H₂ has one σ bond formed by overlap of the two hydrogen 1s orbitals.
In ethane, the C–C σ bond is formed by direct overlap of sp³ hybrid orbitals; each carbon also overlaps sp³ orbitals with hydrogen 1s orbitals to form C–H σ bonds.
Pi overlap lies above and below a sigma bond
A π bond forms by sideways overlap of adjacent p orbitals above and below the σ bond. In ethene, each carbon is sp² hybridised; the C=C double bond contains one σ bond and one π bond.
In HCN and N₂, the relevant atoms use sp hybrid orbitals for the σ framework. A triple bond contains one σ bond and two π bonds. Hybridisation labels the orbitals used in the syllabus model; it is not a count of electron pairs by itself.
Pause and explain
What does an ethene C=C double bond contain?
Worked example
Describe the bonds in the C=C bond of ethene.
Show the worked solution
- Each carbon is sp² hybridised.
- Direct overlap along the internuclear axis forms one σ bond.
- Sideways overlap of adjacent p orbitals forms one π bond above and below the σ bond.
Answer The C=C bond in ethene contains one σ bond from direct sp² overlap and one π bond from sideways p-orbital overlap.
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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.
Compare orbital overlap in H₂ and C₂H₆, naming the sigma bond orbitals.
Give me a hint
Both examples form sigma bonds by direct overlap, but the orbitals differ.
Compare my reasoning
- H₂ uses direct overlap of two 1s orbitals.
- Each ethane carbon is sp³ hybridised.
- The C–C sigma bond is formed by direct overlap of carbon sp³ orbitals.
H₂ has a 1s–1s sigma bond. Ethane's C–C sigma bond forms by direct overlap of sp³ hybrid orbitals; its C–H bonds overlap sp³ and 1s orbitals.
Look for these in your work
- H₂ overlap correct
- Ethane carbon hybridisation correct
- C–C sigma overlap described
Describe the sigma and pi bonds in ethene and name the carbon hybridisation.
Give me a hint
Separate the bond along the axis from the sideways overlap.
Compare my reasoning
- Each carbon is sp² hybridised.
- Direct overlap makes one sigma bond.
- Sideways overlap of the remaining p orbitals makes one pi bond.
Each carbon is sp² hybridised; C=C contains one sigma bond from direct overlap and one pi bond from sideways p-orbital overlap.
Look for these in your work
- sp² identified
- Sigma overlap described
- Pi overlap described
HCN and N₂ each contain a triple bond. State the bond count and the orbital-overlap model for their triple bond.
Give me a hint
A triple bond has one axial overlap and two sideways overlaps.
Compare my reasoning
- Use one sigma bond for direct overlap along the axis.
- Use two pi bonds from sideways p-orbital overlap.
- The syllabus model uses sp hybridisation for the relevant atoms in HCN and N₂.
Each triple bond contains one sigma bond and two pi bonds; the sigma framework uses sp hybrid orbitals, with sideways p overlaps forming the pi bonds.
Look for these in your work
- One sigma counted
- Two pi bonds counted
- sp hybridisation and p overlap linked
Common mistakes
- Calling direct overlap a pi bond
- Placing a pi bond along the internuclear axis
- Saying a double bond is two sigma bonds
- Using hybridisation labels as though they name whole molecules
What can you explain now?
How do orbitals overlap to make a sigma bond?
Compare with the explanation
Directly along the internuclear axis
The direct overlap is centred on that axis.
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Make it stick
Which overlap forms sigma bonds, and which overlap forms pi bonds?
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.