Explain molecular polarity and intermolecular forces
Use electronegativity and molecular shape to explain dipoles, London forces, permanent dipoles and hydrogen bonding.
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
What two features determine whether a molecule has a net dipole?
What you will learn
- Explain bond polarity and molecular dipoles
- Describe instantaneous dipole–induced dipole and permanent dipole–permanent dipole forces
- Classify hydrogen bonding as a special case
A polar bond does not guarantee a polar molecule
A difference in electronegativity makes the shared electron distribution uneven, giving a bond dipole. A molecular dipole depends on both the bond dipoles and the molecular shape; symmetrical dipoles can cancel.
CO₂ has polar C=O bonds but a linear, symmetrical shape, so the bond dipoles cancel. H₂O is bent, so its O–H bond dipoles do not cancel and the molecule has a dipole.
Van der Waals forces describe intermolecular attractions
In this syllabus, van der Waals' forces is used generically for intermolecular forces. Instantaneous dipole–induced dipole (id-id), or London dispersion, forces arise from temporary uneven electron distributions that induce dipoles in neighbours.
Permanent dipole–permanent dipole (pd-pd) attractions occur between polar molecules. Hydrogen bonding is a special case of pd-pd attraction where hydrogen is bonded to a highly electronegative atom and interacts with a lone pair on a neighbouring molecule.
Pause and explain
Why is CO₂ non-polar overall although each C=O bond is polar?
Worked example
Compare molecular dipoles in CO₂ and H₂O.
Show the worked solution
- Both have polar bonds because their bonded atoms have different electronegativities.
- CO₂ is linear, so its equal C=O dipoles cancel.
- H₂O is bent, so its O–H dipoles do not cancel.
Answer CO₂ has no overall dipole because its linear bond dipoles cancel. H₂O has a molecular dipole because its bent shape prevents cancellation.
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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.
Explain why H₂O has a molecular dipole while CO₂ does not, despite polar bonds in both.
Give me a hint
Compare the molecular geometries before adding the bond dipoles.
Compare my reasoning
- H₂O is bent, so its O–H dipoles do not cancel.
- CO₂ is linear with equal C=O dipoles in opposite directions.
- The resulting dipoles are non-zero for H₂O and cancel for CO₂.
H₂O has a net dipole because its bent shape prevents O–H dipoles cancelling. CO₂ is linear and symmetrical, so its equal C=O dipoles cancel.
Look for these in your work
- Both bond polarities recognised
- Shapes compared
- Net dipole conclusion correct
Distinguish instantaneous dipole–induced dipole forces from permanent dipole–permanent dipole forces.
Give me a hint
Ask whether the interacting dipoles are temporary or permanent.
Compare my reasoning
- A temporary electron fluctuation creates an instantaneous dipole.
- It can induce a dipole in a neighbouring particle: id-id.
- Polar molecules with permanent dipoles can attract by pd-pd forces.
Id-id (London dispersion) forces arise from temporary dipoles inducing neighbours. Pd-pd forces act between molecules with permanent dipoles.
Look for these in your work
- Temporary origin stated
- Induction explained
- Permanent case contrasted
Classify hydrogen bonding within the syllabus terminology and explain what makes it a special case.
Give me a hint
State the generic term, the dipole type and the bonded hydrogen condition.
Compare my reasoning
- Van der Waals' forces is used as a generic term for intermolecular forces.
- Hydrogen bonding is a special case of permanent dipole–permanent dipole attraction.
- The H is bonded to a highly electronegative atom and interacts with a neighbouring lone pair.
Hydrogen bonding is a special case of pd-pd intermolecular attraction involving H bonded to a highly electronegative atom and attraction to a lone pair on a neighbouring molecule.
Look for these in your work
- Generic use of van der Waals' stated
- pd-pd classification correct
- Donor and neighbouring lone pair described
Common mistakes
- Equating polar bonds with a polar molecule
- Forgetting shape when adding bond dipoles
- Calling London forces permanent dipole attractions
- Treating hydrogen bonding as a covalent bond between molecules
What can you explain now?
What two features determine whether a molecule has a net dipole?
Compare with the explanation
Its bond dipoles and molecular shape
Shape determines whether the bond dipoles cancel.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do bond polarity, molecular geometry and intermolecular dipoles fit together?
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.