Explain standard molecular shapes with VSEPR
Use electron-pair repulsion to explain the specified molecular geometries and bond angles.
Section 3 · Chemical 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 does VSEPR use to predict a molecular shape?
What you will learn
- Apply VSEPR to stated molecules
- State and explain the specified molecular shapes and bond angles
Electron pairs repel around a central atom
VSEPR theory predicts a shape by arranging regions of electron density around a central atom to reduce repulsion. A lone pair occupies space but is not shown as an atom-to-atom bond; it can compress bond angles relative to an all-bonding-pair arrangement.
The named simple examples are CO₂ linear 180°, BF₃ trigonal planar 120°, CH₄ tetrahedral 109.5°, NH₃ pyramidal 107°, H₂O non-linear 104.5°, PF₅ trigonal bipyramidal with 90° and 120° angles, and SF₆ octahedral 90°.
Count electron regions before naming the shape
For the syllabus model, count the bonding regions and lone pairs at the central atom. Double or triple bonds occupy one region for this basic shape prediction. Then arrange the regions and report the molecular shape and relevant bond angle.
Do not confuse electron-pair arrangement with molecular shape: only atom positions define the named molecular shape, while lone pairs affect the arrangement and angles.
Pause and explain
What is the molecular shape of NH₃?
Worked example
Use VSEPR to explain why NH₃ is pyramidal with a bond angle near 107°.
Show the worked solution
- Nitrogen has three bonding pairs and one lone pair.
- Four electron regions arrange approximately tetrahedrally.
- The lone pair repels more strongly, compressing the H–N–H angles from 109.5° to about 107°.
Answer NH₃ is pyramidal with an angle of about 107° because one lone pair compresses the tetrahedral arrangement of three N–H bonds.
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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.
Give the shape and bond angle of CO₂, then explain the central-atom arrangement.
Give me a hint
Carbon has two regions of bonding electron density.
Compare my reasoning
- Two regions repel to opposite sides.
- The molecule is linear.
- The angle is 180°.
CO₂ is linear with a 180° bond angle because the two electron regions around carbon lie opposite one another.
Look for these in your work
- Two regions counted
- Linear shape stated
- 180° angle stated
Compare the shapes and angles of BF₃, CH₄ and H₂O.
Give me a hint
Count bonding regions and lone pairs at each central atom.
Compare my reasoning
- BF₃ has three bonding regions: trigonal planar, 120°.
- CH₄ has four bonding regions: tetrahedral, 109.5°.
- H₂O has two bonds and two lone pairs: non-linear, about 104.5°.
BF₃ is trigonal planar at 120°; CH₄ is tetrahedral at 109.5°; H₂O is non-linear at about 104.5° because lone pairs compress the bond angle.
Look for these in your work
- All three shapes correct
- Angles correct
- Lone-pair effect included
State the shapes and stated angles for PF₅ and SF₆.
Give me a hint
Use the named electron-region arrangements.
Compare my reasoning
- PF₅ has five bonding regions and a trigonal-bipyramidal shape.
- PF₅ includes 90° and 120° bond angles.
- SF₆ is octahedral with 90° angles.
PF₅ is trigonal bipyramidal with 90° and 120° angles; SF₆ is octahedral with 90° angles.
Look for these in your work
- PF₅ shape and angles correct
- SF₆ shape correct
- SF₆ angle correct
Common mistakes
- Calling NH₃ trigonal planar because it has three H atoms
- Forgetting the central atom's lone pairs
- Reporting tetrahedral electron arrangement as the shape of H₂O
- Swapping PF₅ angles or SF₆ geometry
What can you explain now?
What does VSEPR use to predict a molecular shape?
Compare with the explanation
Repulsion between electron regions around a central atom
The regions arrange to reduce repulsion.
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Make it stick
How do electron regions determine the arrangement, and how do lone pairs affect the named molecular shape?
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.