A LevelChemistryAQARevision NotesPhysical ChemistryMolecules: Shapes & ForcesShapes of Simple Molecules & Ions

Shapes of Simple Molecules & Ions (AQA A Level Chemistry): Revision Note

Exam code: 7405

Stewart Hird

Written by: Stewart Hird

Reviewed by: Caroline Carroll

Updated on

Electron Pairs

  • The valence shell electron pair repulsion theory (VSEPR) predicts the shape and bond angles of molecules

  • Electrons are negatively charged and will repel other electrons when close to each other

  • In a molecule, the bonding pairs of electrons will repel other electrons around the central atom, forcing the molecule to adopt a shape in which these repulsive forces are minimised

  • When determining the shape and bond angles of a molecule, the following VSEPR rules should be considered:

    • Valence shell electrons are the electrons in the outer shell of the central atom

    • Electron pairs repel each other because they are negatively charged

    • The molecule adopts the shape that minimises these repulsions

    • Lone pairs repel more strongly than bonding pairs because their electron clouds are more concentrated and lie closer to the nucleus

    • Multiple bonds count as a single region of electron density, but repulsion between two multiple bonds is greater than between two single bonds

    • The order of repulsion is:
      lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

Diagram of water molecule showing two lone pairs on oxygen, two O–H bonds, bond angle 104.5°, and notes on greatest, intermediate and least electron pair repulsion
Different types of electron pairs have different repulsive forces

Shapes of Molecules & Ions

  • Molecules can adopt the following shapes and bond angles:

Diagram of VSEPR molecular shapes: linear, trigonal planar, bent, pyramidal, tetrahedral, trigonal bipyramidal and octahedral with typical bond angles.
Molecules of different shapes with their corresponding bond angles

Examples

Diagram explaining electron pair repulsion and shapes of BF₃ (trigonal planar, 120°) and CO₂ (linear, 180°) with dot‑and‑cross structures and notes.
Trigonal and linear shaped molecules
Diagram comparing CH₄ tetrahedral shape and NH₃ pyramidal shape, showing bonding and lone electron pairs, repulsions and bond angles 109.5° and 107°.
Tetrahedral and pyramidal shaped molecules
Diagram comparing shapes: bent H₂O with two lone pairs causing 104.5° bond angle, and octahedral SF₆ with six bonding pairs and 90° equal repulsion angles.
Bent(non-linear, V-shaped) and octahedral molecules
Diagram of PF₅ showing electron dot structure and trigonal bipyramidal shape, with 90° and 120° F–P–F bond angles and three equatorial plus two axial fluorines
Trigonal bipyramidal shaped molecules

Summary of the VSEPR theory

Electron pairs

Bonding pairs

Lone pairs

Molecular geometry

Bond angles

2

2

0

Linear

180°

3

3

0

Trigonal planar

120°

3

2

1

Bent

<120°

4

4

0

Tetrahedral

109.5°

4

3

1

Trigonal pyramidal

107°

4

2

2

Bent

104.5°

5

5

0

Trigonal bipyramidal

120° (equatorial)

90° (axial)

5

4

1

Seesaw

<120° (equatorial)

<90° (axial)

5

3

2

T-shaped

<90°

5

2

3

Linear

180°

6

6

0

Octahedral

90°

6

5

1

Square pyramidal

<90°

6

4

2

Square planar

90°

Worked Example

VSEPR & shapes of molecules

Draw the shape of the following molecules:

  1. Phosphorus(V) chloride

  2. N(CH3)3

  3. CCl4

Answer 1:

  • Phosphorus is in group 15, so has 5 valence electrons; Cl is in group 17, so has 7 valence electrons

  • All 5 electrons are used to form covalent bonds with Cl, and there are no lone pairs

  • This gives a trigonal bipyramidal shape:

Worked Example Answer 1 - VSEPR for Phosphorous(V) chloride, downloadable AS & A Level Chemistry revision notes

Phosphorus pentachloride or phosphorus (V) chloride

Answer 2: 

  • Nitrogen is in group 15, so has 5 valence electrons; carbon is in group 14, so has 4 valence electrons, 3 of which are already used in the covalent bonds with hydrogen

  • Three of the valence electrons in N are used to form bonding pairs, so there is one lone pair left

  • N(CH3)3 has a trigonal pyramid shape:

Worked Example Answer 2 - VSEPR for N(CH3), downloadable AS & A Level Chemistry revision notes

Trimethylamine

Answer 3: 

  • Carbon is in group 14, so has 4 valence electrons; chlorine is in group 17, so has 7 valence electrons

  • All four valence electrons are used to bond with chlorine, and there are no lone pairs

  • The shape of CCl4 is tetrahedral

Worked Example Answer 3 - VSEPR for CCl4, downloadable AS & A Level Chemistry revision notes

Tetrachloromethane

Related topics

Examiner Tips and Tricks

To explain reduced bond angles: start from the parent geometry (109.5° tetrahedral) and deduct ~2.5° per lone pair (NH3 107°, H2O 104.5°). Examiners report that students wrongly start from 180°.

Always name VSEPR and state the repulsion order — lone pair–lone pair > lone pair–bond pair > bond pair–bond pair — to explain why an angle is reduced; if there are no lone pairs (e.g., SF₆, CCl₄), state that all bonding pairs repel equally. A common lost mark is not mentioning VSEPR at all.

Drawing 3D shapes: use at least one wedge (coming out) and one dash (going in), show the lone pair(s) on the central atom, and don't draw bonds at 180° when the true angle is smaller.

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Author
Reviewer
Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.