In simple terms
A friendly intro before the formal notes — no formulas yet.
Electron Repulsion Rules
The shape of a molecule is determined by the repulsion between electron pairs in the central atom's valence shell. These pairs arrange themselves to be as far apart as possible, with lone pairs repelling more strongly than bonding pairs.
Imagine you are holding several inflated balloons tied together at a central point. The balloons (electron pairs) will automatically push each other away to take up positions that are as far apart as possible. If you have two balloons, they'll point in opposite directions (linear); if you have four, they'll form a tetrahedron. Now, if one balloon is much larger (a lone pair), it will squash the others (bonding pairs) closer together, changing the angles between them.
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VSEPR: electron pairs repel — arrange to minimise repulsion.
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Lone pairs occupy more space than bonding pairs — compress bond angles.
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Common shapes: linear (2 bp), trigonal planar (3 bp), tetrahedral (4 bp).
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Bond angle evidence distinguishes shapes in Paper 2 structured questions.
Explore the concept
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VSEPR: electron pairs repel
VSEPR: electron pairs repel — arrange to minimise repulsion.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Core Principles of VSEPR Theory
The VSEPR theory is based on a simple but powerful idea: negatively charged electron pairs in the outer shell of a central atom will repel each other. To achieve the most stable arrangement, these electron pairs will position themselves as far apart as possible, which in turn dictates the molecule's shape. This applies to both bonding pairs (involved in covalent bonds) and lone pairs (non-bonding electrons).
The shape of a molecule is determined by the number of electron pairs in the valence shell of the central atom.
Electron pairs arrange themselves to minimise electrostatic repulsion.
The final shape of the molecule is described by the positions of the atomic nuclei, not the electron pairs.
Predicting Shapes: A Step-by-Step Guide
To predict the shape of a molecule or ion, we can follow a logical sequence. First, draw the Lewis structure to identify the central atom and the atoms bonded to it. Then, count the total number of electron pairs around the central atom, being careful to distinguish between bonding pairs and lone pairs. This total number determines the basic geometry of the electron pairs. Finally, the presence of lone pairs modifies this basic geometry to give the final molecular shape.
Step 1: Determine the central atom and count its valence electrons.
Step 2: Add the electrons from the bonded atoms (and adjust for any charge on an ion).
Step 3: Divide the total by two to find the number of electron pairs.
Step 4: Determine the number of bonding pairs and lone pairs to predict the shape and bond angles.
The Influence of Lone Pairs on Bond Angles
Not all electron pairs are equal in their repulsive force. A lone pair is held by only one nucleus, so its electron cloud is more spread out and occupies more volume than a bonding pair, which is tightly held between two positive nuclei. Consequently, lone pairs exert a stronger repulsive force than bonding pairs. This leads to a specific hierarchy of repulsion.
Repulsion strength: LP-LP > LP-BP > BP-BP
This means that the presence of a lone pair will compress the bond angles between the bonding pairs. For example, methane (CH₄), ammonia (NH₃), and water (H₂O) all have four electron pairs around the central atom, giving a basic tetrahedral arrangement. However, the increasing number of lone pairs (0 in CH₄, 1 in NH₃, 2 in H₂O) causes the bond angles to decrease progressively from the ideal 109.5°.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Predict the shape and bond angle of a phosphorus trichloride (PCl₃) molecule. Explain your reasoning.
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Identify central atom and valence electrons: The central atom is Phosphorus (P), which is in Group 15. It has 5 valence electrons.
The hexafluorosilicate(IV) ion, SiF₆²⁻, has an octahedral shape. Predict the F-Si-F bond angle. Explain why the shape of sulfur hexafluoride, SF₆, is also octahedral with the same bond angle.
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Analyse SiF₆²⁻: The central atom is Silicon (Si), in Group 14, with 4 valence electrons. It is bonded to 6 Fluorine (F) atoms. The ion has a 2- charge, meaning 2 extra electrons are present.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is the fundamental principle of VSEPR theory?
Electron pairs in the valence shell of a central atom repel each other and arrange themselves in space to be as far apart as possible, minimising repulsion.
Key takeaways
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The shape of a molecule is determined by the number of electron pairs in the valence shell of the central atom.
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Electron pairs arrange themselves to minimise electrostatic repulsion.
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The final shape of the molecule is described by the positions of the atomic nuclei, not the electron pairs.
Practice — then mark it
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Test your knowledge on molecular shapes
Test your knowledge on molecular shapes
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