VSEPR Theory and Molecular Shapes
VSEPR theory predicts the three-dimensional shape of a molecule based on how electron pairs around a central atom arrange themselves to stay as far apart as possible. The name stands for Valence Shell Electron Pair Repulsion, and the core idea is straightforward: electron pairs in the outermost shell of an atom repel each other, so they spread out in space until the repulsion between them is minimized. The resulting arrangement determines the molecular geometry, whether that turns out to be linear, bent, trigonal planar, tetrahedral, or another shape entirely.
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What Is VSEPR Theory?
VSEPR is a model used in chemistry to predict the shape of individual molecules based on the number of electron pairs surrounding the central atom. It was first introduced by Sidgwick and Powell in 1940 and later developed by Ronald Gillespie and Ronald Nyholm in the 1950s, which is why it is sometimes called the Gillespie-Nyholm theory.
With the VSEPR theory explained in its simplest form, the model says: count the electron groups, minimize their repulsion, and the resulting arrangement tells you the shape.
The VSEPR model rests on one central assumption: pairs of electrons in the valence shell of an atom repel each other because they carry the same negative charge, and they will always arrange themselves to minimize that repulsion.
VSEPR makes a key distinction between electron group geometry (how all electron groups are arranged in space) and molecular geometry (the shape defined only by the positions of the atoms), and both geometries follow directly from counting the electron groups around the central atom.
VSEPR Theory Rules
The VSEPR theory rules for predicting molecular shapes can be summarized in a clear set of steps.
- Identify the central atom. This is usually the least electronegative atom in the molecule (hydrogen and halogens are almost always terminal atoms, not central ones).
- Draw the Lewis structure to see how many bonding pairs and lone pairs surround the central atom.
- Count the total number of electron groups. Each single bond, double bond, triple bond, or lone pair counts as one electron group.
- Determine the electron group geometry by matching the total number of groups to the standard arrangements (2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral).
- Determine the molecular geometry by removing the lone pairs from the picture and looking only at where the atoms sit relative to each other.
VSEPR Theory Molecular Geometry: The Full Table
This VSEPR theory molecular geometry table covers the most common molecular shapes, organized by total electron groups, bonding pairs, and lone pairs.
| Total Electron Groups | Bonding Pairs | Lone Pairs | Electron Geometry | Molecular Shape | Bond Angle | Example |
|---|---|---|---|---|---|---|
|
2 |
2 |
0 |
Linear |
Linear |
180° |
BeCl₂, CO₂ |
|
3 |
3 |
0 |
Trigonal planar |
Trigonal planar |
120° |
BF₃ |
|
3 |
2 |
1 |
Trigonal planar |
Bent |
~117° |
SO₂ |
|
4 |
4 |
0 |
Tetrahedral |
Tetrahedral |
109.5° |
CH₄ |
|
4 |
3 |
1 |
Tetrahedral |
Trigonal pyramidal |
~107° |
NH₃ |
|
4 |
2 |
2 |
Tetrahedral |
Bent |
~104.5° |
H₂O |
|
5 |
5 |
0 |
Trigonal bipyramidal |
Trigonal bipyramidal |
90°, 120° |
PF₅ |
|
5 |
4 |
1 |
Trigonal bipyramidal |
Seesaw |
~102°, ~87° |
SF₄ |
|
5 |
3 |
2 |
Trigonal bipyramidal |
T-shaped |
~87° |
ClF₃ |
|
6 |
6 |
0 |
Octahedral |
Octahedral |
90° |
SF₆ |
|
6 |
5 |
1 |
Octahedral |
Square pyramidal |
~85° |
BrF₅ |
|
6 |
4 |
2 |
Octahedral |
Square planar |
90° |
XeF₄ |
Notice that the electron geometry stays the same as long as the total number of electron groups is the same, but the molecular shape changes depending on how many of those groups are lone pairs versus bonding pairs.
VSEPR Theory Formula
The VSEPR theory formula for finding the number of electron pairs around the central atom is:
VSEP number = ½ × (valence electrons of central atom + number of monovalent atoms bonded to it ± charge adjustment for ions)
For negative ions, add electrons equal to the charge. For positive ions, subtract electrons equal to the charge. This formula gives the total number of electron pairs, and from there, the table above tells you the shape.
For example, in water (H₂O), oxygen has 6 valence electrons and 2 hydrogen atoms are bonded to it, so the VSEP number is ½ × (6 + 2) = 4, predicting a tetrahedral electron geometry with two lone pairs, which gives the bent molecular shape that water actually has.
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How to Use VSEPR Theory: Worked Examples
Knowing how to use VSEPR theory in practice means applying the rules above to a specific molecule’s Lewis structure. Here are step-by-step VSEPR theory examples showing how to predict molecular shapes from scratch.
Example 1: Methane (CH₄)
Carbon has 4 valence electrons and forms 4 bonds with hydrogen. Total electron groups: 4, all bonding pairs. Electron geometry: tetrahedral. Molecular shape: tetrahedral with bond angles of 109.5°.
Example 2: Water (H₂O)
Oxygen has 6 valence electrons, forms 2 bonds with hydrogen, and has 2 lone pairs. Total electron groups: 4 (2 bonding, 2 lone). Electron geometry: tetrahedral. Molecular shape: bent with a bond angle of approximately 104.5°, slightly less than 109.5° because lone pairs compress the bond angle.
Example 3: Ammonia (NH₃)
Nitrogen has 5 valence electrons, forms 3 bonds with hydrogen, and has 1 lone pair. Total electron groups: 4 (3 bonding, 1 lone). Electron geometry: tetrahedral. Molecular shape: trigonal pyramidal with bond angles of approximately 107°.
Example 4: Carbon Dioxide (CO₂)
Carbon has 4 valence electrons and forms 2 double bonds with oxygen. Total electron groups: 2 (each double bond counts as one group). Electron geometry: linear. Molecular shape: linear with a bond angle of 180°.
Example 5: Sulfur Hexafluoride (SF₆)
Sulfur has 6 valence electrons and forms 6 bonds with fluorine. Total electron groups: 6, all bonding pairs. Electron geometry: octahedral. Molecular shape: octahedral with bond angles of 90°.
Why Lone Pairs Change the Shape
The VSEPR model treats lone pairs differently from bonding pairs because lone pairs are held closer to the central atom (they are not shared with another atom), so they occupy more angular space. This extra space pushes bonding pairs closer together, which is why the bond angles in ammonia (107°) and water (104.5°) are both smaller than the ideal tetrahedral angle of 109.5°.
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Test Your Understanding
Try answering these before checking the answers below, or browse more chemistry explanations in the Think10x.ai video library.
- What is the molecular shape of BF₃? Answer: Trigonal planar. Boron has 3 bonding pairs, no lone pairs, and all three electron groups are in a plane at 120° to each other.
- Why is water bent instead of linear? Answer: Oxygen has 4 electron groups (2 bonding pairs and 2 lone pairs), giving it a tetrahedral electron geometry. The 2 lone pairs are invisible in the molecular shape, so only the 2 hydrogen atoms are visible, producing a bent shape.
- How many electron groups does CO₂ have? Answer: 2. Each double bond between carbon and oxygen counts as a single electron group, giving CO₂ a linear shape.
How Think10x.ai Helps With Molecular Geometry
VSEPR theory questions require holding several pieces of information at once: the Lewis structure, the electron group count, the lone pair count, and the molecular shape that results from combining all three.
With the VSEPR theory explained through a narrated video rather than a static diagram, each of those layers becomes easier to follow.
Think10x.ai turns a VSEPR theory or molecular geometry question, typed, spoken, or photographed from a textbook page, into a narrated video that walks through each of those steps individually, so the connection between electron count and shape stays visible instead of getting lost in a block of text.
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Frequently Asked Questions
It works well for most main-group element compounds, but it does not accurately predict the structures of many transition metal compounds, since those involve d-orbital electrons that the basic VSEPR model does not account for.
It predicts approximate bond angles based on ideal geometries (120° for trigonal planar, 109.5° for tetrahedral, and so on), but the actual angles are often slightly different due to lone pair compression or differences in electronegativity between bonded atoms.
No. A double bond and a triple bond each count as a single electron group, exactly like a single bond. The number of electron groups is what determines the shape, not the number of shared electrons within each group.
Electron geometry describes the arrangement of all electron groups (both bonding and lone pairs) around the central atom. Molecular geometry describes only the positions of the atoms. When lone pairs are present, the two geometries can be different.