VSEPR Theory: How to Predict Molecular Geometry

What Is VSEPR Theory?

Have you ever wondered why water is bent, carbon dioxide is linear, and methane is tetrahedral? These shapes aren't random — they are determined by a set of rules called VSEPR theory.

VSEPR stands for Valence Shell Electron Pair Repulsion. The core idea is simple: electron pairs around a central atom repel each other and will arrange themselves as far apart as possible to minimise repulsion. This arrangement determines the molecule's 3D shape.

Learning Goals: By the end of this guide, you should be able to:

Use the AXE method to classify any molecule.

Predict molecular geometry and bond angles from electron domain count.

Explain how lone pairs distort ideal bond angles.

Draw and name all common molecular shapes (linear through to octahedral).

Apply VSEPR to exam-style problems with confidence.

The AXE Method: A Step-by-Step System

The AXE method is the systematic way to apply VSEPR theory:

A = the central atom
X = the number of bonding domains (atoms bonded to A). A single, double, or triple bond each count as one domain.
E = the number of lone pairs on A

How to Use It

Draw the Lewis structure of the molecule.
Identify the central atom (usually the least electronegative atom, except H).
Count X (number of atoms bonded to the central atom).
Count E (number of lone pairs on the central atom).
Add X + E to get the total electron domains.
Look up the geometry in the table below.

Complete Geometry Table

2 Electron Domains

AXE	Bonding	Lone Pairs	Geometry	Bond Angle	Example
$AX_2$	2	0	Linear	$180°$	$CO_2$ , $BeCl_2$

3 Electron Domains

AXE	Bonding	Lone Pairs	Geometry	Bond Angle	Example
$AX_3$	3	0	Trigonal Planar	$120°$	$BF_3$ , $SO_3$
$AX_2E$	2	1	Bent (V-shape)	$\approx 117°$	$SO_2$ , $O_3$

4 Electron Domains

AXE	Bonding	Lone Pairs	Geometry	Bond Angle	Example
$AX_4$	4	0	Tetrahedral	$109.5°$	$CH_4$ , $CCl_4$
$AX_3E$	3	1	Trigonal Pyramidal	$\approx 107°$	$NH_3$ , $PCl_3$
$AX_2E_2$	2	2	Bent (V-shape)	$\approx 104.5°$	$H_2O$ , $H_2S$

5 Electron Domains

AXE	Bonding	Lone Pairs	Geometry	Bond Angle	Example
$AX_5$	5	0	Trigonal Bipyramidal	$90°$ and $120°$	$PCl_5$
$AX_4E$	4	1	Seesaw	$\approx 117°$ and $\approx 90°$	$SF_4$
$AX_3E_2$	3	2	T-shaped	$\approx 90°$	$ClF_3$
$AX_2E_3$	2	3	Linear	$180°$	$XeF_2$

6 Electron Domains

AXE	Bonding	Lone Pairs	Geometry	Bond Angle	Example
$AX_6$	6	0	Octahedral	$90°$	$SF_6$
$AX_5E$	5	1	Square Pyramidal	$\approx 85°$	$BrF_5$
$AX_4E_2$	4	2	Square Planar	$90°$	$XeF_4$

Why Do Lone Pairs Reduce Bond Angles?

This is one of the most important concepts in VSEPR and a favourite exam question.

The repulsion order is:

\text{Lone Pair – Lone Pair} > \text{Lone Pair – Bonding Pair} > \text{Bonding Pair – Bonding Pair}

Lone pairs are held closer to the nucleus (they are not shared with another atom), so they occupy more space and push bonding pairs closer together. This compresses the bond angles below their ideal values.

Demonstration: The 4-Domain Family

All three molecules below have 4 electron domains, but their bond angles decrease as lone pairs increase:

Molecule	AXE	Bond Angle	Shape
$CH_4$ (methane)	$AX_4$	$109.5°$	Tetrahedral
$NH_3$ (ammonia)	$AX_3E$	$107°$	Trigonal Pyramidal
$H_2O$ (water)	$AX_2E_2$	$104.5°$	Bent

Each lone pair compresses the bond angle by roughly $2–2.5°$ .

Interactive VSEPR Model

Visualise all molecular shapes in 3D. Rotate molecules, toggle lone pairs, and watch how bond angles change — from linear to octahedral.

Launch VSEPR Visualiser

Worked Examples

Example 1: What is the shape of $PCl_3$ ?

Step 1: Phosphorus is the central atom. It has 5 valence electrons.

Step 2: 3 Cl atoms bond to P → 3 bonding domains (X = 3). P uses 3 electrons for bonding, leaving 1 lone pair (E = 1).

Step 3: Classification: $AX_3E$ → 4 total electron domains.

Step 4: Look up the table → Trigonal Pyramidal, bond angle $\approx 107°$ .

Example 2: Why is $CO_2$ linear but $SO_2$ is bent?

$CO_2$ : Carbon has 4 valence electrons. Two double bonds to O → 2 bonding domains, 0 lone pairs. $AX_2$ → Linear ( $180°$ ).

$SO_2$ : Sulphur has 6 valence electrons. Two double bonds to O → 2 bonding domains, 1 lone pair on S. $AX_2E$ → Bent ( $\approx 117°$ ).

The lone pair on sulphur pushes the two O atoms closer together, giving a bent shape despite having only 2 bonding domains.

Example 3: Predict the shape of $XeF_4$

Step 1: Xenon has 8 valence electrons. 4 F atoms bond → 4 bonding pairs use 4 electrons. Remaining 4 electrons = 2 lone pairs.

Step 2: AXE classification: $AX_4E_2$ → 6 total electron domains.

Step 3: Look up the table → Square Planar, bond angle $90°$ .

The 2 lone pairs sit on opposite sides of the plane (trans arrangement) to minimise lone pair–lone pair repulsion.

Common Mistakes

Counting double bonds as two domains — In VSEPR, a double bond counts as one bonding domain, not two. $CO_2$ has 2 domains, not 4.
Forgetting to count lone pairs — Students often only count bonds. Always draw the Lewis structure first and check for lone pairs on the central atom.
Confusing electron geometry with molecular geometry — The electron geometry of water is tetrahedral (4 domains), but the molecular geometry (shape you actually see) is bent. Exam questions usually ask for molecular geometry.
Using VSEPR for polyatomic systems incorrectly — VSEPR predicts the shape around one central atom. For molecules like ethene ( $C_2H_4$ ), apply VSEPR separately to each carbon.
Saying "lone pairs don't count" — Lone pairs absolutely count for determining the electron geometry. They just aren't "visible" in the molecular shape.

Exam Tips (A-Level / AP / IB)

Always draw the Lewis structure first — you cannot apply VSEPR without knowing the electron arrangement.
When asked to explain a bond angle, mention the repulsion order: LP–LP > LP–BP > BP–BP.
For 5-domain systems, know that lone pairs always go in equatorial positions (to maximise 120° separations rather than 90°).
State both the electron domain geometry and the molecular geometry when asked to "describe the shape" — this shows deeper understanding.
If two molecules have the same formula type (e.g., both $AX_2E$ ), they have the same shape regardless of which elements are involved.

Frequently Asked Questions

What does VSEPR stand for?

VSEPR stands for Valence Shell Electron Pair Repulsion. It is a model that predicts molecular geometry based on the idea that electron pairs around a central atom arrange themselves as far apart as possible to minimise repulsion.

How do you determine the shape of a molecule?

Draw the Lewis structure, count the bonding domains (X) and lone pairs (E) on the central atom, then use the AXE classification to look up the geometry in the VSEPR table.

Why is water bent and not linear?

Water ( $H_2O$ ) has 2 bonding pairs and 2 lone pairs on oxygen ( $AX_2E_2$ ). The 4 electron domains adopt a tetrahedral arrangement, but since lone pairs are invisible, the visible shape is bent with a bond angle of about $104.5°$ .

Do multiple bonds affect molecular shape?

In VSEPR, a double or triple bond counts as a single bonding domain. So $CO_2$ (two double bonds) has only 2 domains and is linear, just like $BeCl_2$ (two single bonds).

Can VSEPR predict the shape of any molecule?

VSEPR works well for main-group elements but has limitations with transition metals and very large molecules. For more advanced predictions, you would use molecular orbital theory.

Chemical Bonds — Understand the forces that hold atoms together before predicting their shapes.
Molecular Coplanarity — Explore when and why atoms in a molecule lie in the same plane.
Orbital Hybridization — How atomic orbitals combine to explain VSEPR geometries at the quantum level.