What Is VSEPR Theory? Predicting Molecular Shapes
A Lewis structure is flat. A real molecule is not. Water isn't a straight line, methane isn't a cross, and ammonia isn't a triangle — and the theory that gets you from the flat drawing to the real three-dimensional shape, in about thirty seconds per molecule, is VSEPR.
The short answer: VSEPR stands for Valence Shell Electron Pair Repulsion, and it says that the groups of electrons around a central atom push each other as far apart as possible. Count the groups, arrange them at maximum separation, and you have the molecule's shape.
What VSEPR actually claims
The whole theory rests on one idea: electrons repel electrons. Every region of electron density around a central atom — every bond and every lone pair — is negatively charged, so they all shove each other away. They settle into whatever arrangement puts them at the greatest possible angular distance from one another, and the atoms come along for the ride.
That's it. There's no orbital mathematics involved and no quantum mechanics required, which is why VSEPR is taught early and why it's astonishingly accurate for a rule this simple.
The useful mental picture is balloons tied at their necks. Tie two together and they point in opposite directions — 180°, linear. Three settle into a flat triangle — 120°. Four spread into a tetrahedron — 109.5°. They do it automatically, because that's the arrangement with the least crowding. Electron domains do the same thing.
Step 1: count the electron domains
An electron domain (also called an electron group or a region of electron density) is:
- a single bond, or
- a double bond, or
- a triple bond, or
- a lone pair.
Notice that a double and a triple bond each count as one domain, not two or three. All that shared density sits between the same pair of atoms, so it points in one direction and it repels as one unit.
Domains on the central atom = (number of atoms attached) + (number of lone pairs on it).
Step 2: read off the arrangement
Domains map straight onto one of five geometries. This table is the core of the whole topic — learn it and you're most of the way there.
| Domains | Electron arrangement | Ideal bond angle |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 120° in the plane, 90° to the axis |
| 6 | Octahedral | 90° |
Step 3: name the shape from the atoms only
The arrangement above describes where the domains go. The molecular shape you actually name describes where the atoms go — lone pairs are invisible. Four domains give a tetrahedral arrangement every time, but the name changes with how many of those domains are lone pairs:
| Domains | Lone pairs | Shape name | Example | Angle |
|---|---|---|---|---|
| 2 | 0 | Linear | CO₂ | 180° |
| 3 | 0 | Trigonal planar | BF₃ | 120° |
| 3 | 1 | Bent | SO₂ | ~119° |
| 4 | 0 | Tetrahedral | CH₄ | 109.5° |
| 4 | 1 | Trigonal pyramidal | NH₃ | ~107° |
| 4 | 2 | Bent | H₂O | ~104.5° |
| 5 | 0 | Trigonal bipyramidal | PCl₅ | 90°, 120° |
| 5 | 1 | Seesaw | SF₄ | <90°, <120° |
| 5 | 2 | T-shaped | ClF₃ | <90° |
| 6 | 0 | Octahedral | SF₆ | 90° |
| 6 | 1 | Square pyramidal | BrF₅ | <90° |
| 6 | 2 | Square planar | XeF₄ | 90° |
Step 4: shrink the angle for each lone pair
The ideal angles assume every domain repels equally. They don't. A lone pair is held by only one nucleus, so it spreads wider and pushes harder:
lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair
Each lone pair on the central atom therefore squeezes the bond angles a little below the ideal — roughly 2–2.5° per lone pair in the four-domain series:
CH₄ 109.5° → NH₃ 107° → H₂O 104.5°
Double and triple bonds are also slightly fatter than single bonds and push a little harder, which is why the O–S–O angle in SO₂ (about 119°) sits just under the ideal 120° rather than well under it.
Where lone pairs go matters too. In a five-domain molecule, lone pairs always take the roomier equatorial positions rather than the cramped axial ones — that's why SF₄ is a seesaw and not something more symmetrical. In a six-domain molecule with two lone pairs, they sit opposite each other, giving XeF₄ its square planar shape.
Worked examples
Work each one out before reading the answer: total the domains, subtract the lone pairs, name the shape.
- CH₄ → 4 bonding, 0 lone. Tetrahedral, 109.5°.
- CO₂ → 2 domains (two double bonds), 0 lone. Linear, 180° — the double bonds don't change anything.
- H₂O → 2 bonding, 2 lone. Four domains, tetrahedral arrangement, bent at 104.5°.
- SO₃ → 3 domains, 0 lone. Trigonal planar, 120°.
- NH₄⁺ → 4 bonding, 0 lone. Tetrahedral, 109.5° — the lone pair became a bond, so the angle opens back up.
- XeF₂ → 2 bonding, 3 lone. Five domains; the three lone pairs take all three equatorial spots, leaving the fluorines axial. Linear, 180°.
That last one is the best advert for VSEPR: a molecule with five electron domains ends up perfectly linear, and you can predict it without knowing anything else about xenon.
Common mistakes to avoid
- Naming the electron arrangement instead of the molecular shape. Water has a tetrahedral arrangement of domains but is bent. Both answers are correct to different questions — read which one is being asked.
- Counting a double bond twice. One domain, always. CO₂ is linear precisely because of this.
- Quoting the ideal angle when there are lone pairs. Saying ammonia is 109.5° throws away the mark. It's about 107°, and the reason — the lone pair repels harder — is usually worth a mark of its own.
FAQ
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion. It's usually pronounced "vesper".
How do you use VSEPR theory?
Draw the Lewis structure, count the electron domains on the central atom (bonds of any order count once each, plus lone pairs), read off the arrangement from the domain count, then name the shape using only the positions of the atoms and shrink the angle slightly for each lone pair.
Do double bonds count as one or two domains in VSEPR?
One. All the electron density in a double or triple bond points in the same direction, so it repels as a single region.
Is VSEPR always right?
It's remarkably good for small main-group molecules, which is what you'll be examined on. It's less reliable for transition-metal complexes and for some heavy p-block molecules, where other effects compete with simple repulsion.
The takeaway
VSEPR is one rule applied carefully: electron domains repel, so they spread as far apart as they can. Count the domains to get the arrangement, ignore the lone pairs to get the name, and shave a couple of degrees off the angle for every lone pair present.
Before this → [Lone Pair vs Bonding Pair] (sibling) and What Is a Lewis Structure? Dots, Bonds, and Octets. After this → [Electron vs Molecular Geometry] (sibling), and What Is a Polar Molecule? Shape, Dipoles, and Water for what the shape then decides.
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