AP Chemistry VSEPR Chart
Every electron-domain and lone-pair combination, mapped to its shape name and bond angle.
VSEPR theory predicts molecular shape from one idea: electron domains around a central atom repel each other and spread out as far apart as possible. The electron-domain count sets the base geometry; lone pairs then change the molecular shape's name (and compress its bond angles) without changing the underlying domain count. Here's the complete chart, from two domains through six.
Electron-Domain Geometry vs. Molecular Geometry
These two terms get mixed up constantly, and they describe genuinely different things. Electron-domain geometry counts every domain, bonding pairs and lone pairs together. Molecular geometry describes only where the atoms actually are, ignoring lone pairs even though they still take up space and push the bonding pairs closer together. Same domain count, different name, once a lone pair enters the picture.
The Full Chart
| Domains | Lone Pairs | Electron-Domain Geometry | Molecular Geometry | Bond Angle(s) | Example |
|---|---|---|---|---|---|
| 2 | 0 | Linear | Linear | 180° | CO₂ |
| 3 | 0 | Trigonal planar | Trigonal planar | 120° | BF₃ |
| 3 | 1 | Trigonal planar | Bent | <120° | SO₂ |
| 4 | 0 | Tetrahedral | Tetrahedral | 109.5° | CH₄ |
| 4 | 1 | Tetrahedral | Trigonal pyramidal | <109.5° | NH₃ |
| 4 | 2 | Tetrahedral | Bent | ~104.5° | H₂O |
| 5 | 0 | Trigonal bipyramidal | Trigonal bipyramidal | 90° & 120° | PCl₅ |
| 5 | 1 | Trigonal bipyramidal | Seesaw | <90° & <120° | SF₄ |
| 5 | 2 | Trigonal bipyramidal | T-shaped | <90° | ClF₃ |
| 5 | 3 | Trigonal bipyramidal | Linear | 180° | XeF₂ |
| 6 | 0 | Octahedral | Octahedral | 90° | SF₆ |
| 6 | 1 | Octahedral | Square pyramidal | <90° | BrF₅ |
| 6 | 2 | Octahedral | Square planar | 90° | XeF₄ |
What AP actually requires: shape identification is tested for all rows in this chart, including the five- and six-domain shapes. What's not tested is the d-orbital hybridization reasoning (sp3d, sp3d2) behind the five- and six-domain rows, only the resulting shape and bond angles.
Why Lone Pairs Shrink Bond Angles
A lone pair sits closer to the central atom than a bonding pair does, since a bonding pair is shared with (and pulled toward) another nucleus. That makes lone-pair repulsion stronger than bonding-pair repulsion, which compresses the remaining bond angles below the electron-domain geometry's ideal value. That's why water's H-O-H angle (~104.5°) is smaller than ammonia's (~107°), even though both start from the same tetrahedral electron-domain geometry, water simply has one more lone pair doing the compressing.
Common Mistakes
- Reporting electron-domain geometry when asked for molecular geometry, or vice versa. They only match when there are zero lone pairs; otherwise they're different names for the same domain count.
- Forgetting that lone pairs still count as domains. A central atom with 2 bonding pairs and 2 lone pairs has 4 total domains (tetrahedral electron-domain geometry), not 2.
- Assuming all bond angles in a shape are identical. Trigonal bipyramidal and its lone-pair variants have two distinct angles (90° and 120°), not one.
- Skipping five- and six-domain shapes because the hybridization isn't tested. The shape identification itself still is.
Related Resources
- Unit 2 Review: Compound Structure and Properties
- AP Chemistry Polyatomic Ions
- Unit 1 Review: Atomic Structure and Properties
- AP Chemistry Reference Sheet
- AP Chemistry Study Guide
Frequently Asked Questions
What is the difference between electron-domain geometry and molecular geometry?
Electron-domain geometry counts every electron domain around the central atom, bonding pairs and lone pairs alike. Molecular geometry describes only the arrangement of atoms, ignoring lone pairs even though they still occupy space and push bonding pairs closer together. A tetrahedral electron-domain arrangement with one lone pair gives a trigonal pyramidal molecular shape, like ammonia, the two labels describe the same domain count but aren't interchangeable.
Why do lone pairs make bond angles smaller?
A lone pair is held closer to the central atom than a bonding pair, since it isn't shared with another nucleus pulling it outward. That makes lone-pair repulsion stronger than bonding-pair repulsion, which compresses the angles between the remaining bonding pairs below the electron-domain geometry's ideal angle.
Does AP Chemistry test five- and six-domain shapes like octahedral and square pyramidal?
Yes, shape identification for five and six electron domains is tested, but the hybridization labels for them (sp3d, sp3d2) are not. You're responsible for naming and predicting shapes like trigonal bipyramidal, seesaw, octahedral, and square pyramidal, just not the d-orbital hybridization reasoning behind them.
How many lone pairs can an electron-domain geometry have before the shape name changes?
It depends on the total domain count, not a fixed number. Four domains can have 0, 1, or 2 lone pairs (tetrahedral, trigonal pyramidal, or bent), while five domains can have up to 3 lone pairs before you run out of bonding pairs entirely (trigonal bipyramidal down to linear).
Why does water have a smaller bond angle than ammonia?
Both start from a tetrahedral electron-domain geometry, but water has two lone pairs to ammonia's one. Two lone pairs push harder on the remaining bonding pairs than one does, compressing water's H-O-H angle to about 104.5° compared to ammonia's roughly 107°, both below the ideal 109.5°.
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