VSEPR Chart
Count the electron domains and lone pairs on the central atom, find the row, and read off the shape and bond angle.
| Domains | Liaisons | Lone pairs | AXE | Forme | Bond angle | Example |
|---|---|---|---|---|---|---|
| 2 | 2 | 0 | AX₂ | Linear | 180° | CO₂ |
| 3 | 3 | 0 | AX₃ | Trigonal planar | 120° | BF₃ |
| 3 | 2 | 1 | AX₂E | Bent | <120° (~119°) | SO₂ |
| 4 | 4 | 0 | AX₄ | Tetrahedral | 109.5° | CH₄ |
| 4 | 3 | 1 | AX₃E | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 | 2 | AX₂E₂ | Bent | ~104.5° | H₂O |
| 5 | 5 | 0 | AX₅ | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 5 | 4 | 1 | AX₄E | Seesaw | <90°, <120° | SF₄ |
| 5 | 3 | 2 | AX₃E₂ | T-shaped | <90° | ClF₃ |
| 5 | 2 | 3 | AX₂E₃ | Linear | 180° | XeF₂ |
| 6 | 6 | 0 | AX₆ | Octahedral | 90° | SF₆ |
| 6 | 5 | 1 | AX₅E | Square pyramidal | <90° | BrF₅ |
| 6 | 4 | 2 | AX₄E₂ | Square planar | 90° | XeF₄ |
How to find a molecule’s shape with VSEPR
- Draw the Lewis structure and pick out the central atom.
- Count electron domains on it: each bonded atom counts once (single, double or triple bond alike), each lone pair counts once.
- The number of domains gives the electron geometry (2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral).
- Ignore the lone pairs when naming the shape: find your bonds + lone pairs row in the chart.
Worked example: NH₃
Nitrogen has 5 valence electrons; three go into N–H bonds and two form one lone pair. That is 3 bonds + 1 lone pair = 4 domains, so the electron geometry is tetrahedral. Naming the shape from the atoms only gives trigonal pyramidal (AX₃E), and the lone pair pushes the H–N–H angle down from 109.5° to about 107°.
See the Lewis structures behind the chart for CO2, XeF4 et HOCl, then build the molecules and rotate their shapes in 3D in the Constructeur de Molécules.
Foire aux questions
What is VSEPR theory?
Valence Shell Electron Pair Repulsion theory: the electron pairs around a central atom (bonds and lone pairs) repel each other and spread out as far apart as possible. That arrangement sets the shape of the molecule.
Why do lone pairs make bond angles smaller?
A lone pair is held only by the central atom, so it sits closer to it and spreads out more than a bonding pair. It repels the bonds more strongly and squeezes them together: CH₄ is 109.5°, NH₃ about 107°, H₂O about 104.5°.
Do double bonds count as one electron domain?
Yes. A single, double or triple bond to the same atom all point in one direction, so each counts as one domain. CO₂ has two double bonds = 2 domains = linear.
What does AXE notation mean?
A is the central atom, X is each atom bonded to it and E is each lone pair on it. Water is AX₂E₂: two bonded H atoms and two lone pairs.
