VSEPR Geometry Calculator
Predict molecular geometry and bond angles using VSEPR theory from bonding and lone electron pairs.
How VSEPR Predicts Shape
VSEPR rests on one idea: electron pairs repel each other and arrange themselves as far apart as possible. Count the regions of electron density around the central atom, place them at maximum separation, then name the shape using only the positions of the atoms — lone pairs are invisible in the name.
That last point is where confusion usually starts. Water and methane both have four electron regions and both adopt tetrahedral electron geometry. But water has two lone pairs, and since the shape is named from atom positions only, water is called bent while methane is tetrahedral. Same underlying arrangement, different name.
Lone pairs also repel more strongly than bonding pairs, because a lone pair is held by one nucleus rather than shared between two, so it spreads wider. This compresses the remaining bond angles below the ideal — which is why methane sits at 109.5°, ammonia at 107°, and water at 104.5° as lone pairs are added one at a time.
| Regions | Electron geometry | Lone pairs | Molecular shape | Approx. angle |
|---|---|---|---|---|
| 2 | Linear | 0 | Linear | 180° |
| 3 | Trigonal planar | 0 | Trigonal planar | 120° |
| 3 | Trigonal planar | 1 | Bent | ~118° |
| 4 | Tetrahedral | 0 | Tetrahedral | 109.5° |
| 4 | Tetrahedral | 1 | Trigonal pyramidal | ~107° |
| 4 | Tetrahedral | 2 | Bent | ~104.5° |
| 5 | Trigonal bipyramidal | 0 | Trigonal bipyramidal | 90° / 120° |
| 5 | Trigonal bipyramidal | 1 | Seesaw | ~90° / 117° |
| 5 | Trigonal bipyramidal | 2 | T-shaped | ~90° |
| 6 | Octahedral | 0 | Octahedral | 90° |
| 6 | Octahedral | 1 | Square pyramidal | ~90° |
| 6 | Octahedral | 2 | Square planar | 90° |
One further rule matters for five-region molecules: lone pairs always occupy equatorial positions in a trigonal bipyramid, never axial. Equatorial sites have two neighbours at 90° whereas axial sites have three, so placing the bulky lone pair equatorially minimises repulsion. This is why SF4 is a seesaw.
Worked Examples
Common Mistakes
A double or triple bond is one region of electron density. CO2 has two double bonds and therefore two regions — it is linear at 180°, not bent.
Molecular shape describes where the atoms are, ignoring lone pairs. Ammonia has tetrahedral electron geometry but is called trigonal pyramidal, because you cannot see the lone pair.
Ideal angles apply only when all regions are identical bonding pairs. Each lone pair compresses the remaining angles by roughly 2–3°, and bulky substituents shift them further.
Lone pairs always take equatorial positions, where they have fewer close neighbours. Getting this wrong turns a seesaw into an incorrect shape.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This relationship uses electronic structure, bonding or molecular geometry to predict a chemical property or structural descriptor. Assumption: The model may be an approximation; resonance, solvent, coordination environment, conformation and experimental conditions can affect real molecules.