Molecular Geometry
Every VSEPR shape by steric number, with electron geometry, bond angles and an example.
Showing 13 of 13.
| SN | Bonding / lone | Molecular shape | Bond angle | Example |
|---|---|---|---|---|
| 2 | 2 / 0 | Linear | 180° | CO₂ |
| 3 | 3 / 0 | Trigonal planar | 120° | BF₃ |
| 3 | 2 / 1 | Bent | < 120° | SO₂ |
| 4 | 4 / 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 3 / 1 | Trigonal pyramidal | ≈ 107° | NH₃ |
| 4 | 2 / 2 | Bent | ≈ 104.5° | H₂O |
| 5 | 5 / 0 | Trigonal bipyramidal | 90° and 120° | PCl₅ |
| 5 | 4 / 1 | Seesaw | < 90° and < 120° | SF₄ |
| 5 | 3 / 2 | T-shaped | < 90° | ClF₃ |
| 5 | 2 / 3 | Linear | 180° | XeF₂ |
| 6 | 6 / 0 | Octahedral | 90° | SF₆ |
| 6 | 5 / 1 | Square pyramidal | < 90° | BrF₅ |
| 6 | 4 / 2 | Square planar | 90° | XeF₄ |
The steric number counts bonding groups plus lone pairs, and a double or triple bond counts as one group. Electron geometry is the arrangement of everything around the central atom; molecular shape is what you see once the lone pairs are made invisible — which is why NH₃ and H₂O are both tetrahedral in electron geometry but pyramidal and bent in shape.
VSEPR predicts a molecule’s shape from one idea: electron groups around the central atom push apart as far as possible. Count bonding groups plus lone pairs to get the steric number, then read off the shape. Four groups with two lone pairs gives bent, which is why water’s angle is about 104.5°.
One principle, thirteen shapes
Valence shell electron pair repulsion says that regions of electron density around a central atom arrange themselves to be as far apart as they can. That single rule generates every geometry in the table. Two groups end up opposite each other at 180°, three spread into a triangle at 120°, four into a tetrahedron at 109.5°, and so on.
What counts as one group is the part worth getting right. A single, double or triple bond each count as one group, because the extra pairs sit between the same two atoms. A lone pair also counts as one group. So CO₂, with two double bonds and no lone pairs, has a steric number of 2 and is linear.
Electron geometry against molecular shape
These are two different answers to two different questions. Electron geometry describes where all the groups sit, lone pairs included. Molecular shape describes only where the atoms are, because lone pairs are invisible. Ammonia and water both have a tetrahedral electron geometry with four groups, but ammonia has one lone pair and looks trigonal pyramidal while water has two and looks bent.
a double or triple bond counts as one bonding group, not two or three
- 1 Draw the Lewis structure. For H₂O, oxygen sits in the centre with two bonds to hydrogen and two lone pairs.
- 2 Count the groups on the central atom. Two bonding groups plus two lone pairs gives a steric number of 4.
- 3 Read the electron geometry from the steric number. Four groups always arrange tetrahedrally, whatever those groups are.
- 4 Ignore the lone pairs to get the shape. With two of the four positions holding invisible lone pairs, the two hydrogens make a bent shape.
- 5 Adjust the angle for lone pair repulsion. Lone pairs push harder than bonds, squeezing the ideal 109.5° down to about 104.5° in water.
Why the angles shrink
All three have a tetrahedral electron geometry; each lone pair compresses the remaining angle further.
| Molecule | Bonding / lone | Shape | Angle |
|---|---|---|---|
| CH₄ | 4 / 0 | Tetrahedral | 109.5° |
| NH₃ | 3 / 1 | Trigonal pyramidal | ≈ 107° |
| H₂O | 2 / 2 | Bent | ≈ 104.5° |
Shape decides polarity
Geometry is not an end in itself — it determines whether a molecule is polar. CO₂ and H₂O both contain polar bonds, but CO₂ is linear so its two bond dipoles point in exactly opposite directions and cancel, leaving a non-polar molecule. Water is bent, so its dipoles do not cancel, and the result is the polar solvent that most of chemistry depends on.
That is why a symmetric shape with identical outer atoms is usually non-polar, and why an asymmetric shape — anything with lone pairs on the central atom, or with different atoms attached — usually is not. Work out the geometry first, then ask whether the dipoles cancel.