Molecular shape
Electron groups around a central atom repel each other and spread as far apart as they can. This idea is called VSEPR, for valence shell electron pair repulsion.
An electron group is a lone pair or a bond. A double or triple bond counts as one group.
Count the groups on the central atom, then name the shape from where the atoms are. Lone pairs take up space but are not part of the shape’s name.
| Bonded atoms | Lone pairs | Shape | Bond angle | Example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | CO2 |
| 3 | 0 | Trigonal planar | 120° | BF3 |
| 4 | 0 | Tetrahedral | 109.5° | CH4 |
| 3 | 1 | Trigonal pyramidal | about 107° | NH3 |
| 2 | 2 | Bent | about 104.5° | H2O |
Lone pairs repel a little more strongly than bonds, which squeezes the bond angles slightly below 109.5°.
Worked example: ammonia, NH3
Nitrogen has 3 bonds and 1 lone pair, so 4 electron groups arranged roughly as a tetrahedron. Only three corners hold atoms.
The shape is trigonal pyramidal.
Common mistake
Predicting the shape from the formula alone. CO2 and H2O both have three atoms, but CO2 is linear and H2O is bent, because the oxygen in water has two lone pairs.