AP Chemistry — Unit 2: Compound Structure and Properties
VSEPR and molecular geometry
Valence Shell Electron Pair Repulsion says something almost embarrassingly simple: electron domains push each other as far apart as they can get. A domain is a single bond, a double bond, a triple bond, or a lone pair — count them, spread them out, and you have the shape.
The catch is that lone pairs are invisible. You count them when working out the arrangement, but you do not see them when you look at the molecule. Toggle them off in the viewer below and watch a tetrahedral arrangement turn into a bent molecule.
What to take away: electron geometry counts every domain; molecular geometry counts only the atoms. They are identical when there are no lone pairs, and they diverge the moment there is one:
| Domains | Electron geometry | Lone pairs | Molecular geometry | Angle |
|---|---|---|---|---|
| 2 | Linear | 0 | Linear | 180° |
| 3 | Trigonal planar | 0 | Trigonal planar | 120° |
| 3 | Trigonal planar | 1 | Bent | ≈119° |
| 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° and 120° |
| 5 | Trigonal bipyramidal | 1 | Seesaw | ≈102° and ≈173° |
| 5 | Trigonal bipyramidal | 2 | T-shaped | ≈87.5° |
| 5 | Trigonal bipyramidal | 3 | Linear | 180° |
| 6 | Octahedral | 0 | Octahedral | 90° |
| 6 | Octahedral | 1 | Square pyramidal | ≈85° and ≈90° |
| 6 | Octahedral | 2 | Square planar | 90° |
An angle marked ≈ is the measured value for the example molecule in the viewer (NH₃, H₂O, SF₄ and so on); an unmarked angle is the ideal one. The viewer draws each model at the angle it prints, so the two always agree.
With five or six domains the lone pairs go where they have the most room. In a trigonal bipyramid that is the equatorial plane, where a lone pair has two neighbours at 90° instead of the three it would have on the axis; in an octahedron two lone pairs go opposite each other, which is why XeF₄ is a flat square.
Notice the angles shrink as lone pairs are added — 109.5° → 107° → 104.5° across CH₄, NH₃, H₂O. A lone pair is held by only one nucleus instead of two, so it spreads out more and squeezes the bonding pairs together.