VSEPR Shapes: A Method That Works for Any Molecule
Count the electron pairs, place the lone pairs where they hurt least, and the shape follows. Every time.
The procedure
- Count sigma bonds to the central atom. Multiple bonds count as one pair.
- Count lone pairs on the central atom.
- Add them to get the total electron pairs, which gives the electron pair geometry.
- Place lone pairs in the positions of least repulsion.
- Name the shape from the positions of the atoms only.
The geometries
| Pairs | Electron geometry | Lone pairs | Molecular shape |
|---|---|---|---|
| 2 | Linear | 0 | Linear |
| 3 | Trigonal planar | 0 | Trigonal planar |
| 3 | Trigonal planar | 1 | Bent |
| 4 | Tetrahedral | 0 | Tetrahedral |
| 4 | Tetrahedral | 1 | Trigonal pyramidal |
| 4 | Tetrahedral | 2 | Bent |
| 5 | Trigonal bipyramidal | 0 | Trigonal bipyramidal |
| 5 | Trigonal bipyramidal | 1 | See-saw |
| 5 | Trigonal bipyramidal | 2 | T-shaped |
| 5 | Trigonal bipyramidal | 3 | Linear |
| 6 | Octahedral | 0 | Octahedral |
| 6 | Octahedral | 1 | Square pyramidal |
| 6 | Octahedral | 2 | Square planar |
Where lone pairs go
Lone pairs occupy more space than bonding pairs, because they are held by only one nucleus and spread out more. They therefore go where they can most easily avoid other pairs.
- In a trigonal bipyramid, lone pairs go equatorial. An equatorial position has only two close neighbours at ninety degrees, while an axial position has three. Fewer close contacts means less repulsion.
- In an octahedron with two lone pairs, they go opposite each other, giving square planar. Adjacent lone pairs would repel far more strongly.
Being asked why lone pairs take those positions is a standard question, and counting the ninety-degree contacts is the expected justification.
Bond angle distortion
Because lone pairs repel more strongly, each one compresses the remaining bond angles below the ideal value. A tetrahedral arrangement with one lone pair has angles slightly under the ideal; with two, smaller still.
Comparing bond angles across a series with increasing lone pairs is a routine question, and the answer follows from this ordering alone.
Electronegativity effects
More electronegative substituents draw bonding pairs away from the central atom, reducing repulsion between them and allowing the angle to close slightly. So replacing a substituent with a more electronegative one generally decreases the bond angle.
Frequently asked questions
Why do lone pairs occupy equatorial positions in a trigonal bipyramid?
Because an equatorial position has only two neighbours at ninety degrees while an axial position has three, so equatorial placement minimises repulsion.
Do multiple bonds count as more than one pair?
No. A double or triple bond counts as one region of electron density for shape purposes, though it does exert slightly greater repulsion than a single bond.
Why is a molecule with two lone pairs bent rather than tetrahedral?
Because the shape is named from atom positions only. The electron geometry is tetrahedral, but with two positions occupied by lone pairs the atoms form a bent arrangement.
Why does higher substituent electronegativity reduce the bond angle?
Because the bonding pairs are pulled further from the central atom, so they repel one another less and the angle between them closes slightly.
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