VSEPR theory
Electron pairs around the central atom push apart as far as they can. Count them, and the shape follows.
Shape explorer
i
Electron pairs are the bond pairs plus the lone pairs on the central atom (the steric number). A double or triple bond counts as one pair.
What VSEPR theory says
- The shape of a molecule depends on the number of electron pairs, bonded and lone, in the valence shell of the central atom.
- Electron pairs repel each other because their clouds are negatively charged.
- The pairs settle as far apart as they can, where the repulsion between them is least.
- The valence shell is treated as a sphere, with the pairs sitting on its surface as far from each other as possible.
- A double or triple bond is treated as one super pair.
- Where a molecule has resonance structures, VSEPR can be applied to any of them.
- Repulsion is strongest between two lone pairs, weaker between a lone pair and a bond pair, and weakest between two bond pairs, because a lone pair is held by one nucleus only and spreads out more.
Class 11, Chapter 4 Chemical Bonding and Molecular Structure.
Every VSEPR shape with examples
| Shape | Class | Pairs | Angle | Hybridisation | Examples |
|---|---|---|---|---|---|
| linear | AB₂ | 2 bond, 0 lone | 180° | sp | BeCl₂, BeF₂, CO₂, CS₂, HCN, NO₂⁺, C₂H₂ |
| trigonal planar | AB₃ | 3 bond, 0 lone | 120° | sp² | BF₃, BCl₃, AlCl₃, SO₃, NO₃⁻, CO₃²⁻, HNO₃, C₂H₄, C₆H₆, Graphite |
| bent | AB₂E | 2 bond, 1 lone | less than 120° | sp² | SO₂, O₃, NO₂, NO₂⁻ |
| tetrahedral | AB₄ | 4 bond, 0 lone | 109.5° | sp³ | CH₄, NH₄⁺, CCl₄, CHCl₃, SiH₄, SiCl₄, C₂H₆, Diamond, SO₄²⁻, ClO₄⁻, PO₄³⁻, XeO₄, H₂SO₄, SO₂Cl₂ |
| trigonal pyramidal | AB₃E | 3 bond, 1 lone | less than 109.5° | sp³ | NH₃, NF₃, PCl₃, PH₃, H₃O⁺, XeO₃, SOCl₂ |
| bent | AB₂E₂ | 2 bond, 2 lone | less than 109.5° | sp³ | H₂O, H₂S, OF₂, Cl₂O |
| trigonal bipyramidal | AB₅ | 5 bond, 0 lone | 120° and 90° | sp³d | PCl₅ |
| see-saw | AB₄E | 4 bond, 1 lone | less than 120° | sp³d | SF₄ |
| T-shaped | AB₃E₂ | 3 bond, 2 lone | less than 90° | sp³d | ClF₃ |
| linear | AB₂E₃ | 2 bond, 3 lone | 180° | sp³d | XeF₂, I₃⁻, ICl₂⁻ |
| octahedral | AB₆ | 6 bond, 0 lone | 90° | sp³d² | SF₆ |
| square pyramidal | AB₅E | 5 bond, 1 lone | less than 90° | sp³d² | BrF₅, XeOF₄ |
| square planar | AB₄E₂ | 4 bond, 2 lone | 90° | sp³d² | XeF₄, ICl₄⁻ |
| pentagonal bipyramidal | AB₇ | 7 bond, 0 lone | 72° and 90° | sp³d³ | IF₇ |
| distorted octahedral | AB₆E | 6 bond, 1 lone | distorted | sp³d³ | XeF₆ |
All molecules by shape: shapes of molecules. Or type any molecule.
VSEPR theory: quick answers
What is VSEPR theory?
Valence Shell Electron Pair Repulsion theory predicts the shape of a molecule from the electron pairs around its central atom. The pairs, bonded or lone, repel each other and settle as far apart as possible, so their number fixes the arrangement and the lone pairs decide the final shape.
What is the order of repulsion in VSEPR theory?
Lone pair to lone pair repulsion is the strongest, then lone pair to bond pair, then bond pair to bond pair. A lone pair is held by only one nucleus, so it spreads out and takes more room. That is why water (104.5 degrees) and ammonia (107 degrees) have smaller angles than methane (109.5 degrees).
Why does the lone pair sit in an equatorial position in SF4 and ClF3?
In a trigonal bipyramid an equatorial position is 90 degrees from only two other pairs, while an axial position is 90 degrees from three. Lone pairs take the equatorial positions, where they have fewer close neighbours and the repulsion is least. That gives SF4 its see-saw shape and ClF3 its T shape.