Placing electron domains…
VSEPR — valence shell electron pair repulsion — says one thing: electron domains around a central atom get as far from each other as they can. Count the domains, and the geometry follows. The hard part on paper is seeing it, because a tetrahedron drawn flat looks like a cross. Here every shape is a real 3D model you can spin, with the lone pairs drawn as the violet lobes that a Lewis structure only shows as dots.
How to predict a molecular shape in three steps
- Draw the Lewis structure and find the central atom.
- Count electron domains on that atom. Single, double, and triple bonds each count once; each lone pair counts once. The total is the steric number (SN).
- Read the shape. The steric number fixes the electron geometry. Then hide the lone pairs: what is left is the molecular shape. The AXₙEₘ label says the same thing — A is the central atom, X a bonded atom, E a lone pair.
Why lone pairs squeeze bond angles
A lone pair is held by one nucleus instead of two, so it spreads out closer to the central atom and pushes harder. The order is lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. Switch on “Lone-pair squeeze” above and the model relaxes the domains under exactly that ranking. Methane stays at 109.5°. Ammonia closes to about 107°, and water to about 104.5° — the same values measured by spectroscopy.
In a trigonal bipyramid, the two kinds of site are not equal. Equatorial positions have two neighbors at 90°; axial ones have three. Lone pairs therefore go equatorial, which is why SF₄ is a seesaw, ClF₃ is T-shaped, and XeF₂ ends up linear. In an octahedron, a second lone pair goes opposite the first, so XeF₄ is square planar.
VSEPR chart: all 13 shapes
| AXE | Electron geometry | Molecular shape | Bond angle | Hybridization | Example |
|---|---|---|---|---|---|
| AX₂ | Linear | Linear | 180° | sp | CO₂ (carbon dioxide) |
| AX₃ | Trigonal planar | Trigonal planar | 120° | sp² | BF₃ (boron trifluoride) |
| AX₂E | Trigonal planar | Bent | 119° | sp² | SO₂ (sulfur dioxide) |
| AX₄ | Tetrahedral | Tetrahedral | 109.5° | sp³ | CH₄ (methane) |
| AX₃E | Tetrahedral | Trigonal pyramidal | 107° | sp³ | NH₃ (ammonia) |
| AX₂E₂ | Tetrahedral | Bent | 104.5° | sp³ | H₂O (water) |
| AX₅ | Trigonal bipyramidal | Trigonal bipyramidal | 90°, 120° | sp³d | PCl₅ (phosphorus pentachloride) |
| AX₄E | Trigonal bipyramidal | Seesaw | 102°, 173° | sp³d | SF₄ (sulfur tetrafluoride) |
| AX₃E₂ | Trigonal bipyramidal | T-shaped | 87.5° | sp³d | ClF₃ (chlorine trifluoride) |
| AX₂E₃ | Trigonal bipyramidal | Linear | 180° | sp³d | XeF₂ (xenon difluoride) |
| AX₆ | Octahedral | Octahedral | 90° | sp³d² | SF₆ (sulfur hexafluoride) |
| AX₅E | Octahedral | Square pyramidal | ≈85° | sp³d² | BrF₅ (bromine pentafluoride) |
| AX₄E₂ | Octahedral | Square planar | 90° | sp³d² | XeF₄ (xenon tetrafluoride) |
Where VSEPR stops working
VSEPR is a counting rule, not a calculation. It misses transition-metal complexes, where d-electrons and ligand-field effects set the geometry, and heavy hydrides such as H₂S (92°) and PH₃ (94°), whose angles sit near 90° because the bonds use almost pure p orbitals. The hybridization column is the usual classroom label; for SN 5 and 6, modern bonding models do not actually need d orbitals. For real measured structures of everyday molecules, open the molecule sandbox. For the orbitals the lobes come from, see hydrogen orbital shapes.
VSEPR questions
What is the difference between electron geometry and molecular shape?
Electron (domain) geometry counts every region of electron density around the central atom — bonds and lone pairs. Molecular shape only describes where the atoms are. Water has four domains, so its electron geometry is tetrahedral. Two of those domains are lone pairs, so the molecule itself is bent.
Why is the bond angle in water 104.5° and not 109.5°?
Lone pairs sit closer to the nucleus and spread wider than bonding pairs, so they push harder. In VSEPR terms, lone pair–lone pair repulsion beats lone pair–bond pair, which beats bond pair–bond pair. Two lone pairs squeeze the H–O–H angle from the ideal 109.5° down to 104.5°. Ammonia, with one lone pair, lands in between at 107°.
How do I find the VSEPR shape of a molecule?
Draw the Lewis structure. Count the electron domains on the central atom: each single, double, or triple bond counts as one domain, and each lone pair counts as one. That steric number picks the electron geometry (2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral). Then remove the lone pairs to read off the molecular shape.
Why do lone pairs go in the equatorial positions of a trigonal bipyramid?
An axial position has three neighbors at 90°; an equatorial position has only two. Lone pairs repel most strongly at 90°, so they take equatorial sites first. That is why SF₄ is a seesaw, ClF₃ is T-shaped, and XeF₂ is linear.
Does a double bond count as one electron domain?
Yes. A double or triple bond points in one direction, so VSEPR counts it as a single domain. It is a slightly larger domain, however, which is why the angles around a C=O carbon are often a degree or two off the ideal 120°.