VSEPR Geometry Predictor – From Formula to 3D Molecular Shape
VSEPR theory (valence shell electron pair repulsion) predicts the shape of a molecule from one idea: the electron domains around a central atom push each other as far apart as they can. A domain is either a bond to another atom or a lone pair. Count the domains, arrange them for the least repulsion, and the positions of the atoms give the molecular geometry. This VSEPR geometry predictor does the counting for you, names the shape, lists the ideal bond angles and draws a rotatable 3D ball-and-stick model.
AXE notation and the steric number
Chemists summarise a VSEPR shape as AXₙEₘ: A is the central atom, X stands for each bonded atom and E for each lone pair. The steric number is SN = X + E. To find E from a formula, add up the valence electrons, adjust for the charge, give every outer atom a full octet (H needs only two) and put what is left on the central atom. For SF₄: 6 + 4 × 7 = 34, then 34 − 4 × 8 = 2, which is one lone pair, so SF₄ is AX₄E.
Electron-domain geometry vs molecular shape
The electron-domain geometry includes the lone pairs; the molecular shape describes only the atoms. Ammonia has four domains in a tetrahedron, but one is a lone pair, so the atoms form a trigonal pyramid. Water has two lone pairs and is bent. Turn on the domain-geometry toggle under the 3D model to see the dashed tetrahedron that both molecules sit inside.
Lone pairs, repulsion and angle compression
Lone pairs are held by only one nucleus, so they spread out and repel more strongly than bonds. The order is lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. Each lone pair squeezes the bonds closer together, which is why the H–X–H angle falls from 109.5° in CH₄ to about 106.7° in NH₃ and 104.5° in H₂O.
With five or more domains the sites are no longer equivalent. In a trigonal bipyramid an equatorial position has two neighbours at 90° while an axial position has three, so lone pairs go equatorial. That gives the seesaw (SF₄), T-shaped (ClF₃) and linear (XeF₂) shapes. In an octahedron two lone pairs sit opposite each other, which makes XeF₄ square planar.
Multiple bonds count as one domain
A double or triple bond points in one direction, so it counts as one electron domain, just like a single bond. CO₂ is linear and formaldehyde is trigonal planar. A multiple bond does take more room, though: in H₂C=O the H–C–H angle is about 116.5°, not 120°. The "Atom in a larger molecule" mode handles one centre of a bigger molecule, such as each carbon in ethene or the oxygen in methanol, from its bond counts and formal charge.
Hypervalent molecules and the hybridization label
PCl₅, SF₆ and IF₇ have more than four domains around the central atom. Textbooks label them sp³d, sp³d² and sp³d³, and the tool shows those labels because most courses use them. Modern bonding analyses find little d-orbital involvement, though, and describe these molecules with three-centre four-electron bonds instead. The geometry prediction does not depend on that choice.
Where VSEPR fails
- Transition metals: their shapes depend on d-electron configuration and ligand-field effects, so the tool rejects d- and f-block centres.
- Heavy hydrides: PH₃ and H₂S have angles near 90°, far below the tetrahedral ideal. Their bonds use almost pure p orbitals (Drago's rule).
- Radicals: a single electron repels much less than a pair. NO₂ opens to about 134°, and the methyl radical is planar rather than pyramidal.
- XeF₆: VSEPR predicts AX₆E, but the real molecule is fluxional and only a distorted octahedron.
- Heavy alkaline-earth dihalides: gas-phase SrF₂ and BaF₂ are bent, although VSEPR predicts linear AX₂ molecules.
The formula mode also assumes one central atom bonded to all the others, with H on oxygen in oxyacids. It always prints the connectivity it assumed, and you can override the central atom when the guess is wrong.