Molecular Geometry Shapes — VSEPR Theory Reference
| Electron Groups | Bond Pairs | Lone Pairs | Electron Geometry | Molecular Shape | Bond Angle | Example | Hybridization |
|---|---|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | Linear | 180° | CO₂, BeCl₂ | sp |
| 3 | 3 | 0 | Trigonal planar | Trigonal planar | 120° | BF₃, SO₃ | sp² |
| 3 | 2 | 1 | Trigonal planar | Bent (V-shaped) | <120° (~117°) | SO₂, O₃ | sp² |
| 4 | 4 | 0 | Tetrahedral | Tetrahedral | 109.5° | CH₄, SiCl₄ | sp³ |
| 4 | 3 | 1 | Tetrahedral | Trigonal pyramidal | <109.5° (~107°) | NH₃, PCl₃ | sp³ |
| 4 | 2 | 2 | Tetrahedral | Bent (V-shaped) | <109.5° (~104.5°) | H₂O, H₂S | sp³ |
| 5 | 5 | 0 | Trigonal bipyramidal | Trigonal bipyramidal | 90°, 120° | PCl₅, AsF₅ | sp³d |
| 5 | 4 | 1 | Trigonal bipyramidal | Seesaw (sawhorse) | ~90°, ~120° | SF₄, TeCl₄ | sp³d |
| 5 | 3 | 2 | Trigonal bipyramidal | T-shaped | ~90° | ClF₃, BrF₃ | sp³d |
| 5 | 2 | 3 | Trigonal bipyramidal | Linear | 180° | XeF₂, I₃⁻ | sp³d |
| 6 | 6 | 0 | Octahedral | Octahedral | 90° | SF₆, [Fe(CN)₆]³⁻ | sp³d² |
| 6 | 5 | 1 | Octahedral | Square pyramidal | ~90° | BrF₅, IF₅ | sp³d² |
| 6 | 4 | 2 | Octahedral | Square planar | 90° | XeF₄, [PtCl₄]²⁻ | sp³d² |
An 'electron domain' or 'electron group' is any region of electron density: a single bond, a double bond, a triple bond, or a lone pair — multiple bonds count as one domain. Lone pairs spread more than bonding pairs because they're held by only one nucleus, so they compress adjacent bond angles (NH₃ 107° vs. ideal 109.5°, H₂O 104.5°). In trigonal bipyramidal arrangements, lone pairs always occupy equatorial positions to avoid two 90° interactions. VSEPR fails for transition-metal complexes with partly filled d-shells (e.g., square planar d⁸ Pt(II), Pd(II)) — for those, use crystal-field or ligand-field arguments. Source: Atkins' Inorganic Chemistry, Gillespie & Hargittai.
Frequently Asked Questions
What is the difference between electron geometry and molecular shape?
Electron geometry is the arrangement of every electron domain around the central atom — bonding pairs and lone pairs alike. Molecular shape is the arrangement of nuclei only; lone pairs are invisible in the geometry name. Water has four electron domains (2 bonds + 2 lone pairs), so the electron geometry is tetrahedral, but with only three nuclei (O, H, H) the molecular shape is bent. The two coincide whenever the central atom has no lone pairs (CH₄, BF₃, PCl₅, SF₆ all have matching electron and molecular geometries).
Why do lone pairs reduce bond angles?
A bonding pair is shared between two nuclei and therefore localized along the bond axis; a lone pair is held by only the central atom and balloons outward in angular space. The lone pair pushes adjacent bonding pairs closer together, narrowing the bond angle below the ideal. The compression scales with lone-pair count: tetrahedral ideal 109.5°, one lone pair (NH₃) 107°, two lone pairs (H₂O) 104.5°. Lone-pair–lone-pair repulsion is stronger than lone-pair–bond, which beats bond–bond.
How do you count electron groups for VSEPR?
Count one domain per region of electron density on the central atom: each single bond, each double bond, each triple bond, and each lone pair counts once. Multiple bonds count the same as single bonds because VSEPR cares about spatial regions, not electron count within a region. CO₂ (O=C=O) has 2 domains → linear (180°). Formaldehyde H₂C=O has 3 → trigonal planar (120°). XeF₂ has 5 domains (2 bonds + 3 equatorial lone pairs) → linear because the lone pairs occupy the equatorial plane.