Chemistry · General chemistry I · Concept
VSEPR theory and molecular shape
VSEPR (valence-shell electron-pair repulsion) theory predicts a molecule’s shape from its Lewis structure. Count the electron domains around the central atom (a bond of any order counts once, and so does each lone pair), spread them as far apart as possible, then name the shape from the positions of the atoms alone.
Electron domains
A single, double or triple bond counts as one domain, and so does each lone pair on the central atom. The domains repel one another and spread out as far as possible.
Electron geometry
The number of domains fixes their arrangement.
| Domains | Electron geometry | Ideal angle |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 90° and 120° |
| 6 | Octahedral | 90° |
Molecular geometry
Lone pairs take up space but are left out of the shape’s name, which describes the atoms only. With four domains, no lone pairs gives a tetrahedral molecule (CH₄), one gives a trigonal pyramid (NH₃) and two give a bent molecule (H₂O).
| Domains | Lone pairs | Molecular geometry | Example |
|---|---|---|---|
| 2 | 0 | Linear | CO₂ |
| 3 | 0 | Trigonal planar | BF₃ |
| 3 | 1 | Bent | SO₂ |
| 4 | 0 | Tetrahedral | CH₄ |
| 4 | 1 | Trigonal pyramidal | NH₃ |
| 4 | 2 | Bent | H₂O |
Lone pairs squeeze bond angles
A lone pair spreads out more than a bonding pair, so it pushes the bonds closer together. The bond angles in NH₃ and H₂O are therefore smaller than the ideal 109.5°.
Shape decides polarity
A molecule is polar when its bond dipoles do not cancel. CO₂ has two polar carbon–oxygen bonds pointing in opposite directions, so it is nonpolar. H₂O is bent, so its two bond dipoles add and the molecule is polar.
Common mistakes
- Counting a double or triple bond as two or three domains: each multiple bond is one domain.
- Naming the shape from all the domains: the molecular geometry ignores lone pairs, so NH₃ is trigonal pyramidal, not tetrahedral.
- Forgetting the central atom’s lone pairs, which come from the Lewis structure.
- Deciding polarity from the bonds alone: symmetric molecules such as CO₂ and CCl₄ are nonpolar despite polar bonds.
Key terms
- VSEPR
- Valence shell electron pair repulsion: electron groups around a central atom spread as far apart as possible, which predicts the molecule’s shape. The shape’s name describes where the atoms are, not the lone pairs.
- Electron domain
- A region of electrons around a central atom in VSEPR: a lone pair or a bond. A double or triple bond counts as one domain, just like a single bond.
- Molecular geometry
- The 3D arrangement of the atoms in a molecule, such as bent or trigonal pyramidal. Lone pairs shape it but aren’t part of the name, and they can squeeze bond angles.
- Tetrahedral
- A shape with four groups around a central atom, pointing to the corners of a tetrahedron about 109.5° apart, as in CH₄.
- Trigonal pyramidal
- A shape with three atoms around a central atom and a lone pair on top, like a short pyramid, as in NH₃ (about 107°).
- Molecular polarity
- An uneven share of electrons that gives a bond or molecule a slightly positive end and a slightly negative end. A molecule with polar bonds can still be nonpolar if its shape cancels them, as in CO₂.
Work through an example
Predict the electron geometry, molecular geometry and polarity of ammonia, NH₃.
Predict the shape of ammonia with VSEPR →Sources and scope
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