Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the three-dimensional shape of a molecule based on the principle that electron pairs - both bonding pairs and lone pairs - repel each other and arrange themselves as far apart as possible. The electron geometry describes the arrangement of all electron pairs around the central atom; the molecular geometry describes only the positions of the atoms (ignoring lone pairs).
Lone pairs exert more repulsion than bonding pairs, compressing bond angles below ideal values. Water (2 bonding pairs, 2 lone pairs) has tetrahedral electron geometry but bent molecular geometry with a 104.5° angle, less than the 109.5° ideal. Ammonia (3 bonding pairs, 1 lone pair) is trigonal pyramidal with 107° angles.
Molecular geometry directly determines polarity: a symmetric molecule (like CO₂, linear) is nonpolar even if individual bonds are polar, because the dipoles cancel.
Understanding Molecular Geometry and VSEPR
The most reliable way to predict a shape is to begin with a correct Lewis structure. Count the valence electrons from every atom, adjust for any overall charge, then place bonds and remaining electrons. The central atom is usually the least electronegative atom, except hydrogen, which is always on the outside.
After completing outer atoms, put leftover electrons on the central atom. Each bond to the central atom counts as one electron region, even when it is a double bond or triple bond.
A carbon dioxide molecule therefore has two regions around carbon, not four. Multiple bonds can push a little harder than single bonds, so real angles may differ slightly from simple predicted values.
Five and six electron regions add useful patterns beyond the common shapes. Five regions form a trigonal bipyramidal arrangement. Three positions lie around the middle in one flat plane.
Two positions lie above and below that plane. These sites are not equally crowded. Lone pairs prefer an outer position in the flat plane because it gives them fewer close interactions.
This helps explain the shapes of molecules containing sulfur or phosphorus. With six regions, positions point toward the corners of an octahedron.
If lone pairs replace atoms, the visible shape can become square pyramidal or square planar. Naming the molecular shape means looking only at the atoms that remain attached.
Polarity needs more than a list of polar bonds. Each polar bond has a pull toward the more electronegative atom. Think of these pulls as directions in three-dimensional space.
In a balanced shape, equal pulls may cancel. In an unbalanced shape, they leave a net pull across the molecule. Carbon dioxide is balanced because its two bond pulls point in opposite directions.
Water is unbalanced because its bent shape puts both oxygen hydrogen bond pulls partly in the same general direction. This overall polarity affects boiling point, solubility, and how substances interact. Water mixes well with many ionic or polar substances, while nonpolar oils tend to separate from it.
VSEPR is a strong starting model, not a perfect measurement tool. Bond angles in real molecules change when surrounding atoms have different sizes or electronegativities. A bond to a very electronegative atom can draw bonding electrons away from the central atom, reducing its repulsion near the center.
Atoms from the third period and below can form structures that do not fit the simplest octet pattern. Transition metal compounds often need other bonding models entirely. When solving school problems, first draw the Lewis structure, count regions around the central atom, identify the electron arrangement, then remove lone pairs from the name of the visible shape.
Finally, inspect symmetry before deciding whether the whole molecule is polar. Keeping these steps separate prevents many common mistakes.
Key Facts
- VSEPR: electron pairs arrange to minimize repulsion; lone pairs repel more than bonding pairs
- 2 groups (0 lone pairs): linear, 180°
- 3 groups: trigonal planar (0 LP) 120°; bent (1 LP) ~117°
- 4 groups: tetrahedral (0 LP) 109.5°; trigonal pyramidal (1 LP) ~107°; bent (2 LP) ~104.5°
- 5 groups: trigonal bipyramidal; 6 groups: octahedral
- Polarity requires both polar bonds AND asymmetric geometry
Vocabulary
- VSEPR
- Valence Shell Electron Pair Repulsion theory; predicts molecular geometry by minimizing repulsion among electron pairs around the central atom.
- Electron geometry
- The three-dimensional arrangement of all electron pairs (bonding and lone) around the central atom.
- Molecular geometry
- The three-dimensional arrangement of atoms only (not lone pairs) around the central atom.
- Lone pair
- A pair of valence electrons on an atom that is not involved in a covalent bond; exerts greater repulsion than a bonding pair.
- Dipole moment
- A measure of the polarity of a bond or molecule; depends on charge separation and direction. Nonzero net dipole = polar molecule.
Common Mistakes to Avoid
- Confusing electron geometry with molecular geometry. If a central atom has 4 electron domains but 2 are lone pairs, the electron geometry is tetrahedral but the molecular geometry is bent.
- Forgetting to count lone pairs when applying VSEPR. Lone pairs are just as important as bonding pairs in determining the shape - the shape of water is not linear.
- Assuming a molecule with polar bonds is always polar. If the molecule is symmetric (e.g. BF₃ is trigonal planar, CO₂ is linear), the bond dipoles cancel and the molecule is nonpolar.
- Using bond order instead of electron domains for VSEPR. A double or triple bond counts as ONE electron domain (one bonding region), not two or three.
Practice Questions
- 1 Draw the Lewis structure of NH₃. Predict its electron geometry, molecular geometry, and approximate bond angles.
- 2 CO₂ has two polar C=O bonds. Explain why CO₂ is a nonpolar molecule using molecular geometry.
- 3 Predict the molecular geometry of SF₄ (sulfur with 4 F and 1 lone pair). What are the approximate F–S–F bond angles?