VSEPR theory helps predict the three-dimensional shape of a molecule from its Lewis structure. The name means Valence Shell Electron Pair Repulsion, which reflects the idea that electron domains around a central atom spread out to reduce repulsion. Molecular shape matters because it affects polarity, reactivity, boiling point, solubility, and how molecules interact in living systems.
It gives chemists a practical bridge from a flat drawing to a real 3D structure.
An electron domain is any region of electron density around the central atom, including a single bond, double bond, triple bond, or lone pair. VSEPR first finds the electron geometry from the total number of electron domains, then finds the molecular geometry by considering only the positions of atoms. Lone pairs usually repel more strongly than bonding pairs, so they can compress bond angles and change the observed shape.
Common patterns such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral form a shape map for many molecules.
Key Facts
- VSEPR principle: electron domains around a central atom arrange themselves as far apart as possible.
- Steric number = number of bonded atoms on the central atom + number of lone pairs on the central atom.
- 2 electron domains: linear electron geometry, ideal bond angle = 180 degrees.
- 3 electron domains: trigonal planar electron geometry, ideal bond angle = 120 degrees.
- 4 electron domains: tetrahedral electron geometry, ideal bond angle = 109.5 degrees.
- Repulsion strength is usually lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair.
Vocabulary
- VSEPR theory
- A model that predicts molecular shape by arranging valence electron domains around a central atom to minimize repulsion.
- Electron domain
- A region of electron density around a central atom, counted as one domain whether it is a lone pair, single bond, double bond, or triple bond.
- Electron geometry
- The arrangement of all electron domains around the central atom, including both bonds and lone pairs.
- Molecular geometry
- The arrangement of only the atoms in a molecule, ignoring lone pairs when naming the visible shape.
- Lone pair
- A pair of valence electrons on an atom that is not shared in a chemical bond.
Common Mistakes to Avoid
- Counting a double bond as two electron domains: VSEPR counts each multiple bond as one region of electron density, so a double or triple bond counts as one domain.
- Confusing electron geometry with molecular geometry: electron geometry includes lone pairs, while molecular geometry describes only where the atoms are.
- Ignoring lone pairs on the central atom: lone pairs occupy space and repel bonding pairs, so they can change the molecular shape and reduce bond angles.
- Using ideal angles without checking lone pairs: shapes with lone pairs often have smaller bond angles than the ideal electron-geometry angle because lone pairs repel more strongly.
Practice Questions
- 1 CH4 has a central carbon atom with four bonded hydrogen atoms and no lone pairs. What are the steric number, electron geometry, molecular geometry, and approximate H-C-H bond angle?
- 2 NH3 has three N-H bonds and one lone pair on the central nitrogen atom. What are the steric number, electron geometry, molecular geometry, and approximate bond angle compared with 109.5 degrees?
- 3 CO2 and H2O both have three atoms, but CO2 is linear while H2O is bent. Use VSEPR theory to explain why their shapes are different.