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.
Understanding Chemistry: VSEPR Theory
A reliable VSEPR prediction starts before any shape is named. Draw a careful Lewis structure, count the valence electrons, and check that the central atom has the expected number of electrons around it. Hydrogen is never the central atom.
Halogens usually sit at the outside. Carbon, nitrogen, oxygen, phosphorus, sulfur, and many less electronegative atoms often form the center.
If more than one Lewis structure is possible, use formal charges to choose the most reasonable one. A poor Lewis structure gives a poor shape prediction, even if the later counting steps are correct.
After the structure is drawn, treat every atom attached to the center as one occupied direction. A double or triple bond still uses just one direction because its electrons lie between the same two atoms. However, multiple bonds can push a little harder than single bonds.
This can make nearby angles slightly smaller. For centers with five occupied directions, the positions are not all equal. Three positions lie around the middle in a flat triangle, while two point above and below that plane.
Lone pairs prefer the middle positions because those positions have fewer close interactions. This detail explains why some five direction molecules have different shapes even when they contain the same total number of electron domains.
The shape name describes where the atoms are, not every electron pair. For example, a central atom with four occupied directions can appear tetrahedral when all four directions contain bonds. With one lone pair, it becomes trigonal pyramidal.
With two lone pairs, it becomes bent. Ammonia is trigonal pyramidal, and its bond angles are smaller than the ideal tetrahedral angle because the lone pair crowds the bonding pairs. Water is bent for the same reason, with two lone pairs causing an even greater compression.
Carbon dioxide is linear because its central carbon has two occupied directions and no lone pairs. These differences matter for polarity.
In carbon dioxide, the bond effects cancel along a straight line. In water, the bent arrangement leaves an uneven charge distribution, helping water dissolve many ionic and polar substances.
VSEPR is a useful model, not a rule that gives exact angles for every substance. Real bonds are affected by atom size, electronegativity, resonance, and nearby groups. Some molecules with transition metals need other bonding models because their electron arrangements are more complex.
Students often make two mistakes. One is counting each line in a multiple bond as a separate domain. The other is forgetting that lone pairs belong to the central atom only when determining its shape.
Practice by writing the Lewis structure first, identifying the central atom, counting occupied directions, placing lone pairs, then naming the arrangement of atoms. Building models or using ball and stick diagrams helps because a flat page can hide the angles that control a molecule's behavior.
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.