Theory — Valence Shell Electron Pair Repulsion
The VSEPR model, short for valence shell electron pair repulsion, says that the groups of valence electrons around a central atom arrange themselves to be as far apart as possible, because like charges repel. Each group, whether it is a bond or a lone pair, is called an electron domain. A single, double, or triple bond all count as one domain.
From Lewis structure to domains
Start by drawing the Lewis structure. Add up the valence electrons from every atom, connect the atoms with bonds, and place the remaining electrons as lone pairs so that each atom reaches a full shell. Then count, on the central atom, the number of bonding groups and the number of lone pairs. Their sum is the steric number, the total number of electron domains.
Electron-domain geometry and molecular shape
The steric number fixes the electron-domain geometry, the arrangement of all the domains. The molecular shape is the arrangement of just the atoms, found by ignoring the lone pairs but keeping the positions they force. Lone pairs take up more room than bonds, so they squeeze the bond angles slightly smaller than the ideal value.
| Steric number | Electron-domain geometry | Ideal angle | Molecular shapes (by lone pairs) |
|---|---|---|---|
| 2 | Linear | 180° | linear |
| 3 | Trigonal planar | 120° | trigonal planar (0), bent (1) |
| 4 | Tetrahedral | 109.5° | tetrahedral (0), trigonal pyramidal (1), bent (2) |
| 5 | Trigonal bipyramidal | 90° and 120° | trigonal bipyramidal (0), seesaw (1) |
| 6 | Octahedral | 90° | octahedral (0), square planar (2) |
Polarity
A bond between two different atoms is polar. A whole molecule is polar only if those bond dipoles do not cancel. When the shape is symmetric and all the outer atoms are the same, the dipoles cancel and the molecule is nonpolar, as in carbon dioxide or methane. When a lone pair makes the shape lopsided, as in water or ammonia, the dipoles do not cancel and the molecule is polar.
Count the domains
Bonding groups plus lone pairs on the central atom give the steric number, which sets the geometry.
Atoms, not pairs
The molecular shape describes where the atoms sit; the lone pairs are invisible but still push the atoms into place.
Symmetry sets polarity
If the bond dipoles cancel by symmetry the molecule is nonpolar; a lone pair that breaks the symmetry makes it polar.
Apparatus
The equipment a real molecular-shape experiment uses. In the simulation these are modelled for you, but the readings correspond to what each instrument would measure.
Instructions — Running the Virtual Experiment
This is a predict, reveal, and compare lab. For each molecule you predict the answer yourself, enter it, and only then does the simulation reveal the result so you can compare. Work through at least six of the molecules and record every prediction in your worksheet.
Simulation — The VSEPR Bench
| Molecule | Your VE | VE | Your groups | Bonding groups | Your LP | LP |
|---|---|---|---|---|---|---|
| No rows yet — choose a molecule, predict the counts, and check. | ||||||
Molecule
Molecule
Molecule
Team Questions
Example Lab Report
A worked example showing the expected format and the predict, reveal, and compare workflow.
Molecular Shape
Chemistry | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Objective
To draw Lewis structures, predict the electron-domain geometry and molecular shape of a set of molecules using VSEPR, and decide the polarity of each, comparing every prediction with the simulation.
Results Table (worked example)
| Molecule | Bonds / LP | Steric no. | Electron geometry | Molecular shape | Polar? |
|---|---|---|---|---|---|
| CO₂ | 2 / 0 | 2 | Linear | Linear | No |
| BF₃ | 3 / 0 | 3 | Trigonal planar | Trigonal planar | No |
| H₂O | 2 / 2 | 4 | Tetrahedral | Bent | Yes |
| NH₃ | 3 / 1 | 4 | Tetrahedral | Trigonal pyramidal | Yes |
| CH₄ | 4 / 0 | 4 | Tetrahedral | Tetrahedral | No |
| SF₆ | 6 / 0 | 6 | Octahedral | Octahedral | No |
Worked example for water: O contributes 6 valence electrons and each H contributes 1, giving 8 in total. The central oxygen has 2 bonding groups and 2 lone pairs, so the steric number is 4. Four domains give a tetrahedral electron geometry with an ideal angle of 109.5°, but with two lone pairs the molecular shape is bent and the angle is squeezed to about 104.5°. Because the two O–H dipoles do not cancel, water is polar.
Discussion and Conclusion
Every prediction agreed with the simulation. The steric number set the electron-domain geometry, and removing the lone pairs gave the molecular shape: the same tetrahedral arrangement produced tetrahedral methane, trigonal pyramidal ammonia, and bent water as lone pairs were added. Symmetric molecules with identical outer atoms were nonpolar, while lone pairs that broke the symmetry made the molecule polar.