AlCl₃ ↗
Three Al–Cl bonds form a flat, symmetric 120° arrangement. The equal bond dipoles cancel in the isolated monomer.
Trigonal planar · nonpolarFIELD GUIDE / MOLECULAR SHAPE
VSEPR theory, made visible.
VSEPR predicts a molecule’s shape by placing electron regions around a central atom as far apart as possible. Count the bonds and lone pairs, then name the arrangement of the atoms. That shape helps you decide whether bond dipoles cancel.
01 / METHOD
Predict a shape in three steps
- 01
Draw the Lewis structure.
Identify the central atom and every atom bonded to it. A single, double or triple bond each counts as one electron region.
- 02
Count lone pairs on the central atom.
Add these to the bonding regions. Two through six total regions give the basic electron geometries in the chart below.
- 03
Name the molecular shape.
Ignore lone pairs when naming the visible atom arrangement, but keep their effect on bond angles in mind. Then inspect the bond dipoles to decide polarity.
02 / SHAPE FINDER
Count the regions.
Choose the total electron regions around one central atom and how many of those regions are lone pairs.
3 REGIONS / 0 LONE PAIRSTrigonal planar
- Electron geometry
- Trigonal planar
- AXE notation
- AX₃
- Ideal bond angle
- 120°
- Example
- AlCl₃ ↗
Angles are idealized. Real molecules can differ because bonds and lone pairs do not repel equally.
03 / REFERENCE CHART
VSEPR shapes and bond angles
In AXE notation, A is the central atom, X is the number of bonded atoms and E is the number of lone pairs on A.
| Regions | Lone pairs | AXE | Electron geometry | Molecular shape | Typical angle | See it |
|---|---|---|---|---|---|---|
| 2 | 0 | AX₂ | Linear | Linear | 180° | CO₂ ↗ |
| 3 | 0 | AX₃ | Trigonal planar | Trigonal planar | 120° | AlCl₃ ↗ |
| 3 | 1 | AX₂E | Trigonal planar | Bent | Less than 120° | SO₂ ↗ |
| 4 | 0 | AX₄ | Tetrahedral | Tetrahedral | 109.5° | CH₄ ↗ |
| 4 | 1 | AX₃E | Tetrahedral | Trigonal pyramidal | Less than 109.5° | NH₃ ↗ |
| 4 | 2 | AX₂E₂ | Tetrahedral | Bent | Less than 109.5° | H₂O ↗ |
| 5 | 0 | AX₅ | Trigonal bipyramidal | Trigonal bipyramidal | 90°, 120°, 180° | PF₅ ↗ |
| 5 | 1 | AX₄E | Trigonal bipyramidal | Seesaw | Distorted 90° and 120° | — |
| 5 | 2 | AX₃E₂ | Trigonal bipyramidal | T-shaped | Near 90° and 180° | ClF₃ ↗ |
| 5 | 3 | AX₂E₃ | Trigonal bipyramidal | Linear | 180° | — |
| 6 | 0 | AX₆ | Octahedral | Octahedral | 90° and 180° | SF₆ ↗ |
| 6 | 1 | AX₅E | Octahedral | Square pyramidal | Near 90° and 180° | — |
| 6 | 2 | AX₄E₂ | Octahedral | Square planar | 90° and 180° | XeF₄ ↗ |
“Typical angle” describes the ideal arrangement or a useful range; it is not a measured value for every molecule.
04 / APPLY IT
Same method. Different answer.
NH₃ ↗
Four electron regions give a tetrahedral electron arrangement. The lone pair leaves three N–H bonds in a pyramid, so their dipoles do not cancel.
Trigonal pyramidal · polarVSEPR predicts shape; polarity also depends on which atoms are bonded and the direction and size of their bond dipoles. A shape name alone is not a polarity verdict.
Further reading: OpenStax Chemistry 2e, §7.6 ↗