VSEPR Theory
- The valence shell electron pair repulsion (VSEPR) theory is based on the principle that electron groups such as single bonds, double bonds, triple bonds, or single electrons repel one another through Coulombic forces
- The minimize this repulsion, the valence shell electrons should be placed as far apart in the space as possible
- VSEPR theory consist of three rules:
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- Bonding pairs and lone pairs should be arranged as far apart in space as possible
- Double and triple bonds are one electron group
- The Coulombic repulsion forces from lone pairs are stronger than the bonding pairs
- The VSEPR theory and the number of electron groups in an atoms is a useful tool that can be used to predict the structural and electron properties of the molecules
Two electron groups
- If there are two electron groups around the central atom, the angle between the bond is 180° which maximize their separation
- The geometry of this molecules is LINEAR
- g. BeCl2, CO2, and ethyne (HC≡CH)
Examples of Linear Molecules
Beryllium chloride, carbon dioxide and ethyne all have two electron groups
Three electron groups
- If there are three electron groups around the central atom, the angle between the bonds is 120° which maximize their separation
- The geometry of this molecule is TRIGONAL PLANAR
- E.g. BF3, CH2CH2 and CH2O
Examples of Trigonal Planar Molecules
Boron trifluoride, ethene and methanal all have three electron groups
Four electron groups
- Molecules with four electron groups have three dimensional geometries
- If there are four electron groups around the central atom, there are three case scenarios:
- If the four electron groups are bonding groups, the angle between the bonds is 109.5° which maximize their separation
- The geometry of this molecules is TETRAHEDRAL
- E.g. CH4, NH4+
- If the four electron groups are bonding groups, the angle between the bonds is 109.5° which maximize their separation
Examples of Tetrahedral Molecules
Methane and ammonium ions have four electron domains
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- If three electron groups are bonding groups and one is a lone pair, the angle is approximately 107° which is slightly less than 109.5° due to the Coulombic force of repulsion generated by the lone pair
- The geometry of this molecules is TRIGONAL PYRAMIDAL
- E.g. NH3
- If three electron groups are bonding groups and one is a lone pair, the angle is approximately 107° which is slightly less than 109.5° due to the Coulombic force of repulsion generated by the lone pair
NH3 has a Trigonal Pyramidal Geometry
The molecular geometry of ammonia
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- If two electron groups are bonding groups and two are lone pairs, the angle is approximately 104.5° which is less than 109.5° due to the Coulombic force of repulsion generated by the two lone pairs
Bond Angles in Water
The order of electron pair repulsion is lone pairs > lone pair: bonding pair > bonding pairs
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-
- The geometry of this molecules is BENT or ANGULAR
- E.g. H2O
-
Water Has a Bent Geometry
The molecular geometry of water
Five electron groups
- Molecules with five electron groups have three dimensional geometries
- If there are five electron groups around the central atom, there are four case scenarios:
- If the five electron groups are bonding groups, the angle between the equatorial bonds is 120° and the angle between the axial bonds is 90° which maximize their separation
- The geometry of this molecules is TRIGONAL BIPYRAMIDAL
- E.g. PCl5
- If the five electron groups are bonding groups, the angle between the equatorial bonds is 120° and the angle between the axial bonds is 90° which maximize their separation
PCl5 Has a Trigonal Bipyramidal Geometry
The molecular geometry of phosphorus pentachloride
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- If four electron groups are bonding groups and one of them is a lone pair, the angle between the equatorial bonds is slightly less than 120° and the angle between the axial bonds is slightly less than 90° due to the Coulombic force of repulsion generated by the lone pair
- The geometry of this molecules is SEESAW
- E.g. SF4
- If four electron groups are bonding groups and one of them is a lone pair, the angle between the equatorial bonds is slightly less than 120° and the angle between the axial bonds is slightly less than 90° due to the Coulombic force of repulsion generated by the lone pair
SF4 Has a Seesaw Geometry
The molecular geometry sulfur tetrafluoride
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- If three electron groups are bonding groups and two of them are lone pairs, it is a flat molecule
- The angle between bonds is slightly less than 90° due to the Coulombic force of repulsion generated by the lone pairs
- The geometry of this molecules is T-SHAPED
- E.g. ClF3
ClF3 Has a T-shaped Geometry
The molecular geometry of chlorine trifluoride
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- If two electron groups are bonding groups and three of them are lone pairs, it is a flat molecule
- The angle between bonds is slightly 180° due to the Coulombic force of repulsion generated by the lone pairs
- The geometry of this molecules is LINEAR
- E.g. I3-
I3- Has a Linear Geometry
The molecular geometry of the triiodide ion
Six electron groups
- Molecules with five electron groups have three dimensional geometries
- If there are six electron groups around the central atom, there are three case scenarios:
- If the six electron groups are bonding groups, the angle between the bonds is 90° which maximize their separation
- The geometry of this molecules is OCTAHEDRAL
- E.g. SF6
- If the six electron groups are bonding groups, the angle between the bonds is 90° which maximize their separation
SF6 Has an Octahedral Geometry
The molecular geometry of sulfur hexafluoride
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- If five electron groups are bonding groups and one of them is a lone pair, the angle between the bonds is slightly less than 90° due to the Coulombic force of repulsion generated by the lone pair
- The geometry of this molecules is SQUARE PYRAMIDAL
- E.g. BrF5
- If five electron groups are bonding groups and one of them is a lone pair, the angle between the bonds is slightly less than 90° due to the Coulombic force of repulsion generated by the lone pair
BrF5 Has a Square Pyramidal Geometry
The molecular geometry of bromine pentafluoride
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- If four electron groups are bonding groups and two of them are lone pairs, it is a flat molecule
- The angle between the bonds is 90° and the lone pairs are place at 180° minimizing the repulsion interaction between them
- The geometry of this molecules is SQUARE PLANAR
- E.g. XeF4
XeF4 Has a Square Planar Geometry
The molecular geometry of xenon tetrafluoride
In the table below it is a summary of the molecules that are part of the VSEPR
Summary of the VSEPR theory
Electron groups |
Bonding groups |
Lone Pairs |
Molecular Geometry |
Bond angles |
2 |
2 |
0 |
Linear |
180° |
3 |
3 |
0 |
Trigonal planar |
120° |
3 |
2 |
1 |
Bent |
<120° |
4 |
4 |
0 |
Tetrahedral |
109.5° |
4 |
3 |
1 |
Trigonal pyramidal |
107° |
4 |
2 |
2 |
Bent |
104.5° |
5 |
5 |
0 |
Trigonal bipyramidal |
120° (equatorial) 90° (axial) |
5 |
4 |
1 |
Seesaw |
<120° (equatorial) <90° (axial) |
5 |
3 |
2 |
T-shaped |
<90° |
5 |
2 |
3 |
Linear |
180° |
6 |
6 |
0 |
Octahedral |
90° |
6 |
5 |
1 |
Square pyramidal |
<90° |
6 |
4 |
2 |
Square planar |
90° |
Exam Tip
VSEPR theory is one of the most assessed topics in AP Chemistry. The molecular geometry, bond angles, bond order, relative bond energies, relative bond lengths, presence of dipole moment, and hybridization of valence orbitals can be predicted by using this model. Therefore, it is important that you understand its main principles and know how to explain them in terms of the Coulombic repulsion between the electron groups