VSEPR Theory
Chemistry: IB Diploma Higher Level, topic 2.3.3
Valence Shell Electron-Pair Repulsion (VSEPR) Theory
The final geometry of a covalent molecule depends on the number and type of electron domains around a central atom.
Electron domains
- Electrons are attracted to and constrained around positive nuclei in electron domains.
- When determining covalent molecular geometries, the repulsion between electron domains (electron pairs) around a central atom needs to be considered.
- These electron domains repel each other until the repulsive forces are minimised to give an optimised geometry.
VSEPR theory
- The final geometry of a covalent molecule depends on:
- The number of electron domains
- The identity of these electron domains (non-bonding vs. bonding)
- Considering these effects when determining the molecular shape and bond angles of a molecule is called Valence-Shell Electron-Pair Repulsion (VSEPR) theory.
Electron Domain and Molecular Geometries
A summary of how the number and type of electron domains relate to the electron domain and molecular geometries is shown below:
Linear
- Total electron domains: 2
- bonding domains: 2
- non-bonding domains: 0
- Electron domain geometry: Linear
- Molecular geometry: Linear
- Bond angle: 180°
Trigonal planar
- Total electron domains: 3
- bonding domains: 3
- non-bonding domains: 0
- Electron domain geometry: Trigonal planar
- Molecular geometry: Trigonal planar
- Bond angle: 120°
Trigonal planar - bent
- Total electron domains: 3
- bonding domains: 2
- non-bonding domains: 1
- Electron domain geometry: Trigonal planar
- Molecular geometry: Bent
Tetrahedral
- Total electron domains: 4
- bonding domains: 4
- non-bonding domains: 0
- Electron domain geometry: Tetrahedral
- Molecular geometry: Tetrahedral
- Bond angle: 109.5°
Tetrahedral - trigonal pyramidal
- Total electron domains: 4
- bonding domains: 3
- non-bonding domains: 1
- Electron domain geometry: Tetrahedral
- Molecular geometry: Trigonal pyramidal
Tetrahedral - bent
- Total electron domains: 4
- bonding domains: 2
- non-bonding domains: 2
- Electron domain geometry: Tetrahedral
- Molecular geometry: Bent
Types of Bonding Domains
When considering molecular geometry, the type of electron domains present needs to be considered.
Bonding and non-bonding domains
- Electron domains can be either bonding or non-bonding.
- Non-bonding domains (lone pairs) are attracted to only one nucleus, whereas a bonding domain is attracted to two nuclei.
- The attraction of a bonding domain (bonding pair) by two nuclei causes the orbital to elongate and occupy less space around each nucleus compared to non-bonding domains.
- Therefore, non-bonding electron domains repel more than bonding domains and inflict wider angles in molecular geometries.
Example
- This effect is summarised well by comparing the different molecular structures of methane (CH4), ammonia (NH3), and water (H2O), which all have four electron domains.
Example
Test yourself on VSEPR Theory
Which of these statements is NOT correct?
What is the correct bond angle for a molecule with a tetrahedral molecular geometry?
What is the predicted molecular geometry for a molecule with 3 bonding domains and 1 non-bonding domain?
Which of these statements is NOT correct?
Which of these combinations is likely to produce the smallest bond angle?