Predicting Molecular Geometry and the Role of Lone Pairs
The VSEPR Model: Minimizing Electron Repulsion
- The VSEPR model is based on a straightforward idea: electron pairs repel each other and arrange themselves as far apart as possible around a central atom.
- These electron pairs can be bonding pairs (shared between atoms) or lone pairs (non-bonding pairs localized on the central atom).
- Both types of pairs create regions of electron density, called electron domains, which determine the geometry of a molecule.
Definition
Electron domain
An electron domain is a region in which electrons are most likely to be found (bonding and nonbonding)
Step-by-Step Process to Predict Geometry
- Count Electron Domains:
- Identify the central atom in the molecule and count the total number of electron domains around it.
- Each single bond, double bond, triple bond, or lone pair counts as one domain.
- Determine Electron Domain Geometry:
- Arrange the electron domains to minimize repulsion, leading to specific geometries:
- 2 domains: Linear (180° bond angles).
- 3 domains: Trigonal planar (120° bond angles).
- 4 domains: Tetrahedral (109.5° bond angles).
- Arrange the electron domains to minimize repulsion, leading to specific geometries:
- Adjust for Lone Pairs:
- Replace bonding domains with lone pairs as necessary.
- Lone pairs exert stronger repulsion than bonding pairs, which reduces bond angles and alters the molecular geometry.
Example
In methane (CH₄), the central carbon has four single bonds, so there are four electron domains.
Tip
Always begin by determining the total number of electron domains, as this sets the foundation for predicting the molecule’s geometry.
Key Electron Domain Geometries
Linear Geometry
- Electron Domains: 2
- Bond Angle: 180°
- Example: Carbon dioxide (CO₂).
- CO₂ has two double bonds around the central carbon, creating two electron domains. The domains align in a straight line, resulting in a linear shape.
Trigonal Planar Geometry
- Electron Domains: 3
- Bond Angle: 120°
- Example 1: Boron trifluoride (BF₃).
- BF₃ has three single bonds and no lone pairs around boron, forming a flat triangular shape.
- Example 2: Sulfur dioxide (SO₂).
- SO₂ has two bonding pairs and one lone pair. The lone pair reduces the bond angle slightly to less than 120°, resulting in a bent (V-shaped) molecular geometry.
Example
In sulfur dioxide (SO₂), the lone pair pushes the bonding pairs closer together, reducing the bond angle to about 119°.
Tetrahedral Geometry
- Electron Domains: 4
- Bond Angle: 109.5°
- Example 1: Methane (CH₄).
- CH₄ has four single bonds around carbon, forming a tetrahedral shape.
- Example 2: Ammonia (NH₃) and Water (H₂O).
- NH₃: One lone pair reduces the bond angle to ~107°, resulting in a trigonal pyramidal shape.
- H₂O: Two lone pairs reduce the bond angle further to ~104.5°, resulting in a bent (V-shaped) geometry.
Common Mistake
Students often confuse electron domain geometry with molecular geometry. Remember, molecular geometry describes the shape formed by the atoms, not the lone pairs.
Why Do Lone Pairs Reduce Bond Angles?
- Lone pairs occupy more space than bonding pairs because they are localized closer to the nucleus of the central atom.
- This increased repulsion pushes bonding pairs closer together, reducing bond angles.
- The more lone pairs present, the smaller the bond angles.
Example
Water (H₂O)
- Electron Domain Geometry: Tetrahedral (4 domains: 2 bonding pairs, 2 lone pairs).
- Molecular Geometry: Bent (V-shaped).
- Bond Angle: 104.5° (less than the ideal 109.5° due to lone pair repulsion).
Tip
To predict the geometry of a molecule with lone pairs, first determine the electron domain geometry, then adjust for lone pair repulsion.
VSEPR theory and Molecular Shapes
Summary Table: Electron Domain and Molecular Geometries
| Electron domains | Bonding domains | Lone pair | Electron dom ain geometry | Molecular geometry | Bond angle |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | Linear | $180^\circ$ |
| 3 | 3 | 0 | Trigonal planar | Trigonal planar | $120^\circ$ |
| 3 | 2 | 1 | Trigonal planar | Bent | $<120^\circ$ |
| 4 | 4 | 0 | Tetrahedral | Tetrahedral | $109.5^\circ$ |
| 4 | 3 | 1 | Tetrahedral | Trigonal pyramidal | $\sim 107^\circ$ |
| 4 | 2 | 2 | Tetrahedral | Bent | $\sim 104.5^\circ$ |
Self review
- Predict the molecular geometry of NF₃ and state its bond angle.
- Explain why ammonia (NH₃) has a smaller bond angle than methane (CH₄).
