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.
In methane (CH₄), the central carbon has four single bonds, so there are four electron domains.
TipAlways 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°
