IB Chemistry Topic S2.2.4 VSEPR Theory Revision Notes (SL/HL)

IB Chemistry Topic S2.2.4 VSEPR Theory Revision Notes (SL/HL) | RevisionDojo

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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:
1. Identify the central atom in the molecule and count the total number of electron domains around it.
2. Each single bond, double bond, triple bond, or lone pair counts as one domain.
Determine Electron Domain Geometry:
1. Arrange the electron domains to minimize repulsion, leading to specific geometries:
  1. 2 domains: Linear (180° bond angles).
  2. 3 domains: Trigonal planar (120° bond angles).
  3. 4 domains: Tetrahedral (109.5° bond angles).
Adjust for Lone Pairs:
1. Determine the molecular geometry by considering only the positions of the atoms after accounting for any lone pairs on the central atom.
2. 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.

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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:
1. Identify the central atom in the molecule and count the total number of electron domains around it.
2. Each single bond, double bond, triple bond, or lone pair counts as one domain.
Determine Electron Domain Geometry:
1. Arrange the electron domains to minimize repulsion, leading to specific geometries:
  1. 2 domains: Linear (180° bond angles).
  2. 3 domains: Trigonal planar (120° bond angles).
  3. 4 domains: Tetrahedral (109.5° bond angles).
Adjust for Lone Pairs:
1. Determine the molecular geometry by considering only the positions of the atoms after accounting for any lone pairs on the central atom.
2. 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.

Definition

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry S2.2.4 VSEPR Theory Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Predicting Molecular Geometry and the Role of Lone Pairs

The VSEPR Model: Minimizing Electron Repulsion

Step-by-Step Process to Predict Geometry

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Predicting Molecular Geometry and the Role of Lone Pairs

The VSEPR Model: Minimizing Electron Repulsion

Step-by-Step Process to Predict Geometry

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