Molecular Geometry and Bonding Theories: VSEPR and Electron Domains

Molecular Geometry and Bonding Theories

Introduction

This study guide summarizes the foundational concepts of molecular geometry and bonding theories, focusing on the Valence-Shell Electron-Pair Repulsion (VSEPR) model and electron domains. These principles are essential for predicting molecular shapes and understanding chemical bonding in general chemistry.

Molecular Shapes

Overview of Molecular Shapes

Molecular shapes describe the three-dimensional arrangement of atoms in a molecule. While Lewis structures show bonding and lone pairs, they do not directly indicate the actual shape of the molecule. However, Lewis structures are useful for determining molecular shapes by identifying the number and type of electron domains around a central atom.

• Common shapes for molecules with two or three atoms connected to a central atom:

  • CO2: AB2, linear

  • SO2: AB2, bent

  • SO3: AB3, trigonal planar

  • NF3: AB3, trigonal pyramidal

  • ClF3: AB3, T-shaped

• Key Point: The arrangement of atoms and electron pairs around a central atom determines the molecular shape.

The VSEPR Model

Principles of VSEPR

The Valence-Shell Electron-Pair Repulsion (VSEPR) model is used to predict the shape of molecules based on the repulsion between electron pairs in the valence shell of the central atom.

• Bond angles and bond lengths determine the shape and size of molecules.

• Electron pairs repel each other and arrange themselves as far apart as possible to minimize repulsions.

• This arrangement allows for the prediction of molecular shape.

Examples of VSEPR Geometries

• AB2: Linear (180°)

• AB3: Trigonal planar (120°)

• AB4: Tetrahedral (109.5°)

• AB5: Trigonal bipyramidal (90°, 120°)

• AB6: Octahedral (90°)

Electron Domains

Definition and Types

Electron domains are regions where electrons are most likely to be found around a central atom. They include both bonding and nonbonding (lone pair) electron pairs.

• Bonding electron domains: Single, double, or triple bonds between two atoms.

  • Multiple bonds count as one electron domain.

• Nonbonding electron domains: Lone pairs centered on the atom.

• Example: In BF3, the central atom (B) has three electron domains.

Valence-Shell Electron-Pair Repulsion (VSEPR) Model

Minimizing Electron Pair Repulsions

The best arrangement of a given number of electron domains is the one that minimizes repulsions among them. The balloon analogy demonstrates how electron domains maximize their distances to minimize repulsions.

• Two domains: Linear

• Three domains: Trigonal planar

• Four domains: Tetrahedral

Electron-Domain Geometries

Determining Electron-Domain Geometry

Electron-domain geometry is determined by counting the total number of bonding and nonbonding electron domains around the central atom. Each geometry gives rise to specific bond angles and molecular shapes.

• Multiple bonds count as one electron domain.

• Bond angles:

  • Linear: 180°

  • Trigonal planar: 120°

  • Tetrahedral: 109.5°

  • Trigonal bipyramidal: 90°, 120°

  • Octahedral: 90°

Electron-Domain Geometries Table

Number of Electron Domains	Electron-Domain Geometry	Bond Angles
2	Linear	180°
3	Trigonal planar	120°
4	Tetrahedral	109.5°
5	Trigonal bipyramidal	90°, 120°
6	Octahedral	90°

Summary

Understanding molecular geometry and bonding theories is essential for predicting the physical and chemical properties of molecules. The VSEPR model and electron-domain concept provide a systematic approach to determining molecular shapes based on electron pair repulsions and domain arrangements.

Additional info: These notes are based on textbook slides and may be supplemented with further examples and applications in a full textbook chapter.