Covalent Bonding II: Molecular Shapes, VSEPR Theory, and Advanced Bonding Models

Topic 4: Covalent Bonding II — Molecular Shapes, VBT and MO Theory

Overview

This section covers advanced theories of covalent bonding, focusing on how molecular shapes are determined and explained using VSEPR theory, Valence Bond Theory (VBT), and Molecular Orbital (MO) Theory. These models provide insight into the three-dimensional structure of molecules, which is not revealed by Lewis structures alone.

• VSEPR Theory: Predicts molecular shapes based on electron group repulsions.

• Valence Bond Theory: Explains bonding through orbital overlap and hybridization.

• Molecular Orbital Theory: Describes electron delocalization in molecules.

10.2 VSEPR Theory: The Five Basic Shapes

Introduction to VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the geometry of molecules based on the repulsion between electron groups around a central atom. The optimal arrangement minimizes repulsions, resulting in specific molecular shapes.

• Electron groups are regions of high electron density, including lone pairs and bonds (single, double, or triple).

• Double and triple bonds count as a single electron group for geometry purposes.

Counting Electron Domains

To determine molecular shape, count the number of electron domains (groups) around the central atom:

• Lone pairs (non-bonding electrons)

• Bonds (single, double, triple all count as one domain each)

Basic Electron Domain Geometries

# of electron domains	Electron domain geometry	Predicted bond angles
2	Linear	180°
3	Trigonal planar	120°
4	Tetrahedral	109.5°
5	Trigonal bipyramidal	90°, 120°
6	Octahedral	90°

Examples of Basic Shapes

• Linear geometry: 2 electron groups (e.g., BeCl2), bond angle = 180°

• Trigonal planar geometry: 3 electron groups (e.g., BF3), bond angle = 120°

• Tetrahedral geometry: 4 electron groups (e.g., CH4), bond angle = 109.5°

• Trigonal bipyramidal geometry: 5 electron groups (e.g., PCl5), bond angles = 90°, 120°

• Octahedral geometry: 6 electron groups (e.g., SF6), bond angle = 90°

Line-Dash-Wedge Notation

This notation is used to represent the 3D structure of molecules in 2D drawings:

• Solid line: Bond in the plane of the page

• Dashed line: Bond going into the page (away from viewer)

• Wedge: Bond coming out of the page (towards viewer)

It is best to maximize the number of bonds in the plane of the page for clarity.

Example: PH5 Structure

Draw the VSEPR structure for PH5 using wedge, dash, and solid bonds to represent the 3D arrangement.

10.3 VSEPR Theory: The Effect of Lone Pairs

Introduction

Lone pairs of electrons influence molecular geometry by occupying more space than bonding pairs, leading to deviations from ideal bond angles. This effect is crucial for understanding real molecular shapes.

• Lone pairs cause greater repulsion than bonding pairs, reducing bond angles between atoms.

• Double and triple bonds also increase repulsion, affecting bond angles.

Examples of Lone Pair Effects

• NH3 (Ammonia): 4 electron domains (3 bonds, 1 lone pair) — expected tetrahedral geometry, but bond angle is reduced from 109.5° to 107° due to lone pair repulsion.

• H2O (Water): 4 electron domains (2 bonds, 2 lone pairs) — bond angle further reduced to about 104.5°.

Placing Lone Pairs in Geometries

• Trigonal bipyramidal: Place lone pairs in equatorial positions to minimize repulsion.

• Octahedral: Place lone pairs in axial positions first.

• Other geometries: Lone pairs can be placed in any position.

Relative Repulsion Order

• Lone pair > triple bond > double bond > single bond

Strategy for Determining VSEPR Structures

1. Draw the Lewis structure if not provided.

2. Count electron groups on the central atom and determine electron group geometry.

3. Count lone pairs and determine molecular geometry.

4. Draw the 3D structure using line-dash-wedge notation.

Example: Determining Geometry

• Given molecules such as PH3, phosgene (COCl2), and NO2-, identify the molecular geometry and draw the VSEPR structure.

• For AlCl3 and CH4, give the electron-pair geometry, bond angles, and VSEPR structures.

Additional info:

• Later sections (10.6–10.8) will cover Valence Bond Theory and Molecular Orbital Theory, which further explain bonding and electron delocalization.