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Module 1 Chemical Bonding and Molecular Geometry

 

CHEM-1312 M1L2 Explore: Molecular Geometry

Building on Lewis structures from M1L1, you will now learn to predict three-dimensional molecular shapes using VSEPR theory. This systematic approach transforms 2D Lewis structures into accurate 3D geometry predictions.


Module Competencies

A ★ indicates that this page contains an activity related to that LO.

CC1.1 Determine qualifications for molecular bonding based on geometric shapes

✅ LO1.1.1 Draw Lewis structures for molecules and ions (Completed in M1L1)

★ LO1.1.2 Apply VSEPR theory to predict molecular geometry

LO1.1.3 Determine molecular polarity using geometry and electronegativity (M1L3)

LO1.1.4 Explain bonding using valence bond theory and hybridization (Future lesson)

 

Overview

What This Lesson Is About

VSEPR theory application for predicting three-dimensional molecular geometry from Lewis structures.

What You Will Learn

Building on your Lewis structure foundation from M1L1, you'll master the systematic prediction of molecular shapes:

  • LO1.1.2: Apply VSEPR theory to predict molecular geometry

You'll learn the step-by-step VSEPR methodology: counting electron groups, determining electron-pair geometry, identifying molecular geometry, and predicting bond angles. This systematic approach transforms 2D Lewis structures into accurate 3D molecular shape predictions.

Why This Matters: Molecular geometry determines physical properties, chemical behavior, and biological activity. Every prediction about molecular polarity, intermolecular forces, and reactivity depends on understanding 3D molecular shape.

How to Succeed: Master the systematic VSEPR process step-by-step. Practice distinguishing between electron-pair geometry and molecular geometry. Work through progressively complex molecules, always starting with accurate Lewis structures.

What You Will Read

Overby/Chang: Chemistry, 14th Ed. - Chapter 10 (Sections 10.1-10.3)

VSEPR Theory and Molecular Geometry

  • VSEPR Theory Fundamentals (10.1)
    • Electron-pair repulsion principles
    • Electron-pair vs. molecular geometry distinction
    • Bond angle prediction methodology
  • Molecular Geometries (10.2-10.3)
    • Five basic electron-pair geometries
    • Molecular shapes with lone pairs
    • Bond angle modifications by lone pairs
    • Systematic geometry prediction process

📖 Reading Strategy: Focus on understanding how electron-pair repulsion creates specific 3D arrangements. Practice the systematic VSEPR methodology with the examples provided, always beginning with accurate Lewis structures from M1L1.

 

VSEPR Theory and Molecular Geometry Videos

The tabs below contain essential videos for mastering VSEPR theory and molecular geometry prediction. Watch each video to build your systematic understanding before working through the practice sections that follow.

1. VSEPR Theory Fundamentals (LO1.1.2)

Learning Objective Focus

LO1.1.2: Apply VSEPR theory to predict molecular geometry

Master the fundamental principles of electron-pair repulsion and how they determine molecular shapes.

Valence Shell Electron Pair Repulsion Theory

Learn the core principles of VSEPR theory: how electron pairs around a central atom repel each other and arrange themselves to minimize repulsion, creating predictable 3D molecular geometries.

Video Duration: 9:29 min.

Credit: Agapito Serrato III, TSTC produced

Key VSEPR Principles:
  • Electron pairs repel: Both bonding and lone pairs repel each other
  • Maximum separation: Electron pairs arrange for minimum repulsion
  • Lone pairs dominate: Lone pairs occupy more space than bonding pairs
  • Predictable shapes: Specific electron counts create specific geometries

2. Basic Molecular Geometries

Five Fundamental Geometries

Explore the five basic electron-pair geometries that form the foundation for all molecular shape predictions: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

Video Duration: 5:52 min.

Credit: Agapito Serrato III, TSTC produced

Electron Groups Electron-Pair Geometry Bond Angles Example
2 Linear 180° BeCl₂, CO₂
3 Trigonal Planar 120° BF₃, SO₃
4 Tetrahedral 109.5° CH₄, CF₄
5 Trigonal Bipyramidal 120°, 90° PF₅
6 Octahedral 90° SF₆

3. How Lone Pairs Modify Molecular Shape

Shape Modification by Lone Pairs

Understand how lone pairs change molecular geometry from the basic electron-pair geometry, creating bent, trigonal pyramidal, T-shaped, and other molecular shapes.

Video Duration: 2:35 min.

Credit: Agapito Serrato III, TSTC produced

Lone Pair Effects on Bond Angles

Why Lone Pairs Matter:

  • Lone pairs occupy more space than bonding pairs
  • They compress bond angles between atoms
  • They create different molecular shapes from electron-pair geometry

Common Examples:

  • H₂O: Tetrahedral e⁻ pairs → Bent molecular shape
  • NH₃: Tetrahedral e⁻ pairs → Trigonal pyramidal
  • SF₄: Trigonal bipyramidal → Seesaw

4. Complete Guide to Molecular Geometries

Comprehensive Molecular Shape Catalog

Survey all possible molecular geometries systematically, including how to predict them from Lewis structures and electron group counts.

Video Duration: 10:30 min.

Credit: Agapito Serrato III, TSTC produced

Systematic VSEPR Prediction Process:
  1. Draw Lewis structure (from M1L1 skills)
  2. Count electron groups around central atom (bonding + lone pairs)
  3. Determine electron-pair geometry from electron group count
  4. Identify molecular geometry considering only bonded atoms
  5. Predict bond angles accounting for lone pair compression

 

 

Practice: VSEPR Theory and Molecular Geometry

Systematic VSEPR Practice Process

For each molecule below, follow these steps:

  1. Draw the Lewis structure (using skills from M1L1)
  2. Count total electron groups around the central atom
  3. Determine electron-pair geometry from electron group count
  4. Identify molecular geometry (considering only bonded atoms)
  5. Predict bond angles, accounting for lone pair effects
Progressive VSEPR Practice Problems
Level 1: Basic Shapes
  • BeCl₂: Linear geometry
  • BF₃: Trigonal planar
  • CH₄: Tetrahedral
Level 2: Lone Pair Effects
  • H₂O: Bent geometry
  • NH₃: Trigonal pyramidal
  • SO₂: Bent with double bonds
Level 3: Complex Geometries
  • SF₄: Seesaw geometry
  • ClF₃: T-shaped
  • XeF₄: Square planar
Interactive Molecular Shapes Simulation

Using the PhET simulation below:

  1. Select "Real Molecules" and then choose "Model" view
  2. Enable "Show lone pairs" and "Show bond angles"
  3. Work through each molecule systematically
  4. Compare your predictions with the simulation results
  5. Complete the data table that follows
VSEPR Analysis Table
Complete this table using the PhET simulation and VSEPR methodology
Molecule Lewis Structure Total e⁻ Groups Bonding Groups Lone Pairs Electron-Pair Geometry Molecular Geometry Bond Angles
CH₄
NH₃
H₂O
BF₃
SO₂
CO₂

Supplemental Resources