VSEPR Theory and Molecular Geometry

🔬 VSEPR Theory and Molecular Geometry

Part 1 of 7 — Introduction to VSEPR

VSEPR stands for Valence Shell Electron Pair Repulsion. It is one of the most powerful tools in chemistry for predicting the three-dimensional shapes of molecules.

The core idea is simple: electron groups around a central atom repel each other and arrange themselves as far apart as possible to minimize repulsion.

Why Does Shape Matter?

Molecular geometry determines:

• Polarity of the molecule (and therefore solubility, boiling point, etc.)
• Reactivity and how molecules interact with each other
• Biological function — even slight shape changes in proteins can cause disease

In this lesson series, you'll learn to predict the geometry of any molecule from its Lewis structure.

What Is an Electron Domain?

An electron domain (also called an electron group or region of electron density) is any of the following around a central atom:

Critical Rule

A double bond counts as ONE electron domain. A triple bond also counts as ONE electron domain. Only the number of regions of electron density matters, not the total number of electrons.

Examples:

• CO₂: C has 2 double bonds → 2 electron domains
• H₂O: O has 2 single bonds + 2 lone pairs → 4 electron domains
• NH₃: N has 3 single bonds + 1 lone pair → 4 electron domains
• HCN: C has 1 single bond + 1 triple bond → 2 electron domains

Identify the number of electron domains around the central atom.

The Steric Number

The steric number is the total number of electron domains around the central atom. It is calculated as:

$\text{Steric Number} = \text{(number of atoms bonded to central atom)} + \text{(number of lone pairs on central atom)}$

The steric number determines the electron domain geometry — the arrangement of ALL electron groups (both bonding and lone pairs) in 3D space.

Determine the steric number for each central atom.

Two Types of Geometry

This is one of the most important distinctions in VSEPR theory:

Electron Domain Geometry

• Describes the arrangement of all electron domains (bonding + lone pairs)
• Determined solely by the steric number
• Think of it as the "invisible scaffolding"

Molecular Geometry

• Describes the arrangement of only the atoms (ignoring lone pairs)
• This is the actual shape of the molecule
• It's what we observe experimentally

When Are They Different?

They are the same when there are no lone pairs on the central atom.

They are different when lone pairs are present — because lone pairs take up space in the electron domain geometry but are invisible in the molecular shape.

Example: CH₄ vs. NH₃ vs. H₂O

Test your understanding of the difference between electron domain and molecular geometry.

Select the correct answers for each scenario.

📐 Linear, Trigonal Planar, and Tetrahedral Geometries

Part 2 of 7 — The Core Geometries

Linear Geometry

When a central atom has 2 electron domains (steric number = 2), they arrange themselves 180° apart on opposite sides of the atom.

$\text{Bond angle} = 180°$

Characteristics

• Shape: straight line through all three atoms
• Bond angle: exactly 180°
• All atoms in a straight line

Examples

🔷 Trigonal Bipyramidal and Octahedral Geometries

Part 3 of 7 — 5 and 6 Electron Domains

Elements in Period 3 and beyond can accommodate more than 8 electrons in their valence shell because they have access to empty d orbitals. This allows steric numbers of 5 and 6.

Which Elements Can Expand?

• Must be in Period 3 or higher (have d orbitals available)
• Common elements: P, S, Cl, Br, I, Xe, Se
• Elements in Period 2 (C, N, O, F) cannot expand their octet

Examples of Expanded Octets

Trigonal Bipyramidal Geometry

When a central atom has 5 electron domains, they arrange in a trigonal bipyramidal shape. This geometry has two distinct types of positions:

👁️ Lone Pair Effects on Molecular Geometry

Part 4 of 7 — Bent, Trigonal Pyramidal, Seesaw, T-Shaped, Square Pyramidal, and Square Planar

When lone pairs occupy electron domain positions, they are "invisible" to molecular geometry but still exert repulsive forces. This creates molecular shapes that differ from the electron domain geometry.

Key Principle: Lone Pair Repulsion is Stronger

Lone pairs repel more strongly than bonding pairs because they are held closer to the central atom and spread out more. The repulsion strength order is:

$\text{Lone pair–Lone pair} > \text{Lone pair–Bond pair} > \text{Bond pair–Bond pair}$

This means:

• Lone pairs compress bond angles slightly below the ideal values
• The more lone pairs present, the smaller the bond angles become

Molecular Shapes from Steric Number 4

All of these have tetrahedral electron domain geometry but different molecular geometries:

Tetrahedral (0 lone pairs)

🧭 Predicting Molecular Geometry

Part 5 of 7 — From Lewis Structure to 3D Shape

Follow this systematic process to predict the geometry of any molecule:

Step 1: Draw the Lewis Structure

• Count total valence electrons
• Place bonds and lone pairs
• Check octets (and expanded octets for Period 3+)

Step 2: Identify the Central Atom

• Usually the least electronegative atom
• Usually the atom that can form the most bonds
• Hydrogen and fluorine are NEVER central atoms

Step 3: Count Electron Domains on the Central Atom

• Count each bond (single, double, or triple) as ONE domain
• Count each lone pair as ONE domain
• Sum = steric number

Step 4: Determine Electron Domain Geometry

Step 5: Determine Molecular Geometry

• Remove lone pairs from the picture
• The remaining atom positions define the molecular geometry
• Name the shape based on atom positions only

Worked Example: SO₂ (Sulfur Dioxide)

⚡ Polarity of Molecules

Part 6 of 7 — From Bond Dipoles to Molecular Dipoles

Understanding molecular geometry is essential because it determines whether a molecule is polar or nonpolar — a property that affects solubility, boiling point, intermolecular forces, and biological behavior.

Review: Bond Polarity

A bond dipole exists whenever two atoms with different electronegativities share electrons unequally. The more electronegative atom pulls electron density toward itself.

$\text{Bond dipole: } \delta^+ \text{—} \delta^-$

• Larger electronegativity difference → stronger bond dipole
• Equal electronegativity (e.g., C–C, O–O) → nonpolar bond

From Bond Dipoles to Molecular Dipoles

🎯 Synthesis & AP Exam Review

Part 7 of 7 — Comprehensive Review

You've now learned the complete VSEPR framework:

1. Draw a Lewis structure → identify bonds and lone pairs
2. Count electron domains → determine the steric number
3. Identify electron domain geometry → the 3D arrangement of all electron groups
4. Identify molecular geometry → the shape based on atom positions only
5. Predict polarity → vector sum of bond dipoles based on geometry

AP Exam Tips

• VSEPR questions appear in multiple-choice and free-response
• You must be able to go from a chemical formula to a 3D shape quickly
• Know the connection between geometry, polarity, and intermolecular forces
• Understand how geometry affects physical properties (boiling point, solubility)
• Be able to explain why a molecule has its shape (electron pair repulsion)

Let's do a comprehensive review with AP-style problems.

Identify the molecular geometry of each species from its Lewis structure.

Predict the approximate bond angle for each molecule. Use the ideal angle for the geometry (don't worry about small lone pair compressions unless specified).

For each molecule, predict whether it is polar or nonpolar based on its geometry.

How to Answer VSEPR Free-Response Questions

AP Chemistry FRQs often ask you to:

1. Draw or describe the Lewis structure
2. Predict the molecular geometry
3. Explain whether the molecule is polar or nonpolar
4. Relate geometry/polarity to a physical property

Template Answer