VSEPR Theory: Predicting Molecular Shapes

What is VSEPR Theory?

VSEPR stands for Valence Shell Electron Pair Repulsion. It's a model used in chemistry to predict the 3D shape of individual molecules.

The main idea is simple but powerful: Electron pairs in the outermost shell (valence shell) of an atom repel each other.

Whether they are in chemical bonds (bonding pairs) or not (lone pairs), these groups of electrons will spread out as far as possible from each other to minimize this repulsion. This arrangement of electron pairs determines the entire geometry of the molecule.

Key Concepts: Electron vs. Molecular Geometry

This is the most important distinction in VSEPR theory.

• Electron Domain Geometry: This is the 3D arrangement of all the electron domains (both bonding and lone pairs) around the central atom.

• Molecular Geometry: This is the 3D arrangement of just the atoms in the molecule. You essentially ignore the lone pairs when naming the final shape, but the lone pairs are still there, influencing the shape by repelling the bonding pairs.

What is an "Electron Domain"?

An electron domain is any region of high electron density around a central atom.

• A single bond = 1 domain

• A double bond = 1 domain

• A triple bond = 1 domain

• A lone pair of electrons = 1 domain

(Note: Double and triple bonds count as only one domain because all their electrons are located in the same region between the two atoms.)

The Common Shapes

Let's look at the most common shapes based on the number of electron domains around the central atom.

Two (2) Electron Domains

• Electron Geometry: Linear

• Molecular Geometry: Linear

• Example: CO₂ (Carbon Dioxide)

• The central carbon atom has two double bonds to oxygen. That's two electron domains.

• To get as far apart as possible, they go to opposite sides of the carbon atom, creating a 180° bond angle.

• Shape: Linear

Three (3) Electron Domains

• Electron Geometry: Trigonal Planar

• The three domains form a flat triangle shape around the central atom, with 120° angles between them.

• Case 1: 3 Bonding Domains, 0 Lone Pairs

• Molecular Geometry: Trigonal Planar

• Example: BF₃ (Boron Trifluoride)

• All three domains are bonds, so the molecular shape is the same as the electron geometry. The molecule is flat with 120° bond angles.

• Case 2: 2 Bonding Domains, 1 Lone Pair

• Molecular Geometry: Bent

• Example: SO₂ (Sulfur Dioxide)

• The lone pair is still there, repelling the two S-O bonds. This pushes the bonds closer together. We don't "see" the lone pair in the final shape, so the molecule just looks bent. The bond angle will be slightly less than 120° because lone pairs repel more strongly than bonding pairs.

Four (4) Electron Domains

• Electron Geometry: Tetrahedral

• To get as far apart as possible in 3D space, four domains form a shape called a tetrahedron, with ideal bond angles of 109.5°.

• Case 1: 4 Bonding Domains, 0 Lone Pairs

• Molecular Geometry: Tetrahedral

• Example: CH₄ (Methane)

• The classic example of a tetrahedral molecule. All four H atoms are spaced out equally with 109.5° bond angles.

• Case 2: 3 Bonding Domains, 1 Lone Pair

• Molecular Geometry: Trigonal Pyramidal

• Example: NH₃ (Ammonia)

• The lone pair on the nitrogen pushes the three N-H bonds down, creating a short pyramid shape. The bond angles are compressed to about 107°.

• Case 3: 2 Bonding Domains, 2 Lone Pairs

• Molecular Geometry: Bent

• Example: H₂O (Water)

• The two lone pairs on the oxygen repel the two O-H bonds very strongly, pushing them even closer together. This results in a bent shape with a bond angle of about 104.5°.

Practice Problems

For each molecule, draw the Lewis structure, determine the number of electron domains on the central atom, and then state the molecular geometry (shape).

1. CCl₄ (Carbon tetrachloride)

2. H₂S (Hydrogen sulfide)

3. PH₃ (Phosphine)

Answers

1. CCl₄ (Carbon tetrachloride)

• Lewis Structure: Carbon is the central atom, single-bonded to four Chlorine atoms. Each Chlorine has 3 lone pairs. Carbon has no lone pairs.

• Electron Domains: The central Carbon atom has 4 electron domains (four single bonds).

• Breakdown: 4 bonding domains, 0 lone pairs.

• Molecular Geometry: With 4 bonding domains and 0 lone pairs, the shape is Tetrahedral.

2. H₂S (Hydrogen sulfide)

• Lewis Structure: Sulfur is the central atom, single-bonded to two Hydrogen atoms. The Sulfur atom also has two lone pairs.

• Electron Domains: The central Sulfur atom has 4 electron domains (two single bonds and two lone pairs).

• Breakdown: 2 bonding domains, 2 lone pairs.

• Molecular Geometry: The electron geometry is tetrahedral, but the shape considering only the atoms is Bent. (It is very similar to a water molecule).

3. PH₃ (Phosphine)

• Lewis Structure: Phosphorus is the central atom, single-bonded to three Hydrogen atoms. The Phosphorus atom also has one lone pair.

• Electron Domains: The central Phosphorus atom has 4 electron domains (three single bonds and one lone pair).

• Breakdown: 3 bonding domains, 1 lone pair.

• Molecular Geometry: The electron geometry is tetrahedral, but the shape considering only the atoms is Trigonal Pyramidal. (It is very similar to an ammonia molecule).