S2.2.4 VSEPR theory

Predicting Molecular Geometry and the Role of Lone Pairs

You're arranging chairs around a table for a group meeting. To ensure everyone has enough personal space, you spread the chairs as far apart as possible. Now, replace the chairs with electron pairs around an atom, and the table with the atom's nucleus. The "meeting" is the molecule's geometry! But what determines the exact arrangement of these "electron chairs"? And how do lone pairs—those extra chairs no one sits on—affect the meeting's layout?

These questions are at the heart of molecular geometry, a concept that explains why water forms a "V" shape and why carbon dioxide is linear.

The VSEPR Model: Minimizing Electron Repulsion

The VSEPR model is based on a straightforward idea: electron pairs repel each other and arrange themselves as far apart as possible around a central atom.

These electron pairs can be bonding pairs (shared between atoms) or lone pairs (non-bonding pairs localized on the central atom). Both types of pairs create regions of electron density, called electron domains, which determine the geometry of a molecule.

Definition

Electron domain

An electron domain is a region in which electrons are most likely to be found (bonding and nonbonding)

Step-by-Step Process to Predict Geometry

Count Electron Domains:
- Identify the central atom in the molecule and count the total number of electron domains around it. Each single bond, double bond, triple bond, or lone pair counts as one domain.

Example

In methane (CH₄), the central carbon has four single bonds, so there are four electron domains.

Determine Electron Domain Geometry:
- Arrange the electron domains to minimize repulsion, leading to specific geometries:
- 2 domains: Linear (180° bond angles).
- 3 domains: Trigonal planar (120° bond angles).
- 4 domains: Tetrahedral (109.5° bond angles).
Adjust for Lone Pairs:
- Replace bonding domains with lone pairs as necessary. Lone pairs exert stronger repulsion than bonding pairs, which reduces bond angles and alters the molecular geometry.

Tip

Always begin by determining the total number of electron domains, as this sets the foundation for predicting the molecule’s geometry.

Key Electron Domain Geometries

Linear Geometry

Electron Domains: 2
Bond Angle: 180°
- Example: Carbon dioxide (CO₂).
- CO₂ has two double bonds around the central carbon, creating two electron domains. The domains align in a straight line, resulting in a linear shape.

Trigonal Planar Geometry

Electron Domains: 3
Bond Angle: 120°
- Example 1: Boron trifluoride (BF₃).
- BF₃ has three single bonds and no lone pairs around boron, forming a flat triangular shape.
- Example 2: Sulfur dioxide (SO₂).
- SO₂ has two bonding pairs and one lone pair. The lone pair reduces the bond angle slightly to less than 120°, resulting in a bent (V-shaped) molecular geometry.

Example

In sulfur dioxide (SO₂), the lone pair pushes the bonding pairs closer together, reducing the bond angle to about 119°.

Trigonal planar geometry of boron trifluoride.

Trigonal planar geometry of sulfur dioxide.

Tetrahedral Geometry

Electron Domains: 4
Bond Angle: 109.5°
- Example 1: Methane (CH₄).
- CH₄ has four single bonds around carbon, forming a tetrahedral shape.
- Example 2: Ammonia (NH₃) and Water (H₂O).
- NH₃: One lone pair reduces the bond angle to ~107°, resulting in a trigonal pyramidal shape.
- H₂O: Two lone pairs reduce the bond angle further to ~104.5°, resulting in a bent (V-shaped) geometry.

Common Mistake

Students often confuse electron domain geometry with molecular geometry. Remember, molecular geometry describes the shape formed by the atoms, not the lone pairs.

Why Do Lone Pairs Reduce Bond Angles?

Lone pairs occupy more space than bonding pairs because they are localized closer to the nucleus of the central atom. This increased repulsion pushes bonding pairs closer together, reducing bond angles. The more lone pairs present, the smaller the bond angles.

Example

Water (H₂O)

Electron Domain Geometry: Tetrahedral (4 domains: 2 bonding pairs, 2 lone pairs).
Molecular Geometry: Bent (V-shaped).
Bond Angle: 104.5° (less than the ideal 109.5° due to lone pair repulsion).

Tip

To predict the geometry of a molecule with lone pairs, first determine the electron domain geometry, then adjust for lone pair repulsion.

Summary Table: Electron Domain and Molecular Geometries

Electron domains	Bonding domains	Lone pair	Electron dom ain geometry	Molecular geometry	Bond angle
2	2	0	Linear	Linear	$180^{\circ}$
3	3	0	Trigonal planar	Trigonal planar	$120^{\circ}$
3	2	1	Trigonal planar	Bent	$< 120^{\circ}$
4	4	0	Tetrahedral	Tetrahedral	${109.5}^{\circ}$
4	3	1	Tetrahedral	Trigonal pyramidal	$\sim 107^{\circ}$
4	2	2	Tetrahedral	Bent	$\sim {104.5}^{\circ}$

Reflection and Practice

Self review

Predict the molecular geometry of NF₃ and state its bond angle.
Explain why ammonia (NH₃) has a smaller bond angle than methane (CH₄).

Theory of Knowledge

How do the shapes of molecules influence their roles in biological systems, such as enzymes or DNA?
Can molecular geometry be viewed as a bridge between chemistry and biology?

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S2.2.4 VSEPR theory

Predicting Molecular Geometry and the Role of Lone Pairs

The VSEPR Model: Minimizing Electron Repulsion

Step-by-Step Process to Predict Geometry

Key Electron Domain Geometries

Linear Geometry

Trigonal Planar Geometry

Tetrahedral Geometry

Why Do Lone Pairs Reduce Bond Angles?

Water (H₂O)

Summary Table: Electron Domain and Molecular Geometries

Reflection and Practice

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Introduction to VSEPR Theory