Molecular Geometry and VSEPR Theory: A Study Guide

Molecular Geometry

Introduction to Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms within a molecule or polyatomic ion. Understanding molecular geometry is essential for predicting the physical and chemical properties of substances, such as polarity, reactivity, phase of matter, color, magnetism, and biological activity.

• Definition: The spatial arrangement of atoms in a molecule, determined by the number of bonding and nonbonding electron pairs around the central atom.

• Importance: Molecular geometry affects molecular polarity, intermolecular forces, and the overall behavior of molecules in chemical reactions.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the repulsion between electron pairs in the valence shell of the central atom.

• Key Principle: Electron pairs (bonding and nonbonding) around a central atom arrange themselves as far apart as possible to minimize repulsion.

• Types of Electron Domains:

  • Bonding domains: Regions where electrons are shared between atoms (single, double, or triple bonds).

  • Nonbonding domains (lone pairs): Regions where electrons are localized on the central atom and not shared.

Common Molecular Geometries Predicted by VSEPR

The geometry of a molecule depends on the number of electron domains (regions of electron density) around the central atom. Below is a summary of common electron domain geometries, their designations, and resulting molecular shapes:

Electron Domain Geometry	Designation	Molecular Geometry	Shape/Angle
Linear	AB2	Linear	180°
Trigonal Planar	AB3	Trigonal Planar	120°
Trigonal Planar	AB2E	Angular or Bent	<120°
Tetrahedral	AB4	Tetrahedral	109.5°
Tetrahedral	AB3E	Trigonal Pyramidal	<109.5°
Trigonal Bipyramidal	AB5	Trigonal Bipyramidal	90°, 120°
Trigonal Bipyramidal	AB4E	Seesaw	<90°, <120°
Octahedral	AB6	Octahedral	90°
Octahedral	AB5E	Square Pyramidal	<90°
Octahedral	AB4E2	Square Planar	90°

Note: E represents a lone pair (nonbonding domain) on the central atom.

Examples of Molecular Geometries

• Linear: CO2, BeCl2

• Trigonal Planar: BF3

• Bent (Angular): SO2, O3

• Tetrahedral: CH4

• Trigonal Pyramidal: NH3

• Seesaw: SF4

• Octahedral: SF6

• Square Planar: XeF4

Practice Exercise: Determining Molecular Geometry

For each compound, determine the number of bonding and nonbonding domains, the electron domain geometry, and the molecular geometry.

Compound	No. of Bonding Domains	No. of Nonbonding Domains	Electron Domain Geometry	Molecular Geometry
ClF4	4	2	Octahedral	Square Planar
CF4	4	0	Tetrahedral	Tetrahedral
OF2	2	2	Tetrahedral	Bent (Angular)

Summary Table: Electron Domain and Molecular Geometry

Electron Domains	Lone Pairs	Molecular Geometry	Example
2	0	Linear	CO2
3	0	Trigonal Planar	BF3
3	1	Bent	SO2
4	0	Tetrahedral	CH4
4	1	Trigonal Pyramidal	NH3
4	2	Bent	H2O
5	0	Trigonal Bipyramidal	PCl5
5	1	Seesaw	SF4
6	0	Octahedral	SF6
6	1	Square Pyramidal	BrF5
6	2	Square Planar	XeF4

Key Equations and Concepts

• Bond Angle: The angle between two adjacent bonds on the same atom. For example, in a tetrahedral geometry, the bond angle is: $109.5^{\circ}$

• VSEPR Notation: The general formula for designating molecular shapes is $AB_nE_m$, where:

  • $A$ = central atom

  • $B$ = number of atoms bonded to the central atom

  • $E$ = number of lone pairs on the central atom

Additional info:

• Some slides and handwritten notes were unclear; standard VSEPR geometries and examples were inferred for completeness.

• Practice exercise answers were filled in based on standard VSEPR theory.