Covalent Bonding II: Molecular Shapes, VSEPR Theory, and Advanced Bonding Models

Topic 4: Covalent Bonding II — Molecular Shapes, VBT, and MO Theory

Introduction

This section explores advanced theories of covalent bonding, focusing on how molecular shapes are determined and explained using Valence Shell Electron Pair Repulsion (VSEPR) theory, Valence Bond Theory (VBT), and Molecular Orbital (MO) theory. Understanding these models is essential for predicting the three-dimensional structure and properties of molecules.

Advanced Theories of Covalent Bonding

• Lewis structures show the number and type of bonds in a molecule but do not provide information about its 3D shape.

• Three main theories account for molecular shape:

  • Valence Shell Electron Pair Repulsion (VSEPR) model

  • Valence Bond Theory (VBT)

  • Molecular Orbital (MO) Theory

VSEPR Theory: The Five Basic Shapes

Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory states that the best arrangement of a given number of electron groups is the one that minimizes the repulsions among them. Electron groups are regions of high electron density, including:

• Lone pairs

• Bonds (single, double, or triple; all count as one group)

Counting Electron Domains

To determine the shape, count the number of electron domains (regions of electron density) around the central atom. Examples:

• CH4: 4 electron domains (4 single bonds)

• H2O: 4 electron domains (2 bonds, 2 lone pairs)

• COCl2: 3 electron domains (1 double bond, 2 single bonds)

Electron Domain Geometries and Bond Angles

# of electron domains	Electron domain geometry	Predicted bond angles
2	Linear	180°
3	Trigonal planar	120°
4	Tetrahedral	109.5°
5	Trigonal bipyramidal	90°, 120°
6	Octahedral	90°

Examples of Basic Shapes

• Linear geometry (2 electron groups): e.g., BeCl2

• Trigonal planar geometry (3 electron groups): e.g., BF3

• Tetrahedral geometry (4 electron groups): e.g., CH4

• Trigonal bipyramidal geometry (5 electron groups): e.g., PCl5

• Octahedral geometry (6 electron groups): e.g., SF6

3D Representation: Line-Dash-Wedge Notation

• Line: Same plane as the page

• Dash: Goes into the page (away from you)

• Wedge: Comes out of the page (towards you)

It is best to maximize the number of bonds in the plane of the page for clarity.

VSEPR Theory: The Effect of Lone Pairs

Impact of Lone Pairs on Geometry

• Lone pairs occupy more space than bonding pairs, leading to greater repulsion and smaller bond angles.

• Example: In NH3 (ammonia), the ideal tetrahedral angle (109.5°) is reduced to 107° due to the lone pair.

• In H2O (water), two lone pairs reduce the bond angle further to about 104.5°.

Placing Lone Pairs in Larger Geometries

• For trigonal bipyramidal geometry, place lone pairs in equatorial positions to minimize repulsion.

• For octahedral geometry, place lone pairs in axial positions first.

• For other geometries, lone pairs can be placed in any position.

Effect of Multiple Bonds

• Double and triple bonds also increase repulsion and can affect bond angles.

• Order of repulsion: lone pair > triple bond > double bond > single bond

Strategy for Determining VSEPR Structures

1. Draw the Lewis structure if not provided.

2. Count electron groups on the central atom to determine electron geometry.

3. Count lone pairs to determine molecular geometry.

4. Draw the 3D structure using line-dash-wedge notation.

VSEPR Theory: Predicting Molecular Geometries

Steps for Drawing 3D Structures of Larger Molecules

1. Draw the Lewis structure and identify all central atoms.

2. Determine the number of electron pairs and lone pairs on each center.

3. Assign the geometry to each center and draw the structure using line-dash-wedge notation.

Example: Methanol (CH3OH)

• Oxygen: 2 bonds, 2 lone pairs (bent geometry)

• Carbon: 4 bonds (tetrahedral geometry)

Molecular Shape and Polarity

Bond Dipole Moments

• Covalent bonds between atoms with different electronegativities are polar, resulting in a bond dipole moment.

• The dipole moment ($\mu$) is a vector quantity, having both magnitude and direction.

• Formula: $\mu = q \times r$ where $q$ is the magnitude of the charge and $r$ is the distance between charges.

Molecular Dipole Moments

• For molecules with multiple polar bonds, the molecular dipole moment is the vector sum of all individual bond dipole moments.

• If the vector sum is zero, the molecule is nonpolar.

Vector Addition

• Vectors are represented by arrows; their length indicates magnitude, and their direction indicates orientation.

• To add vectors: connect them head-to-tail and draw a resultant vector from the start of the first to the end of the last.

Example: Polarity of Molecules

• CO2: Linear, bond dipoles cancel, nonpolar molecule.

• H2O: Bent, bond dipoles do not cancel, polar molecule.

Summary Table: Electron Domain Geometries

Electron Domains	Geometry	Example	Bond Angles
2	Linear	BeCl2	180°
3	Trigonal planar	BF3	120°
4	Tetrahedral	CH4	109.5°
5	Trigonal bipyramidal	PCl5	90°, 120°
6	Octahedral	SF6	90°

Additional info: Later sections (VBT and MO theory) are listed but not covered in detail in the provided slides. For a complete understanding, students should refer to textbook sections on orbital overlap, hybridization, and molecular orbital theory.