Covalent Bonding II: Molecular Shapes, VSEPR Theory, and Molecular Polarity (Ch. 10)

Topic 4: Covalent Bonding II – Molecular Shapes, VBT and MO Theory (Ch. 10)

Overview

This section explores advanced theories of covalent bonding, focusing on how molecular shapes are determined and how these shapes influence molecular polarity. The main models discussed are the Valence Shell Electron Pair Repulsion (VSEPR) theory, Valence Bond Theory (VBT), and Molecular Orbital (MO) theory. The notes cover the five basic VSEPR shapes, the effect of lone pairs, strategies for predicting molecular geometries, and the relationship between molecular shape and polarity.

Advanced Theories of Covalent Bonding

Introduction to Bonding Models

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

• Three main theories account for molecular shape:

  • Valence Shell Electron Pair Repulsion (VSEPR) model

  • Valence Bond Theory (VBT)

  • Molecular Orbital (MO) Theory

10.2 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 around a central atom is the one that minimizes repulsions among them.

• Electron groups are regions of high electron density:

  • Lone pairs

  • Bonds (single, double, or triple; double and triple bonds ($\pi$bonds) count as one electron group)

Counting Electron Domains – Example

• For each central atom, count the number of electron domains (bonds + lone pairs).

• Examples:

  • CH4: 4 electron domains

  • H2O: 4 electron domains

  • COCl2: 3 electron domains

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	120° (equatorial), 90° (axial)
6	Octahedral	90°

Examples of Basic Shapes

• Linear geometry: 2 electron groups (e.g., BeCl2), bond angle = 180°

• Trigonal planar geometry: 3 electron groups (e.g., BF3), bond angle = 120°

• Tetrahedral geometry: 4 electron groups (e.g., CH4), bond angle = 109.5°

• Trigonal bipyramidal geometry: 5 electron groups (e.g., PCl5), bond angles = 120° (equatorial), 90° (axial)

• Octahedral geometry: 6 electron groups (e.g., SF6), bond angle = 90°

Line-Dash-Wedge Notation

• Line: Bond in the plane of the page

• Dash: Bond going into the page (away from you)

• Wedge: Bond coming out of the page (towards you)

• Maximize the number of bonds in the plane for clarity.

10.3 VSEPR Theory: The Effect of Lone Pairs

Lone Pair Effects on Geometry and Bond Angles

• Lone pairs occupy more space than bonding pairs, increasing repulsion and reducing bond angles.

• Example: NH3 (ammonia) has 4 electron domains (tetrahedral), but the bond angle is 107° (less than 109.5°) due to the lone pair.

• Example: H2O (water) has 2 lone pairs, resulting in a bond angle of 104.5°.

Molecular Geometry vs. Electron Group Geometry

• Electron group geometry considers all electron domains (bonds and lone pairs).

• Molecular geometry considers only the positions of atoms (ignoring lone pairs).

Rules for Placing Lone Pairs

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

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

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

Bond Angle Trends

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

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

Strategy for Determining VSEPR Structures

1. Draw the Lewis structure.

2. Count the number of electron groups on the central atom.

3. Count the number of lone pairs on the central atom.

4. Use the counts to determine electron geometry and molecular geometry.

5. Draw the structure using line-dash-wedge notation.

10.4 VSEPR Theory: Predicting Molecular Geometries

Steps for Drawing 3D Structures of Larger Molecules

1. Draw the Lewis structure and identify the central atoms (centers).

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

3. Determine the geometry of each center and draw the structure using line-dash-wedge notation.

Examples

• Methanol (CH3OH):

  • Carbon: 4 bonds, 0 lone pairs → tetrahedral

  • Oxygen: 2 bonds, 2 lone pairs → bent

• For more complex molecules, assign geometry to each atom based on its electron domains (e.g., linear, bent, tetrahedral).

10.5 Molecular Shape and Polarity

Bond Dipole Moments

• Covalent bonds between atoms with large differences in electronegativity (ΔEN) are polar.

• The separation of charge in a covalent bond is called the bond dipole moment ($\mu$).

• Formula: $\mu = Qr$

  • $Q$ = charge

  • $r$ = bond length

• $\mu$ is a vector (has magnitude and direction).

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.

• If the vector sum is nonzero, the molecule is polar.

Vector Addition

• Vectors are represented by arrows of a particular size and direction.

• To add vectors:

  1. Connect them head to tail.

  2. Draw a new arrow from the tail of the first to the head of the last.

• More than two vectors can be summed.

Examples

• Determine the vector sum for various arrow arrangements.

• Predict whether molecules (e.g., acetone, BF3) have molecular dipole moments and draw the resulting dipole vector.

• Represent molecules using hash, wedge, and solid bonds; determine molecular dipole and polarity for each.

Additional info: These notes provide foundational knowledge for understanding molecular geometry and polarity, which are essential for predicting chemical reactivity and physical properties in General Chemistry.