04 Chapter - Molecular Geometry, Polarity, and intermolecular forces of Attraction

Chapter Overview

• Title: Molecular Geometry, Polarity, and Intermolecular Forces of Attraction

• Source: Guinn, Essentials of General, Organic, and Biochemistry, 3e, 2019 W. H. Freeman and Company

Contents

1. Three-Dimensional Shapes of Molecules

2. Bond Dipoles and Molecular Polarity

3. Intermolecular Forces of Attraction in a Compound

Molecular Geometry

• Importance of molecular shape in biological processes.

  • Influences recognition by receptors (e.g. hormones and drugs).

• Basic Molecular Geometries:

  • Five geometries create complex molecular structures (not specified but typically include linear, trigonal planar, tetrahedral, etc.).

Molecular Models

• Types of Models:

  • Lewis Dot Structure: Representation of valence electrons.

  • Ball-and-Stick Model: Displays bonds and angles.

  • Space-Filling Model: Gives a visual of relative sizes of atoms in a molecule.

Predicting Electron Geometry and Molecular Geometry

• Electron Geometry: Arrangements of electron groups around central atom.

• Molecular Geometry: Arrangement of atoms in a molecule based on electron geometry.

• Types of electron groups include:

  • Single bond.

  • Double bond.

  • Triple bond.

  • Nonbonding pair.

VSEPR Theory

• Valence Shell Electron Pair Repulsion (VSEPR) Theory:

  • Predicts orientation of groups based on repulsion:

    • 2 Electron Groups: Linear.

    • 3 Electron Groups: Trigonal Planar.

    • 4 Electron Groups: Tetrahedral.

Molecular Geometry Determination

• Molecular Geometry from Electron Groups:

  • If all groups are bonding, geometry is the same as electron geometry.

  • If there are nonbonding electrons, geometry differs.

Electron Geometry Examples

• Two Electron Groups:

  • Electron Geometry: Linear.

  • Molecular Geometry: Linear.

• Three Electron Groups:

  • Electron Geometry: Trigonal planar.

  • Molecular Geometry: Trigonal or bent.

• Four Electron Groups:

  • Electron Geometry: Tetrahedral.

  • Molecular Geometry: Tetrahedral, trigonal pyramidal, or bent.

Molecular Bond Angles

• Tetrahedral Angle: 109.5°.

• Trigonal Planar Angle: 120°.

• Linear Angle: 180°.

Molecular Geometry of Larger Molecules

• Larger molecules consist of multiple atom centers, creating various shapes.

Molecular Polarity

• Determines interactions and macroscopic properties of molecules.

Covalent Bonds and Electronegativity

• Covalent Bonds involve sharing valence electrons, sometimes unevenly.

• Electronegativity: Measure of an atom's ability to attract electrons in a bond. Electronegativity trend:

  • Increases from left to right across a period.

  • Decreases from top to bottom within a group.

Bond Polarity

• Polar Bonds: Unequal sharing of electrons due to differences in electronegativity.

• Nonpolar Bonds: Equal sharing of electrons.

• Electronegativity Differences:

  • < 0.5 indicates nonpolar.

  • 0.5-2.0 indicates polar.

  • 2.0 indicates ionic.

Intermolecular Forces of Attraction

1. Dispersion Forces:

   • Present in all substances, weakest.

2. Dipole-Dipole Forces:

   • Occurs between polar molecules with permanent dipoles.

3. Hydrogen Bonding:

   • Strongest intermolecular force, occurs between H and F, O, or N.

Summary of Hydrogen Bonding in Water**

• Ice forms structured hydrogen bonds with four neighbors; liquid water has two to three hydrogen bonds per molecule.

Determining Strongest Intermolecular Forces**

• Utilize electronegativity trends and molecular structure to determine relevant intermolecular forces.

Examples of Polar and Nonpolar Molecules**

• Identifying polar and nonpolar based on symmetry and bond types.

  • Diatomic molecules with identical atoms are nonpolar.

  • Molecules with polar bonds in symmetrical shape can still be nonpolar if dipoles cancel.

Practice Exercises**

• Exercises cover determining molecular geometries, identifying intermolecular forces, and explaining molecular polarity based on structure and electronegativity.