Lecture Notes on Covalent Bonds and Molecular Geometry

Introduction to Covalent Bonds

• Covalent bonds are formed when non-metals share electrons to achieve stable electron configurations.
• These compounds are known as molecules.
• The shape and polarity of molecules determine their chemical behavior.

Lewis Structures and Electron Domains

• Lewis structures show the arrangement of valence electrons around atoms.
• Electron domains include groups of electrons in bonds or lone pairs.
• Example: Sulfur dioxide has three electron domains around sulfur.

VSEPR Theory (Valence Shell Electron Pair Repulsion)

• Electron domains repel each other and arrange in 3D space to minimize repulsion and maximize bond angles.
• Electron geometry vs. Molecular geometry:
  • Electron Geometry: The arrangement of all electron domains.
  • Molecular Geometry: The shape formed by the atoms, influenced by non-visible electron pairs.

Examples of Molecular Shapes

• Methane (CH₄): Four bonds, tetrahedral geometry for both electron and molecular shapes.
• Ammonia (NH₃): Three bonds and one lone pair, tetrahedral electron geometry, trigonal pyramidal molecular geometry.
• Water (H₂O): Two bonds and two lone pairs, tetrahedral electron geometry, bent molecular geometry.

Bond Angles and Lone Pairs

• Tetrahedral molecules have bond angles of 109.5°.
• Lone pairs decrease bond angles due to increased repulsion:
  • Ammonia: 107°
  • Water: 104.5°

Molecules with Three Electron Domains

• Methanal (CH₂O): Trigonal planar geometry, bond angles of 120°.
• Ozone (O₃): Trigonal planar electron geometry, but bent molecular geometry due to lone pair, bond angle approx. 117°.

Molecular Geometry and Polarity

• Polarity is influenced by molecular geometry and bond dipoles.
• Non-polar Examples:
  • Carbon Tetrachloride (CCl₄): Tetrahedral symmetry cancels dipoles.
  • Carbon Dioxide (CO₂): Linear geometry, equal and opposite dipoles.
• Polar Examples:
  • Trichloromethane (CHCl₃): Asymmetrical dipoles lead to polarity.
  • Methanal and Ammonia: Non-polar bonds with overall dipole moments due to geometry.

Allotropes of Carbon

• Graphite: Trigonal planar, conductive, soft, used in pencils.
• Diamond: Tetrahedral, very hard, high melting point.
• Fullerenes: Spherical, semiconductors, light, strong.

Summary

• Molecular shapes (electron and molecular geometry) are determined by repulsion between electron domains.
• Symmetrical bond dipoles can cancel, leading to non-polar molecules.
• Geometry and polarity are crucial for understanding covalent molecule behavior and intermolecular forces.