17 - Molecular Orbital Theory / VSEPR

Molecular Geometry Overview

  • Molecular Geometry: Focuses on the shape of molecules rather than the arrangement of electrons in Lewis structures.

  • Polarity: Defined as separation of electric charge resulting in a dipole moment (partial positive and negative ends).

    • Example: Hydrogen and Fluorine where Fluorine is more electronegative, pulling shared electrons closer.

  • Electron Domains: Refers to any bonds (single, double, triple) or lone pairs around a central atom.

    • Example: NH3 has 4 electron domains (3 H-N bonds and 1 lone pair).

VSEPR Theory

  • VSEPR: Valence Shell Electron Pair Repulsion Theory; states that electron domains around a central atom will maximize their distance apart to minimize repulsion.

    • Molecular Geometries and Bond Angles:

      • 2 Electron Domains: Linear, 180°

      • 3 Electron Domains: Trigonal Planar, 120°

        • If lone pairs are present, it changes geometry (e.g., Bent).

      • 4 Electron Domains: Tetrahedral, 109.5°

        • Changes with lone pairs to Trigonal Pyramidal or Bent.

      • 5 Electron Domains: Trigonal Bipyramidal, 120° and 90°

        • Changes with lone pairs to Seesaw, T-shaped, or Linear.

      • 6 Electron Domains: Octahedral, 90°

        • Changes remain with lone pairs, leading to Square Pyramidal or Square Planar.

Polar vs. Nonpolar Molecules

  • Polarity: Depends on molecular geometry and the presence of electronegative atoms.

  • Example: CO2 is nonpolar (linear) despite having polar bonds; NH3 is polar due to its bent shape.

Valence Bond Theory

  • Hybridization: Mixing atomic orbitals to form new, equivalent hybrid orbitals.

    • Types of Hybridization:

      • 2 Electron Domains: sp

      • 3 Electron Domains: sp²

      • 4 Electron Domains: sp³

      • 5 Electron Domains: sp³d

      • 6 Electron Domains: sp³d²

  • Sigma and Pi Bonds:

    • Sigma bonds: Result from the head-on overlap of orbitals.

      • Form single bonds between atoms.

    • Pi bonds: Result from the side-on overlap of orbitals.

      • Present in double and triple bonds.

Summary & Recommendations

  • Ensure understanding of molecular geometries, bond angles, and how they relate to polarity.

  • Familiarize with hybridization concepts and types of bonds before exams.