Molecular Dipoles & MO Theory [Lecture 17]

Determining Molecular Polarity

  • Molecular (net) dipole:

    • Sum of all bond dipoles in a molecule via vector addition.

  • Bond Dipole (μ):

    • A measure representing the separation of charges due to electronegativity differences between atoms, indicated by δ+ (partial positive) and δ– (partial negative) charges.

  • Polar Molecule:

    • Molecule with non-canceling bond dipoles, resulting in an overall molecular dipole and uneven electron density.

  • Nonpolar Molecule:

    • All bond dipoles cancel out, leading to no net dipole.

Vector Addition of Bond Dipoles

  • Vector Definition:

    • A quantity with both magnitude and direction.

  • Adding Vectors:

    • Use methods like constructing a parallelogram to visualize how bond dipoles combine.

Common Molecular Shapes and Their Polarity

  • Linear (CO2):

    • Nonpolar

  • Trigonal Planar (BH3):

    • Nonpolar

  • Bent (H₂O):

    • Polar

  • Tetrahedral (CH4):

    • Nonpolar

  • Trigonal Pyramidal (NH3):

    • Polar

Steps to Determine Molecular Polarity

  1. Draw Lewis Structure:

    • Identify molecular geometry.

  2. Identify Polar Bonds:

    • Determine the presence of polar bonds and their direction.

  3. Assess Bond Dipole Addition:

    • Check if polar bonds add to create a new dipole moment.

    • Symmetric Dipoles: Nonpolar

    • Asymmetric Dipoles: Polar

Introduction to Molecular Orbital Theory (MO Theory)

  • Description:

    • MO Theory provides a detailed view of how atoms bond by treating electrons as waves and utilizing molecular orbitals that span the entire molecule.

  • Key Principles:

    • Linear Combination of Atomic Orbitals (LCAO):

      • Wavefunctions of atomic orbitals combine to form molecular orbitals through interference.

  • Types of Interference:

    • Constructive: Adds up leads to bonding orbitals, (lower energy).

    • Destructive: Cancels out leads to antibonding orbitals, (higher energy).

Constructing Molecular Orbital Diagrams

  • Molecular Orbitals:

    • Number of MOs formed equals the number of atomic orbitals combined.

    • Distinction between bonding (lower energy) and antibonding (higher energy) orbitals.

  • Bond Order Calculation:

    • Bond Order = 1/2 (Number of electrons in bonding MOs - Number of electrons in antibonding MOs)

  • Electron Configuration Rules:

    • Place electrons in MOs starting from the lowest energy. No more than two electrons per MO, with opposite spins.