Notes on Bonding Theories and Molecular Geometry

Chapter 5: Bonding Theories

Explaining Molecular Geometry

Molecular Geometry Overview

  • molecular geometry involves the spatial arrangement of atoms in a molecule.
  • Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict molecular geometries based on electron pair repulsion.

VSEPR Theory Key Points

  • Electron pairs around a central atom repel each other.
  • Different arrangements lead to different molecular shapes dictated by the number of bonded atoms and lone pairs.

Electron Pair Arrangement and Molecular Geometry

  • Linear Geometry (AB2)

    • # of atoms bonded: 2
    • # of lone pairs: 0
    • Arrangement of electron pairs: linear
    • Molecular Geometry: linear
    • Bond Angle: 180°
    • Examples: BeCl2, HgCl₂

  • Trigonal Planar Geometry (AB3)

    • # of atoms bonded: 3
    • # of lone pairs: 0
    • Arrangement of electron pairs: trigonal planar
    • Molecular Geometry: trigonal planar
    • Bond Angle: 120°
    • Example: BF3

  • Tetrahedral Geometry (AB4)

    • # of atoms bonded: 4
    • # of lone pairs: 0
    • Arrangement of electron pairs: tetrahedral
    • Molecular Geometry: tetrahedral
    • Bond Angle: 109.5°
    • Examples: CH4, NH3

  • Trigonal Bipyramidal Geometry (AB5)

    • # of atoms bonded: 5
    • # of lone pairs: 0
    • Arrangement of electron pairs: trigonal bipyramidal
    • Molecular Geometry: trigonal bipyramidal
    • Bond Angles: 90° and 120°
    • Example: PCl5

  • Octahedral Geometry (AB6)

    • # of atoms bonded: 6
    • # of lone pairs: 0
    • Arrangement of electron pairs: octahedral
    • Molecular Geometry: octahedral
    • Bond Angle: 90°
    • Example: SF6

Effect of Lone Pairs on Geometry

  • Lone pair effects create distortions in bond angles due to increased repulsion.

  • Comparison of repulsions:

    • lone-pair vs. lone-pair repulsion > lone-pair vs. bonding pair repulsion > bonding-pair vs. bonding pair repulsion.

  • VSEPR Examples with Lone Pairs

    • Trigonal Planar Arrangement with 1 Lone Pair (AB2E)
    • Geometry: bent
    • Example: SO₂
    • Tetrahedral Arrangement with 1 Lone Pair (AB3E)
    • Geometry: trigonal pyramidal
    • Example: NH3
    • Tetrahedral Arrangement with 2 Lone Pairs (AB2E2)
    • Geometry: bent
    • Trigonal Bipyramidal Arrangement with 1 Lone Pair (AB4E)
    • Geometry: distorted tetrahedron
    • Trigonal Bipyramidal Arrangement with 2 Lone Pairs (AB3E2)
    • Geometry: T-shaped
    • Octahedral Arrangement with 1 Lone Pair (AB5E)
    • Geometry: square pyramidal
    • Octahedral Arrangement with 2 Lone Pairs (AB4E2)
    • Geometry: square planar

Predicting Molecular Geometry

  1. Draw the Lewis Structure for the molecule.
  2. Count Lone Pairs on the central atom and number of atoms bonded to it.
  3. Use VSEPR Theory to predict molecular geometry.

Dipole Moments and Polar Molecules

  • Dipole Moment (μ) definition: A measure of the polarity of the molecule, calculated as μ = Q × r, where Q is the charge and r is the distance between charges.
  • 1 D (Debye) = 3.36 × 10⁻³⁰ Coulomb-m.

Dipole Moments Table

  • Examples of dipole moments of polar molecules:
    • HF (Linear) - 1.92 D
    • H₂O (Bent) - 1.87 D
    • NH3 (Trigonal pyramidal) - 1.46 D
    • SO₂ (Bent) - 1.60 D

Bonding Theories

  1. Valence Bond Theory

    • Bonds are formed by the overlap of atomic orbitals.
    • Example: H₂ molecule formed by sharing of electrons between hydrogen atoms.

  2. Molecular Orbital Theory

    • Molecular orbitals formed from overlapping atomic orbitals.
    • Key characteristics of MO:
      • A bonding molecular orbital has lower energy than the atomic orbitals from which it formed, increasing stability.
      • An antibonding molecular orbital has higher energy, lowering stability.

Hybridization Theory

  • Definition: Hybridization is the mixing of atomic orbitals to form new hybrid orbitals.
  • Key rules:
    • The number of hybrid orbitals equals the number of atomic orbitals combined.
    • Shapes of hybrid orbitals depend on the types of orbitals mixed (e.g., sp, sp², sp³).
  • Examples of Hybridization:
    • sp Hybridization: Example BeCl₂ (linear, 180° bond angle)
    • sp² Hybridization: Example BF₃ (trigonal planar, 120° bond angle)
    • sp³ Hybridization: Example CH₄, NH₃, H₂O (tetrahedral arrangements with respective angles).

Molecular Orbital Configuration Basics

  1. The number of molecular orbitals equals the number of atomic orbitals combined.
  2. Bonding molecular orbitals are filled before antibonding molecular orbitals.
  3. Each molecular orbital can hold 2 electrons, with Hund’s rule applied when filling equal energy orbitals.