CH 11 Molecular Geometries and Bonding Theories

Chapter 11 Goals

• Predict molecular shapes: Utilize VSEPR theory to determine bond angles in molecules & ions.
• Determine molecular polarity: Assess molecules for symmetry and bond polarities to classify as polar or nonpolar.
• Identify hybridization: Recognize the hybridization states of atoms within a molecule.
• Molecular orbital (MO) theory: Construct and utilize energy level diagrams, and write MO configurations for simple molecules/ions.

Valence Shell Electron-Pair Repulsion Theory (VSEPR)

• Concept: Electron density regions (bonding pairs + lone pairs) are spaced apart to minimize repulsion.
  • Repulsion strength hierarchy:
  • Lone pair to lone pair (strongest)
  • Lone pair to bonding pair (intermediate)
  • Bonding pair to bonding pair (weakest)
• Mnemonic: lp/lp > lp/bp > bp/bp
• Basic shapes: 5 geometrical arrangements exist depending on the number of regions of electron density around the central atom.

Predicting Molecular Geometries and Bond Angles

• Steps: Count bonding pairs (BP) and lone pairs (LP) around the central atom:
  • Each multiple bond or lone pair counts as one region of high electron density.
  • Adjust bond angles for presence of lone pairs (angles slightly reduced), except for linear (180°) and square planar (90°).

Table of Geometries and Bond Angles

Molecular Polarity

• Nonpolar Molecules:
  • All bonds are nonpolar.
  • Polar bonds present but symmetric molecule causes dipoles to cancel (e.g., BF3).
• Polar Molecules:
  • At least one polar bond exists or asymmetric arrangement causes dipoles not to cancel (e.g., H2O, SO2).
• Method:
  • Assess bond angle and symmetry, sum dipole moment vectors to determine if the molecule is polar or nonpolar.

Valence Bond (VB) Theory

• Basic Concept: Atoms bond through overlap of their atomic orbitals to form covalent bonds.
• Hybridization: Predicts bond angles and shapes:
  • sp hybridization: 2 regions -> linear (180°).
  • sp² hybridization: 3 regions -> trigonal planar (120°).
  • sp³ hybridization: 4 regions -> tetrahedral (109.5°).
  • sp³d hybridization: 5 regions -> trigonal bipyramidal (90°, 120°).
  • sp³d² hybridization: 6 regions -> octahedral (90°).

Bonding Example: Methane (CH4)

• Hybridization: sp³ (four equivalent hybrid orbitals contributing to each C-H σ bond).
• Shape: Tetrahedral geometry, all C-H bond angles are approximately 109.5°.

Molecular Orbital Theory

• Molecular orbitals formed from the overlap of atomic orbitals (two MOs created per pair of atomic orbitals).
• Types of MOs: Bonding (σ) and Antibonding (σ*).
• Key Concepts:
  • Bonding MOs: Constructive interference leads to a lower energy state.
  • Antibonding MOs: Destructive interference leads to a higher energy state.
• Filling Order: Follow the Aufbau principle, Pauli Exclusion principle, and Hund’s Rule when populating molecular orbitals.

Magnetism and Bond Order

• Diamagnetic: Molecules with all paired electrons, weakly repelled by a magnetic field (e.g., H2).
• Paramagnetic: Molecules with unpaired electrons, attracted to a magnetic field.
• Bond Order Calculation:
  ext{Bond Order} = rac{1}{2} imes (Nb - Na) where $Nb$ = number of bonding electrons and $Na$ = number of antibonding electrons.

Summary for Complex Molecules: Glycine

• Determine hybridization based on bonding and geometry. Identify sigma (σ) and pi (π) bonds present.
• Important to derive Lewis structures to establish connectivity and hybridization in molecular structure.