Chapter 11 Lecture Notes - CHEM 1113 Broering

Chapter 11: Molecular Shapes and Bonding Theories

Valence-Shell Electron-Pair Repulsion Theory (VSEPR)

• Electron-pair geometry: 3D arrangement of bonding pairs and lone pairs around a central atom.

• Molecular geometry: 3D arrangement or shape of atoms in a molecule.

• Key principle: Negatively charged electrons repel each other, shaping the molecule based on the number of electron domains.

  • Best arrangement minimizes electron repulsions.

Electron Domains

• Definition: A single bond pair, multiple bond, or lone pair(s) on one atom.

  • Notation: "ABx" or AXE, where A is the central atom, B is surrounding atoms, x is the integer (2-6).

  • Arrangement minimizes electron repulsion.

Practice: Counting Electron Domains

• Examples:

  • CO2: 2 electron domains

  • CH4: 4 electron domains

  • SF6: 6 electron domains

  • BF3: 3 electron domains

Concept Test Question

• Question: How many electron domains are on S in SF4?

  • Options: a. 2, b. 3, c. 4, d. 5

Steps for Determining Geometries

1. Draw Lewis structure.

2. Count electron domains around the central atom (Electron-domain geometry).

3. Determine molecular geometry from bonded atom arrangement.

• Often referred to as Steric Number.

Electron Geometries Based on Electron Domains/Steric Number (SN)

• SN = 2: Linear

  • Example: CO2

• SN = 3: Trigonal planar

  • Example: BF3

• SN = 4: Tetrahedral

  • Example: CCl4

• SN = 5: Trigonal bipyramidal

  • Example: PF5

• SN = 6: Octahedral

  • Example: SF6

Example: Predict Electron Geometry

• Formaldehyde (CH2O): Three atoms bonded to central atom with no lone pairs results in a trigonal planar electron-pair and molecular geometry.

Electron Domains with SN = 2, 3, and 4

• SN = 2:

  • Linear geometry

  • Bond angle 180°

  • Examples: BeH2, CO2, OCN−

• SN = 3:

  • Trigonal planar geometry

  • Bond angle 120°

  • Molecular geometry can be bent if there is one lone pair (e.g., NO2−).

• SN = 4:

  • Tetrahedral geometry

  • Bond angle 109.5°

  • Can be trigonal pyramidal or bent with one or two lone pairs (e.g., PCl3, OF2).

Geometry for SN = 5 (Expanded Octets)

• SN = 5:

  • Trigonal bipyramidal geometry

  • Bond angles: 180° (axial), 120° (equatorial)

• Geometrical types:

  • See-saw for 4 bonding pairs + 1 lone pair.

  • T-shaped for 3 bonding pairs + 2 lone pairs.

  • Linear for 2 bonding pairs + 3 lone pairs.

Geometry for SN = 6 (Expanded Octets)

• SN = 6:

  • Octahedral geometry

  • Bond angle: 90°

  • Types:

    • Square pyramidal for 5 bonding pairs + 1 lone pair.

    • Square planar for 4 bonding pairs + 2 lone pairs.

Geometry Examples and Practice

• Example: Determine molecular geometries and bond angles for:

  • (a) Phosphorus trichloride, PCl3; SN = 4, trigonal pyramidal

    • Bond angle: slightly less than 109.5° due to lone pair repulsion.

  • (b) Oxygen difluoride, OF2;

    • Same as above, bent geometry.

  • (c) Dibromodichloromethane, CCl2Br2;

    • SN = 4, tetrahedral geometry.

Hybridization and Bonding

• Hybrid orbitals arise from mixing of atomic orbitals to form new orbitals that maximize overlap in bonds.

• Valence Bond Theory: Covalent bonds form when orbitals of different atoms overlap.

  • Hybridization helps explain molecular shapes and bond characteristics.

• Types of hybridization:

  • sp3: 108.5° bond angles (e.g., methane, CH4).

  • sp2: 120° bond angles (e.g., propene, C3H6).

  • sp: 180° bond angles (e.g., acetylene, C2H2).

Bonding Theories: Sigma and Pi Bonds

• Sigma (σ) bond: Formed by head-to-head overlaps; electron density along the bond axis.

• Pi (π) bond: Formed from side-by-side overlap of p orbitals; exists in areas above and below the bond axis.

Expanded Octets and Hybridization

• Elements with expanded octets involve d orbitals in bonding, forming hybrid orbitals such as sp3d and sp3d2.

Practice and Concept Tests

• Practice predicting hybridizations and determining bond types in various molecules by assessing their Lewis structures and electron domains.

Chapter 11: Molecular Shapes and Bonding Theories

Valence-Shell Electron-Pair Repulsion Theory (VSEPR): Determines how electron pairs around a central atom shape molecular geometry based on repulsion.

• Electron-pair geometry: 3D arrangement, Molecular geometry: Shape of atoms.

• Electron Domains: Includes single/multiple bond pairs or lone pairs (Notation: ABx or AXE).

Electron Geometries by Steric Number (SN):

• SN = 2: Linear (e.g., CO2, angle 180°)

• SN = 3: Trigonal planar (e.g., BF3, angle 120°)

  • Can be bent if a lone pair (e.g., NO2−).

• SN = 4: Tetrahedral (e.g., CCl4, angle 109.5°)

  • Can be trigonal pyramidal or bent with lone pairs (e.g., PCl3, OF2).

• SN = 5: Trigonal bipyramidal (e.g., PF5, angles 180° axial, 120° equatorial); geometries include see-saw, T-shaped, linear.

• SN = 6: Octahedral (e.g., SF6, angle 90°); includes square pyramidal and square planar geometries.

Hybridization and Bonding:

• Orbitals mix to form hybrids for better overlap in bonds (Valence Bond Theory).

• Hybridization types: sp3 (e.g., CH4), sp2 (e.g., C3H6), sp (e.g., C2H2).

Bonding Theories:

• Sigma (σ) bonds: head-on overlaps along bond axis.

• Pi (π) bonds: side-by-side overlaps above/below bond axis.

Practice: Predict hybridizations and bond types from Lewis structures and electron domains.