CEM141 Recitation #11

CEM 141 Recitation 11: Key Concepts in Isomers, Valence Bond Theory, and VSEPR

1. Isomers and Molecular Structures

• Question: Are the following molecules the same or isomers?

• Structures: Molecule comparisons

Same vs Isomers

• Same: The –OH group is positioned on the 2nd carbon in a 4-carbon chain.

  • Explanation: Molecular structure shows identical connectivity of atoms.

• Isomers: Molecules with the same molecular formula but different structures; they can exhibit different chemical properties.

  • Example: Molecules with double bonds can vary in geometry (cis/trans).

Rotation in Bonds

• Single (sigma) bonds:

  • Explanation: Formed by the end-to-end overlap of atomic or hybrid orbitals. The bond can freely rotate, as rotation does not disrupt the overlap.

• Double bonds:

  • Definition: Comprises one sigma bond and one pi bond.

  • Pi bonds: Formed by the side-to-side overlap of parallel p-orbitals, providing bonding above and below the plane of the nuclei.

  • Rotation Limitation: If rotation occurs, the overlap of the p-orbitals would break, thus breaking the bond.

2. Molecular Geometry and Shapes

Tetrahedral Electron Center Geometry

• Construction: Using a molecular model kit to build a tetrahedral model.

• Electron Centers: Can be any combination of lone pairs, single, double, or triple bonds.

• Possible Molecular Shapes:

  • Tetrahedral

  • Trigonal pyramidal

  • Bent

Trigonal Planar Electron Center Geometry

• Construction: Build a trigonal planar model with the molecular model kit.

• Possible Molecular Shapes:

  • Trigonal planar

  • Bent

Comparison: Trigonal Planar vs Trigonal Pyramidal

• Trigonal Planar:

  • Description: Flat molecular shape.

  • Bond Angle: 120 degrees between all electron centers.

• Trigonal Pyramidal:

  • Description: Pyramid-like structure.

  • Bond Angle: Approximately 109 degrees; the fourth electron center repels the three others, creating a pyramid shape.

  • Illustration:

    • Trigonal Planar:
      

      

    • Trigonal Pyramidal:
      

      

3. Electron Pair Geometry and Molecular Shapes Chart

• Instruction: Fill in the chart considering central atom properties and use models for visualization.

• Note: Utilize hands-on models and the PhET simulation application to visualize these structures in 3D. Show your models to the TA for verification before proceeding further.