Unit 4c - VSEPR

VSEPR Model and Molecular Geometry

1. Basic Premises of VSEPR

  • Objective: Understand that a molecule's geometry is dictated by maximizing the space among electron pairs around the central atom.

    • Electron pairs repel each other, leading to specific geometric arrangements.

    • Non-bonding (unshared) electron pairs require more space than bonding (shared) pairs due to their spread.

    • Multiple bonds are considered as a single electron pair for spatial purposes.

2. The 5 Basic Geometries

  • Linear Geometry (2 electron pairs): 180° bond angle (e.g., BeF2, CO2).

  • Trigonal Planar Geometry (3 electron pairs): 120° bond angle (e.g., BF3, formaldehyde).

  • Tetrahedral Geometry (4 electron pairs): 109.5° bond angle (e.g., CH4).

  • Trigonal Bipyramidal Geometry (5 electron pairs): 90° and 120° bond angles (e.g., PF5).

  • Octahedral Geometry (6 electron pairs): 90° bond angle (e.g., SF6).

3. Arrangement of Electron Pairs

  • Different arrangements based on the number of electron pairs:

    • 2 pairs - Linear (180°)

    • 3 pairs - Trigonal planar (120°)

    • 4 pairs - Tetrahedral (109.5°)

    • 5 pairs - Trigonal bipyramidal (90°/120°)

    • 6 pairs - Octahedral (90°)

4. The VSEPR Algorithm

  • Steps to Determine Geometry:

    1. Draw the Lewis structure to visualize electron pairs.

    2. Count the total electron pairs around the central atom to determine basic geometry.

    3. Assess unshared electron pairs, noting that they repel more strongly than bonding pairs.

    4. Select the specific molecular geometry and predict bond angles.

5. Specific Molecular Geometries

  • Linear (2 bonding pairs, 0 lone pairs): e.g., BeF2

  • Trigonal Planar (3 bonding pairs, 0 lone pairs): e.g., BF3

  • Bent (or Angular) (2 bonding pairs, 1 lone pair): e.g., SO2

  • Tetrahedral (4 bonding pairs, 0 lone pairs): e.g., CH4

  • Trigonal Pyramidal (4 bonding pairs, 1 lone pair): e.g., NH3

  • Bent (or Angular) (2 bonding pairs, 2 lone pairs): e.g., H2O

  • Trigonal Bipyramidal (5 bonding pairs, 0 lone pairs): e.g., PF5

  • Seesaw (4 bonding pairs, 1 lone pair): e.g., SF4

  • T-shaped (3 bonding pairs, 2 lone pairs): e.g., ClF3

  • Square Planar (4 bonding pairs, 2 lone pairs): e.g., XeF4

6. Effect of Unshared Electron Pairs on Bond Angles

  • The presence of lone pairs can distort bond angles:

    • CH4 (109.5°)

    • CH3Cl (110°)

    • NH3 (107°)

    • H2O (105°)

7. VSEPR Examples

  • Predict geometries and bond angles for:

    • CH4, NH3, H2O, HF

8. Limitations of the VSEPR Model

  • Bond Equivalence: Does not account for bonded atoms having differing orbital contributions.

  • Multiple Bonds: Fails to adequately explain bond strength between single and double bonds.

  • Resonance: Cannot justify resonance structures in compounds.

  • Magnetism: Does not address para- and diamagnetism.

  • Conclusion: While VSEPR is useful for simplicity in geometrical predictions, more sophisticated models are needed for comprehensive understanding.