VSEPR Theory: Introduction

Introduction to VSEPR Theory

• VSEPR: Valence Shell Electron Pair Repulsion theory used to predict 3D shapes of molecules from Lewis structures.

Key Concepts

Electron Repulsion

• Electrons push away from each other due to negative charge; they arrange to maximize distance (repel).

• Covalent bonds can be visualized as pairs of electrons shared between atoms.

Molecular Shapes from VSEPR

Linear Shape

• Example: Bromine dichloride

  • The linear arrangement of bonds: 180° angle between bonds.

• Applies to molecules with two bonds (e.g., CO2, HCN).

Trigonal Planar Shape

• Example: Boron trifluoride (BF3)

  • Central atom: Boron (B) with three Fluorines (F)

  • Bonds arranged in a plane with 120° angles.

• Double bonds do not alter the shape (e.g., CH2O).

Bent Shape

• Example: Sulfur dioxide (SO2)

  • Contains a bond and an unshared electron pair.

  • Molecule deviates from linear, resulting in a bent structure:

    • Angle < 120° due to repulsion from the unshared pair.

Tetrahedral Shape

• Example: Methane (CH4)

  • Central atom with four bonds; arranged to minimize repulsion at 109.5° angles.

• Example: Ammonia (NH3)

  • Similar to tetrahedral but with one unshared pair, results in trigonal pyramidal shape (bond angle ≈ 107°).

Bent Shape with Two Lone Pairs

• Example: Water (H2O)

  • Four regions: two bonds and two lone pairs.

  • Results in a bent shape with angles ≈ 105°, tighter than trigonal pyramidal.

Additional Considerations

• Distinction between bent shapes:

  • 3 regions with one lone pair yield angles < 120°.

  • 4 regions with two lone pairs yield angles < 109.5°.

Conclusion

• Building a solid foundation in VSEPR involves understanding electron arrangements and molecular geometry through practice and further study.