VSEPR Theory and Molecular Geometry

VSEPR Theory Overview

  • VSEPR (Valence Shell Electron Pair Repulsion) theory helps predict the geometry of molecules based on the number of electron pairs surrounding a central atom.

Electron Group Arrangement

  • Linear (2 groups)

    • Molecular shape: Linear

    • Example: AX_2

    • Bond angle: 180°

  • Trigonal Planar (3 groups)

    • Molecular shape: Trigonal Planar

    • Example: AX_3

    • Bond angle: 120°

  • Tetrahedral (4 groups)

    • Molecular shape: Tetrahedral

    • Example: AX_4

    • Bond angle: 109.5°

    • Other shapes under 4 groups:

      • V shaped (or bent) - AX2E2

      • Trigonal pyramidal - AX_3E

      • Bond angles are usually less than the ideal angle due to lone pairs.

Bond Angles in Tetrahedral Arrangements

  • Bonds and angles:

    • V shaped or bent (due to lone pairs): <109.5°

    • Trigonal pyramidal: <109.5°

Additional Electron Groupings

  • Trigonal Bipyramidal (5 groups)

    • Molecular Shapes:

    • Trigonal Bipyramidal - AX_5

    • Seesaw - AX_4E

    • T-shaped - AX3E2

    • Linear - AX2E3

    • Bond Angles:

    • Axial: 90°

    • Equatorial: 120°

    • Other angles can be lower due to lone pairs.

  • Octahedral (6 groups)

    • Molecular Shapes:

    • Octahedral - AX_6

    • Square pyramidal - AX_5E

    • Square planar - AX4E2

    • Bond Angles:

    • Axial to equatorial: 90°

    • Linear from angular positions: 180°

    • Other angles may vary with the presence of lone pairs.

Key Takeaway

  • The geometry of a molecule can be predicted by determining the number of bonding and nonbonding pairs of electrons around a central atom, leading to the arrangement of atoms and resulting bond angles.