Chapter 5 Study Notes - Bonding Theories and Molecular Geometry

Chapter 5: Bonding Theories: Explaining Molecular Geometry

5.1 Molecular Shape

  • Molecular shape significantly influences the function of molecules, both small and large.

5.2 Valence-Shell Electron-Pair Repulsion Theory (VSEPR)

  • Definition: Electron domains (bonding pairs and lone pairs) around a central atom are arranged to minimize repulsions, maximizing spatial separation.

    • When predicting molecular shapes using VSEPR:

    1. Draw the Lewis Structure.

    2. Determine the steric number (SN):
      SN=extnumberofatomsattachedtocentralatom+extnumberoflonepairsoncentralatomSN = ext{number of atoms attached to central atom} + ext{number of lone pairs on central atom}

    3. Determine Electron-Pair (Group) Geometry (EG): This is the 3D arrangement of bonding pairs and lone pairs around the central atom.

    4. Determine Molecular Geometry (MG): This is the 3D arrangement of the atoms in the molecule.

5.2.1 Bond Angles

  • Bond Angle: The angle defined by lines joining the centers of two atoms to a third atom to which they are bonded.

  • Examples include:

    • Carbon Dioxide (CO₂):

    • Lewis Structure: :O=C=O

    • Bond Angle: 180°.

    • Methane (CH₄):

    • Lewis Structure: H-C-H,

    • Bond Angle: 109.5°.

5.2.2 Central Atoms with No Lone Pairs

  • When there are no lone pairs, the molecular geometry (MG) will match the electron-pair geometry (EG).

    • Example: For a linear structure with an SN of 2, both MG and EG are linear with a 180° bond angle.

5.2.3 Central Atoms with Lone Pairs

  • Lewis Structure Determination: The presence of lone pairs leads to distortions from ideal geometries, affecting the bond angles.

  • Example:

    • Trigonal-bipyramidal Geometry: Involves two bond angles (120°, 90°).

5.2.4 The Effect of Lone Pairs

  • Electron-pair geometry: determined by the total number of electron pairs (bonding + lone), significantly influencing molecular geometry.

    • Lone pairs exert a greater repulsive force on adjacent bonding pairs than bonding pairs do between themselves.

5.2.5 Table of Electron-Pair Geometries and Molecular Geometries

  • SN = 3:

    • Trigonal Planar: 3 bonded atoms, 0 lone pairs; angles 120° (EG & MG).