Molecular Geometry Lecture Notes

Importance of Molecular Geometry

• Understanding molecular geometry is crucial because it affects how molecules interact and perform in chemical reactions.

VSEPR Model

• VSEPR: Valence Shell Electron Pair Repulsion.
• Used to predict the shape of a molecule.

Electron Cloud Repulsion

• Electron clouds around atoms repel each other, leading to specific molecular geometries.
• Molecules arrange themselves to minimize energy by maximizing the distance between electron clouds.

Hybridization and Molecular Geometry

• Hybridization: Mixing of atomic orbitals to form new hybrid orbitals.
• Correlates with electron domain geometry.

Electron Domains and Hybridization

• Count electron domains to determine hybridization:
  • 1s, 3p, 5d: Different orbitals available at energy levels.

Example Molecules

1. Carbon Dioxide (CO2)

   • 2 electron domains.
   • sp hybridized.
   • Linear geometry: 180-degree bond angle.

2. Boron Trifluoride (BF3)

   • 3 electron domains.
   • sp2 hybridized.
   • Trigonal planar geometry: 120-degree bond angles.

3. Methane (CH4)

   • 4 electron domains.
   • sp3 hybridized.
   • Tetrahedral geometry: 109.5-degree bond angles.

4. Trigonal Bipyramidal (sp3d)

   • 5 electron domains.
   • Geometry: Trigonal bipyramidal.
   • Bond angles: 90 and 120 degrees.

5. Octahedral (sp3d2)

   • 6 electron domains.
   • Geometry: Octahedral.
   • Bond angles: 90 degrees.

Lone Pairs and Molecular Geometry

• Lone pairs count as electron domains but affect molecular shape differently.

Examples

1. Ammonia (NH3)

   • 3 bonds, 1 lone pair.
   • sp3 hybridized.
   • Tetrahedral electron domain geometry.
   • Trigonal pyramidal molecular geometry.

2. Water (H2O)

   • 2 bonds, 2 lone pairs.
   • sp3 hybridized.
   • Tetrahedral electron domain geometry.
   • Bent molecular geometry.

Summary

• Number of electron domains (bonds + lone pairs) determines hybridization and geometry.
• Draw Lewis dot structures to count electron domains and predict molecular shapes.

Conclusion

• Understanding the concepts of electron domain and molecular geometry is crucial for predicting the shapes of molecules and their chemical behavior.