Understanding Molecular Geometries with VSEPR Theory

Nov 10, 2024

VSEPR Theory and Molecular Geometries

Introduction to VSEPR Theory

  • VSEPR Model: A tool used to predict the shape of molecules based on the repulsion of valence shell electron pairs.
  • Valence Shell Electrons: Electrons on the outermost shell of an atom, generally come in pairs.
  • Electron Repulsion: Electrons repel each other, leading to the arrangement of atoms that minimize repulsion.

Major Molecular Geometries

Linear Molecular Geometry

  • Conditions: Steric number of 2 with no lone pairs.
  • Example: CO2 has two atoms bonded to a central carbon atom, with a bond angle of 180°.
  • Note: Type of bond (single, double, triple) does not affect linear geometry.

Trigonal Planar Molecular Geometry

  • Conditions: Steric number of 3 with no lone pairs.
  • Example: BF3 with three fluorine atoms bonded to a central boron atom, bond angles of 120°.

Bent Molecular Geometry

  • Conditions: Steric number of 3 with two atoms and one lone pair.
  • Example: SO2 with two oxygen atoms and a lone pair on central sulfur.
  • Lone Pairs: Influence shape by repelling bonded atoms.

Tetrahedral Molecular Geometry

  • Conditions: Steric number of 4 with four atoms and no lone pairs.
  • Example: CH4 with four hydrogen atoms bonded to a central carbon.
  • Bond Angle: Approximately 109.5°.

Trigonal Pyramidal Molecular Geometry

  • Conditions: Steric number of 4 with three atoms and one lone pair.
  • Example: NH3 (ammonia) with three hydrogen atoms and one lone pair on nitrogen.

Bent Molecular Geometry (Water Example)

  • Conditions: Steric number of 4 with two atoms and two lone pairs.
  • Example: H2O, leading to a bent shape due to lone pairs.
  • Bond Angle: General model for bent geometry.

Advanced Geometries

Trigonal Bipyramidal Molecular Geometry

  • Conditions: Steric number of 5 with five atoms and no lone pairs.

Octahedral Molecular Geometry

  • Conditions: Steric number of 6 with six atoms.

Practice and Application

  • Practice: Drawing Lewis structures, identifying steric numbers, and predicting molecular geometry.
  • Visualization: Envisioning 3D structures based on steric number and lone pairs.

Conclusion

  • Understanding: Developing an intuitive sense of molecular shapes using VSEPR theory.
  • Application: Using Lewis structures and steric numbers to predict molecular geometries.

This lecture was presented by Dr. B, explaining the VSEPR model for understanding molecular shapes.

Full transcript