Lecture Notes: Steric Number and Molecular Geometry

Key Concepts

• Steric Number: Useful in determining the number of hybridized orbitals in a molecule.
  • Calculate by adding the number of sigma bonds (single bonds) and the number of lone pairs of electrons.

Methane (CH₄)

• Steric Number Calculation:
  • Carbon has 4 sigma bonds (C-H single bonds).
  • No lone pairs.
  • Steric Number = 4 + 0 = 4.
• Hybridization:
  • SP³ hybridized: Involves 1 s orbital and 3 p orbitals to form 4 SP³ hybrid orbitals.
• Geometry:
  • Electron Pair Geometry: Tetrahedral.
  • Molecular Geometry: Tetrahedral.
  • Bond Angle: Approximately 109.5 degrees.

Ammonia (NH₃)

• Steric Number Calculation:
  • Nitrogen has 3 sigma bonds (N-H single bonds).
  • 1 lone pair of electrons.
  • Steric Number = 3 + 1 = 4.
• Hybridization:
  • SP³ hybridized.
• Geometry:
  • Electron Pair Geometry: Tetrahedral.
  • Molecular Geometry: Trigonal-pyramidal.
  • Bond Angle: Approximately 107 degrees (reduced due to lone pair repulsion).

Water (H₂O)

• Steric Number Calculation:
  • Oxygen has 2 sigma bonds (O-H single bonds).
  • 2 lone pairs of electrons.
  • Steric Number = 2 + 2 = 4.
• Hybridization:
  • SP³ hybridized.
• Geometry:
  • Electron Pair Geometry: Tetrahedral.
  • Molecular Geometry: Bent or angular.
  • Bond Angle: Approximately 105 degrees (further reduced due to two lone pairs).

VSEPR Theory

• Valence Shell Electron Pair Repulsion (VSEPR) Theory:
  • Electron pairs repel each other and arrange themselves as far apart as possible.
  • Determines the shape and bond angles of molecules.

General Observations

• SP³ hybridization is common in molecules with a steric number of 4.
• The presence of lone pairs affects bond angles and molecular geometry.
• Understanding hybridization and steric numbers helps predict molecule shapes and their chemical behavior.