Lecture Notes: Drawing Dot Structures and VSEPR Theory

Introduction to VSEPR Theory

• VSEPR stands for Valence Shell Electron Pair Repulsion.
• Valence electrons repel each other and determine the molecule's geometry.

Boron Trifluoride (BF₃) Dot Structure

• Valence Electrons Calculation:
  • Boron in group 3: 3 valence electrons.
  • Fluorine in group 7: 7 valence electrons each.
  • Total: 3 (Boron) + 21 (Fluorine) = 24 valence electrons.
• Dot Structure:
  • Boron is the central atom, less electronegative than fluorine.
  • Bonded to three fluorine atoms, using 6 valence electrons.
  • Remaining 18 electrons fill the octet for each fluorine.
• Formal Charge:
  • Boron has a formal charge of 0, even though it does not follow the octet rule.
• Molecular Geometry:
  • Determine the number of electron clouds around the central atom.
  • Results in a trigonal planar shape with 120° bond angles.

Sulfur Dioxide (SO₂) Dot Structure

• Valence Electrons Calculation:
  • Sulfur in group 6: 6 valence electrons.
  • Oxygen in group 6: 6 valence electrons each.
  • Total: 6 (Sulfur) + 12 (Oxygen) = 18 valence electrons.
• Dot Structure:
  • Sulfur is the central atom, less electronegative than oxygen.
  • Bonded to two oxygen atoms, using 4 valence electrons.
  • Remaining electrons assigned to oxygens to fill octet.
  • Sharing electrons between sulfur and oxygen to minimize formal charge.
• Resonance Structures:
  • Possibility of different resonance structures.
• Molecular Geometry:
  • Three electron clouds (including a lone pair) around sulfur.
  • Trigonal planar geometry of electron clouds.
  • Lone pairs affect the final shape, resulting in a bent or angular shape of the molecule with bond angles slightly less than 120°.

Key Takeaways

• VSEPR Theory: Determines molecular geometry based on electron repulsions.
• Electron Clouds: Include bonding and non-bonding electrons.
• Geometry Prediction: Consider electron clouds and ignore lone pairs for final molecular shape.
• Formal Charge: Important to check for minimizing it in dot structures.
• Resonance Structures: Provide different possible structures for certain molecules.