Lecture on Visualizing Chemical Bonds and Molecules in Three Dimensions

Introduction

• Presenter: Jeremy Krug
• Topic: Visualizing chemical bonds and molecules in three dimensions

Types of Chemical Bonds

Single Bonds

• Consist of two electrons shared straight between two atoms.

Double Bonds

• Have two bonds:
  • First bond is similar to a single bond, shared in the middle between atoms.
  • Second bond is looped, indicating a different configuration.

Triple Bonds

• Composed of three bonds:
  • One straight bond.
  • Two looped bonds, one of which comes out in the third dimension.

Bond Types: Sigma and Pi

Sigma Bonds (σ)

• Represented by the Greek letter σ (lowercase Sigma).
• Present in every type of bond:
  • Single bond: one sigma bond.
  • Double bond: one sigma and one pi bond.
  • Triple bond: one sigma and two pi bonds.
• Result from overlapping s orbitals (spherical shape).

Pi Bonds (π)

• Represented by the Greek letter π (lowercase Pi).
• Result from overlapping p orbitals (looped shape).

Bond Lengths

• Single bonds: Longest.
• Double bonds: Intermediate length.
• Triple bonds: Shortest.

Counting Sigma and Pi Bonds

• Example with HCN:
  • Single bond = 1 sigma.
  • Triple bond = 1 sigma and 2 pi.
  • Total: 2 sigma and 2 pi bonds.
• Complex Molecule Example:
  • Six single bonds (sigmas).
  • Two double bonds (each with one sigma and one pi).
  • One triple bond (one sigma and two pis).
  • Total: 9 sigma and 4 pi bonds.

Hybridization of Orbitals

Understanding Hybridization

• Example with Methane (CH₄):
  • Carbon forms four sigma bonds despite having only four valence electrons.
  • Undergoes hybridization to form sp³ hybrid orbitals.

Determining Hybridization

• Sum the number of sigma bonds and unshared electron pairs touching an atom to find the steric number.
• Steric Number and Hybridization:
  • 2: sp
  • 3: sp²
  • 4: sp³
  • 5: sp³d
  • 6: sp³d²
• Example Calculations:
  • HCN: 2 sigma bonds, zero unshared pairs → steric number 2 → sp hybridization.
  • Water (H₂O): 2 sigma bonds, 2 unshared pairs → steric number 4 → sp³ hybridization.
  • Nitrate Ion (NO₃⁻): 3 sigma bonds, zero unshared pairs → steric number 3 → sp² hybridization.

AP Chemistry Notes

• AP exams cover hybridizations up to sp³.

Conclusion

• The lecture covered visualization of bonds, types of bonds, bond lengths, counting bonds, and hybridization.
• Next focus on molecular geometry and bond angles.
• For further learning, engage with the AP Chemistry material or follow the next sections of the unit.