Molecular Orbitals Lecture Notes

Introduction to Molecular Orbitals

• Atomic Orbitals: Atoms possess atomic orbitals that can hybridize to form molecular orbitals.
• Hybridized Orbitals: Derived shapes resemble combinations of atomic orbitals used in covalent bonds.

Example: Hydrogen Molecule (H2)

• Each hydrogen atom has one electron in the 1s orbital.
• To form a sigma bond:
  • Atomic orbitals overlap to create one molecular orbital.
  • Both electrons reside in this new orbital.
• Energy Consideration:
  • H2 is at lower energy than two separate hydrogen atoms due to increased electrostatic interactions.
  • This leads to the spontaneous formation of diatomic hydrogen molecules.
• Orbital Conservation:
  • Two atomic orbitals lead to:
    • One bonding orbital
    • One antibonding orbital (notated as sigma star)
• Electrons occupy the lower energy bonding orbital, while excited electrons can move to the antibonding orbital.

Hybridization and Bonding

• Hybridization: Necessary for forming multiple bonds.
• Carbon Example:
  • Carbon typically forms four bonds by using its 2s and 2p orbitals.
  • To hybridize:
    • One 2s electron is promoted to a vacant 2p orbital.
    • Creates four sp3 hybridized orbitals (degenerate).
• Types of Hybridization:
  • sp3 Hybridization (4 sigma bonds)
  • sp2 Hybridization (3 electron domains with a double bond)
    • Example: C2H4 (ethylene)
      • 1 electron is promoted, uses 2s and 2p to create 3 sp2 orbitals. One p orbital remains unhybridized.
      • The unhybridized p orbitals overlap to create a pi bond.
  • sp Hybridization (2 electron domains with a triple bond)
    • Example: C2H2 (acetylene)
      • 2 electrons are promoted, uses 2s and one 2p to create 2 sp orbitals.
      • Two unhybridized p orbitals form two pi bonds.

Orbital Diagrams and Bond Order Calculation

• Orbital Diagrams: Visual representation of atomic and molecular orbitals.

  • Example with H2:
    • Each hydrogen has 1 electron in 1s orbital.
    • Fill bonding orbital:
      • Bond order = (electrons in bonding orbitals - electrons in antibonding orbitals) / 2
      • For H2: 1 (single covalent bond).

• Oxygen Molecule (O2):

  • Each oxygen has 4 electrons in the 2p orbitals.
  • Fill molecular orbitals:
    • Bond order = (10 - 6) / 2 = 2 (double bond).

• Nitrogen Molecule (N2):

  • Bond order calculation = 3 (triple bond).

Conclusion

• Understanding molecular orbitals and hybridization allows prediction of covalent bonding behavior.
• Encouragement to subscribe for more content and engage with questions.