Molecular Orbital Energy Diagram for He₂

Molecular Orbital Theory

  • Definition of Molecular Orbital Theory:

    • Molecular Orbital (MO) Theory describes the behavior of electrons in a molecule by considering the combination of atomic orbitals to form molecular orbitals.

  • Diatomic Molecule Concept:

    • A diatomic molecule consists of two atoms. For example, helium (He) forms the diatomic molecule He₂.

Molecular Orbital Energy Diagram for He₂

  • Steps to Construct the Molecular Orbital Energy Diagram:

    1. Identify Atomic Orbitals:
    • Each helium atom has the following atomic orbitals:
      • $1s$ orbital
      • $2s$ orbital
    1. Combine Atomic Orbitals:
    • The atomic orbitals combine to form molecular orbitals.
      • Each $1s$ atomic orbital from both atoms will combine to form a bonding molecular orbital ($ ext{MO}{1s}$) and an antibonding molecular orbital ($ ext{MO}{1s}^*$).
      • The same occurs for the $2s$ orbitals where a bonding ($ ext{MO}{2s}$) and antibonding ($ ext{MO}{2s}^*$) orbital are formed.

  • Energy Level Arrangement:

    • In the case of He₂, the order of energy levels is as follows:
    • The MO energy diagram for helium can be depicted as:
      • $ ext{MO}{1s}$ (bonding) < $ ext{MO}{1s}^$ (antibonding) < $ ext{MO}{2s}$ (bonding) < $ ext{MO}{2s}^$ (antibonding)

  • Filling Molecular Orbitals:

    • Helium has an atomic number of 2, meaning it has 2 total electrons that need to be placed in molecular orbitals.
    • The electrons are filled starting from the lowest energy level:
    • Both electrons will occupy the $ ext{MO}_{1s}$ bonding orbital.

  • Unoccupied Orbital:

    • The $ ext{MO}{1s}^*$ (antibonding) and $ ext{MO}{2s}$ (bonding) orbitals remain unoccupied in He₂.

Summary of Observations

  • He₂ Formation:
    • He₂ is chemically stable because it has a filled bonding molecular orbital, which indicates bonding interactions between the two helium atoms.
  • Energy Representation:
    • The molecular orbital diagram visually represents the relative energies of the MOs, indicating energetics that affect molecular stability.

Conclusion

  • Overview:
    • The molecular orbital energy diagram for He₂ is essential for understanding its bonding characteristics and stability.
    • He₂ has 2 bonding electrons in the $ ext{MO}_{1s}$ with higher energy orbitals being unoccupied, which emphasizes its simple diatomic structure.