What does HOMO stand for in molecular orbital theory?

HOMO stands for Highest Occupied Molecular Orbital in molecular orbital theory. This term is critical as it identifies the most energetic molecular orbital that contains electrons within a molecule. The highest occupied molecular orbital plays an important role in determining the chemical properties and reactivity of a molecule. It helps in understanding electron transitions during chemical reactions and absorption of light, as the electrons in the HOMO can be promoted to higher energy levels, such as the Lowest Unoccupied Molecular Orbital (LUMO), during such processes.

In contrast, the other definitions provided do not accurately describe HOMO. "Highest orbital molecular energy" does not reflect the concept of occupancy, while "Hybrid orbital mixture" pertains to hybridization in valence bond theory rather than molecular orbitals. "High orbital molecular overlap" similarly relates more to the interaction between atomic orbitals during bond formation than to the specific identification of an occupied molecular orbital. This makes the understanding of HOMO essential for the application of molecular orbital theory in predicting chemical behavior and properties.