Final answer:
Stable molecules usually do not have their highest-energy electrons in antibonding orbitals, as these are high in energy and lead to instability in the molecule. The ground state of H₂ is an example where the bonding orbital is occupied and the antibonding orbital is unoccupied, reflecting typical stable molecular structures.
Step-by-step explanation:
Stable molecules typically do not have their highest-energy electrons in antibonding orbitals. Instead, these molecular orbitals are higher in energy and are associated with instability in the bonding of atoms within a molecule. Antibonding orbitals, designated as sigma-star (σ*) or pi-star (π*), are always higher in energy compared to the parent atomic orbitals or bonding molecular orbitals.
When atomic orbitals combine, they form molecular orbitals where the bonding orbital is stabilized and the antibonding orbital is destabilized. The interaction between atomic orbitals is greatest when they are of similar energy and the correct spatial orientation for overlap. Bonding molecular orbitals, such as the bonding σ (sigma) orbitals, are lower in energy and contribute to the stabilization of the molecule because electrons in these orbitals are attracted by both atomic nuclei. In contrast, electrons in antibonding orbitals, such as σ*, are found in regions of space that do not enhance the attractive interactions between atoms and can actually contribute to repulsion between them.
Molecular hydrogen (H₂) serves as a classic example, where the ground state configuration has electrons paired in the lower-energy bonding orbital, while the antibonding orbital remains unoccupied. In the molecular orbital (MO) theory, the presence of electrons in antibonding orbitals cancels out the stabilization from bonding orbitals, potentially leading to unstable structures with a bond order of zero, meaning these typically do not exist in nature as stable entities. Therefore, the highest occupied molecular orbital (HOMO) is normally a bonding orbital, and the lowest unoccupied molecular orbital (LUMO) is the antibonding orbital.