In molecular orbital theory (MOT), the atomic orbitals of different atoms combine to form molecular orbitals (MOs), which are spread out over the entire molecule. This mixing of atomic orbitals occurs because the wave functions of the atomic orbitals overlap in space.
When atoms come together to form a molecule, their atomic orbitals interact and combine to form bonding and antibonding molecular orbitals. The bonding molecular orbitals are lower in energy and stabilize the molecule, while the antibonding molecular orbitals are higher in energy and are less stable.
In MOT, all atomic orbitals, including both core and valence orbitals, can mix together to form molecular orbitals. However, the key factor that determines the nature of the chemical bond is the interaction of the valence orbitals, which are the outermost orbitals involved in bonding. Valence orbitals are the most important because they are the ones primarily involved in the formation of chemical bonds.
The mixing of atomic orbitals leads to the formation of molecular orbitals with different energy levels and spatial distributions. The number of molecular orbitals formed is equal to the number of atomic orbitals involved in the mixing process. The molecular orbitals can be filled with electrons following the Aufbau principle and the Pauli exclusion principle.
Bonding occurs when the molecular orbitals formed by the valence orbitals have a lower energy than the original atomic orbitals. Electrons fill the lower-energy bonding orbitals, resulting in a more stable molecule. In contrast, antibonding orbitals have higher energy and are less stable, so they are typically unoccupied or only partially filled.
Overall, while all atomic orbitals can mix in MOT, it is the valence orbitals that play the most significant role in forming chemical bonds because they are the outermost orbitals involved in bonding interactions.