In molecular orbital theory (MOT), the atomic orbitals of different atoms combine to form molecular orbitals (MOs). When atomic orbitals combine, they interact and mix together to create bonding and antibonding molecular orbitals. The atomic orbital with lower energy makes a larger contribution to the bonding molecular orbital due to two main factors: overlap and energy matching.
- Overlap: The extent of overlap between atomic orbitals determines the strength of their interaction. When two atomic orbitals overlap, their electron densities combine, leading to the formation of a molecular orbital. The extent of overlap is influenced by the shape and size of the atomic orbitals involved.
Atomic orbitals with lower energy, such as the 1s orbital, tend to be smaller and more concentrated around the nucleus compared to higher-energy orbitals. This concentrated electron density allows for a more effective overlap with other atomic orbitals, leading to a stronger interaction and larger contribution to the bonding molecular orbital.
- Energy Matching: The energy of an atomic orbital determines its compatibility with other orbitals. In bonding, the atomic orbitals must have similar energies to form a stable molecular orbital. When the energies of two atomic orbitals are well-matched, their mixing is more favorable, resulting in a greater contribution to the bonding molecular orbital.
Atomic orbitals with lower energy are generally closer in energy to each other, facilitating a better energy match. This energy match enhances the stability of the bonding molecular orbital and promotes a larger contribution from the lower-energy atomic orbital.
Overall, the combination of effective overlap and favorable energy matching between atomic orbitals results in the lower-energy orbital making a larger contribution to the bonding molecular orbital. This contribution leads to a more stable bonding interaction, thereby favoring the formation of a chemical bond.