Valence Bond Theory vs. Molecular Orbital Theory — What's the Difference?

Valence Bond Theory and Molecular Orbital Theory are both foundational concepts in understanding molecular structure and behavior. Valence Bond Theory (VBT) posits that a chemical bond results from the overlap of half-filled atomic orbitals from individual atoms. This overlap leads to the pairing of electrons from the participating atoms. Thus, according to Valence Bond Theory, molecules are formed when atomic orbitals come close enough to share electrons.

Molecular Orbital Theory (MOT), on the other hand, delves deeper into the quantum mechanical nature of molecules. Instead of simply overlapping atomic orbitals, the Molecular Orbital Theory suggests that atomic orbitals merge or combine to produce new orbitals called molecular orbitals. These molecular orbitals span the entire molecule and can be either bonding (where electrons are likely found between the atoms) or antibonding (where electrons are more likely to be found outside the space between the atoms).

A noticeable distinction between Valence Bond Theory and Molecular Orbital Theory lies in their descriptions of electron placement. In VBT, electrons remain localized between the two bonding atoms, while in MOT, electrons can exist in orbitals that encompass the entire molecule. This makes MOT better equipped to describe phenomena like delocalized electrons in some molecules.

Another significant differentiation is in their treatment of molecular shapes. Valence Bond Theory focuses on the hybridization of atomic orbitals to explain molecular shapes, suggesting that atomic orbitals merge to create equivalent hybrid orbitals. In contrast, Molecular Orbital Theory doesn't concern itself with hybridization; it looks at how atomic orbitals combine to form molecular orbitals, which can be bonding or antibonding.