Molecular Orbital Theory — Core Principles

Core Principles

Molecular Orbital Theory (MOT) describes chemical bonding by forming molecular orbitals (MOs) from the linear combination of atomic orbitals (AOs). Unlike VBT, MOT considers electrons to be delocalized over the entire molecule.

When AOs combine, they form an equal number of MOs: bonding molecular orbitals (BMOs), which are lower in energy and stabilize the molecule, and antibonding molecular orbitals (ABMOs), which are higher in energy and destabilize it.

The combination requires AOs of comparable energy, proper symmetry, and significant overlap. Electrons fill these MOs according to the Aufbau principle, Pauli exclusion principle, and Hund's rule. The 'bond order' is calculated as half the difference between bonding and antibonding electrons ($BO = \frac{1}{2}(N_b - N_a)$), indicating molecular stability and bond strength.

MOT successfully explains magnetic properties (paramagnetism/diamagnetism) and the existence/non-existence of various diatomic species, such as the paramagnetism of $O_2$ and the non-existence of $He_2$.

The energy order of MOs varies for lighter ($B_2, C_2, N_2$) versus heavier ($O_2, F_2$) diatomic molecules due to s-p mixing.

Important Differences

vs Valence Bond Theory (VBT)

Aspect	This Topic	Valence Bond Theory (VBT)
Electron Localization	Electrons are localized between two atoms, forming specific bonds.	Electrons are delocalized over the entire molecule, occupying molecular orbitals.
Orbital Formation	Atomic orbitals overlap to form hybrid orbitals, which then form bonds.	Atomic orbitals combine (LCAO) to form new molecular orbitals.
Nature of Orbitals	Atomic orbitals retain their identity to a large extent, forming localized bonds.	Atomic orbitals lose their individual identity, forming polycentric molecular orbitals.
Magnetic Properties	Often fails to explain magnetic properties (e.g., predicts $O_2$ as diamagnetic).	Accurately predicts magnetic properties (e.g., explains paramagnetism of $O_2$).
Bond Order	Typically predicts integer bond orders (single, double, triple).	Can predict integer or fractional bond orders, providing a more nuanced view.
Stability of species	Struggles with species like $H_2^+$ or $He_2$.	Clearly explains stability based on bond order (e.g., $H_2^+$ exists, $He_2$ doesn't).

While both Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT) describe chemical bonding, they approach it from fundamentally different perspectives. VBT focuses on localized electron pairs formed by overlapping atomic orbitals, often employing hybridization to explain molecular geometry. In contrast, MOT considers electrons to be delocalized across the entire molecule, residing in molecular orbitals formed by the linear combination of atomic orbitals. This delocalized view allows MOT to more accurately predict magnetic properties, such as the paramagnetism of oxygen, and explain the stability of various diatomic species, including those with fractional bond orders, which VBT struggles with.