##  [Molecular Orbital](/molecular-orbital-0) 

 Definition

A quantum‑mechanical function (orbital) that describes the allowed spatial distribution and energy of electrons delocalized over a molecule, constructed as combinations of atomic orbitals and categorized into bonding, antibonding, and nonbonding types.

 

 

 

 

 

 





## Principle

Principle

Molecular orbitals form by linear combination of atomic orbitals (LCAO) or other basis functions; occupancy of MOs by electrons determines bond order, electronic excitations, and reactivity, while symmetry and energy ordering constrain feasible transitions.

 

 

 

 

 





## Demonstration

Demonstration

In H2, the in‑phase combination of 1s orbitals produces a bonding σMO with increased electron density between nuclei, while the out‑of‑phase combination produces an antibonding σ*MO; occupation of σ* weakens or prevents bonding.

 

 

 

 

## Misapplication

Misapplication

Treating MOs as permanently localized electron pairs identical to Lewis bonds, or assuming a single MO picture always captures correlated electron effects—this neglects electron correlation, resonance averaging, and environment-induced orbital mixing.

 

 

 

 

 





## Consequence

Consequence

Using MO theory correctly predicts spectroscopic transitions, frontier orbitals (HOMO/LUMO) that guide chemical reactivity, and delocalization effects that explain conjugation and aromaticity.

 

 

 

 

## Reversal

Reversal

The inverted viewpoint emphasizes localized orbitals or valence‑bond structures that represent electrons as pairwise localized bonds and lone pairs rather than delocalized MOs; both pictures can be complementary.

 

 

 

 

 





## Boundary

Boundary

Applies to electronic states describable by one‑electron-like orbitals in a given basis; excludes phenomena requiring explicit multiconfigurational treatment without reference to a single‑determinant MO picture and excludes strictly classical charge‑separation models.

 

 

 

 

 





## Semantic Tension

Semantic Tension

Tension lies between MO and valence‑bond descriptions: MO emphasizes delocalized energy eigenfunctions and is powerful for spectroscopy and delocalization, while valence‑bond emphasizes localized bonding patterns and resonance structures.

 

 

 

 

 





## Synthesis

Synthesis

A molecular orbital is a quantum function built from atomic orbitals whose occupancy and symmetry determine bonding character, electronic excitations, and reactivity patterns across a molecule.