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1.11: MO Theory and Covalent Bonding

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Organic Chemistry

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MO Theory and Covalent Bonding

1.11: MO Theory and Covalent Bonding

The molecular orbital theory describes the distribution of electrons in molecules in a manner similar to the distribution of electrons in atomic orbitals. The region of space in which a valence electron in a molecule is likely to be found is called a molecular orbital. Mathematically, the linear combination of atomic orbitals (LCAO) generates molecular orbitals. Combinations of in-phase atomic orbital wave functions result in regions with a high probability of electron density, while out-of-phase waves produce nodes or regions of no electron density.

The in-phase combination of two atomic s orbitals on adjacent atoms produces a lower energy σs bonding molecular orbital in which most of the electron density is directly between the nuclei. The out-of-phase addition produces a higher energy σs* antibonding molecular orbital, in which there is a node between the nuclei.

Similarly, the wave function of p orbitals gives rise to two lobes with opposite phases. When p orbitals overlap end to end, they create σ and σ* orbitals. The side-by-side overlap of two p orbitals generates π bonding and π* antibonding molecular orbitals.

The filled molecular orbital diagram shows the number of electrons in bonding and antibonding molecular orbitals. An electron contributes to a bonding interaction only if it occupies a bonding orbital. The net contribution of the electrons to the bond strength of a molecule is determined from the bond order, which is calculated as follows:


The bond order is a guide to the strength of a covalent bond; a bond between two given atoms becomes stronger as the bond order increases. If the distribution of electrons in the molecular orbitals yields a bond order of zero, a stable bond does not form.

The molecular orbital theory is also useful for polyatomic molecules. The Lewis model of benzene (C6H6), which has a planar hexagonal structure with sp2 hybridized carbon atoms, cannot accurately represent its delocalized electrons. However, the molecular orbital theory assigns those electrons to three π bonding molecular orbitals covering the entire carbon ring. This results in a fully occupied (6 electrons) set of bonding molecular orbitals that endow the benzene ring with additional thermodynamic and chemical stability.

Suggested Reading


MO Theory Covalent Bonding Molecular Orbital Valence Electron LCAO Atomic Orbitals Electron Density Bonding Molecular Orbital Antibonding Molecular Orbital P Orbitals Overlap Bonding Orbital Antibonding Orbital Molecular Orbital Diagram Bond Strength Bond Order

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