Chemistry » Advanced Theories of Covalent Bonding » Molecular Orbital Theory

# Summarizing Molecular Orbital Theory

## Key Concepts and Summary

Molecular orbital (MO) theory describes the behavior of electrons in a molecule in terms of combinations of the atomic wave functions. The resulting molecular orbitals may extend over all the atoms in the molecule. Bonding molecular orbitals are formed by in-phase combinations of atomic wave functions, and electrons in these orbitals stabilize a molecule.

Antibonding molecular orbitals result from out-of-phase combinations of atomic wave functions and electrons in these orbitals make a molecule less stable. Molecular orbitals located along an internuclear axis are called σ MOs. They can be formed from s orbitals or from p orbitals oriented in an end-to-end fashion. Molecular orbitals formed from p orbitals oriented in a side-by-side fashion have electron density on opposite sides of the internuclear axis and are called π orbitals.

We can describe the electronic structure of diatomic molecules by applying molecular orbital theory to the valence electrons of the atoms. Electrons fill molecular orbitals following the same rules that apply to filling atomic orbitals; Hund’s rule and the Aufbau principle tell us that lower-energy orbitals will fill first, electrons will spread out before they pair up, and each orbital can hold a maximum of two electrons with opposite spins.

Materials with unpaired electrons are paramagnetic and attracted to a magnetic field, while those with all-paired electrons are diamagnetic and repelled by a magnetic field. Correctly predicting the magnetic properties of molecules is in advantage of molecular orbital theory over Lewis structures and valence bond theory.

## Key Equations

• $$\text{bond order}=\phantom{\rule{0.2em}{0ex}}\frac{\left(\text{number of bonding electron}\right)-\left(\text{number of antibonding electrons}\right)}{2}$$

## Glossary

### antibonding orbital

molecular orbital located outside of the region between two nuclei; electrons in an antibonding orbital destabilize the molecule

### bond order

number of pairs of electrons between two atoms; it can be found by the number of bonds in a Lewis structure or by the difference between the number of bonding and antibonding electrons divided by two

### bonding orbital

molecular orbital located between two nuclei; electrons in a bonding orbital stabilize a molecule

### degenerate orbitals

orbitals that have the same energy

### diamagnetism

phenomenon in which a material is not magnetic itself but is repelled by a magnetic field; it occurs when there are only paired electrons present

### homonuclear diatomic molecule

molecule consisting of two identical atoms

### linear combination of atomic orbitals

technique for combining atomic orbitals to create molecular orbitals

### molecular orbital

region of space in which an electron has a high probability of being found in a molecule

### molecular orbital diagram

visual representation of the relative energy levels of molecular orbitals

### molecular orbital theory

model that describes the behavior of electrons delocalized throughout a molecule in terms of the combination of atomic wave functions

### paramagnetism

phenomenon in which a material is not magnetic itself but is attracted to a magnetic field; it occurs when there are unpaired electrons present

### π bonding orbital

molecular orbital formed by side-by-side overlap of atomic orbitals, in which the electron density is found on opposite sides of the internuclear axis

### π* bonding orbital

antibonding molecular orbital formed by out of phase side-by-side overlap of atomic orbitals, in which the electron density is found on both sides of the internuclear axis, and there is a node between the nuclei

### σ bonding orbital

molecular orbital in which the electron density is found along the axis of the bond

### σ* bonding orbital

antibonding molecular orbital formed by out-of-phase overlap of atomic orbital along the axis of the bond, generating a node between the nuclei

### s-p mixing

change that causes σp orbitals to be less stable than πp orbitals due to the mixing of s and p-based molecular orbitals of similar energies.