Molecular Orbital Theory: Bonding and Antibonding Orbitals

Molecular Orbital Theory

Introduction

Molecular orbital theory is a theoretical framework used to explain the bonding within molecules. It posits that molecules consist of molecular orbitals made up of linear combinations of atomic orbitals. These molecular orbitals provide a more comprehensive description of the electron distribution within molecules compared to traditional valence bond models.

Bonding Orbitals

Bonding orbitals are molecular orbitals formed by the constructive overlap of atomic orbitals. They are regions of low electron density and high probability of finding electrons. The interaction between atomic orbitals in these regions contributes to the overall stability of the molecule. Bonding orbitals typically involve atomic orbitals of similar energy and orientation.

Sigma ((σ)) Bonds

Sigma bonds, also known as (σ)-bonds, are formed when the axial nodes of the molecular orbitals lie on the molecular axis. They exhibit cylindrical symmetry and contribute to the attraction between nuclei of the bonded atoms. In sigma bonds, orbitals tend to be concentrated near the line connecting the atomic nuclei.

Pi ((\pi)) Bonds

Pi bonds, also known as (\pi)-bonds, are formed when the axial nodes of the molecular orbitals do not lie on the molecular axis. They exhibit planar symmetry and contribute to the repulsion between the outer shells of the bonded atoms. In pi bonds, orbitals tend to be concentrated perpendicular to the line connecting the atomic nuclei.

Hybridization

Hybrid orbitals are formed when atomic orbitals undergo (sp)-hybridization or other types of hybridization. These new orbitals exhibit symmetries not present in the combining atomic orbitals. The resulting molecular orbitals from hybridized atoms can form either bonding or antibonding orbitals.

Antibonding Orbitals

Antibonding orbitals are molecular orbitals formed by the destructive overlap of atomic orbitals. They are regions of high electron density and low probability of finding electrons. The interaction between atomic orbitals in these regions contributes to the repulsion between nuclei, preventing further attractive forces between them. Antibonding orbitals typically involve atomic orbitals of similar energy and orientation.

Sigma ((σ))* Bonds

Sigma star bonds, also known as (σ)*-bonds, are formed when the axial nodes of the molecular orbitals lie on the molecular axis but the electrons occupying these orbitals are attracted to one nucleus more than the other. These orbitals exhibit cylindrical symmetry and contribute to secondary bonding interactions between atoms.

Pi ((\pi))* Bonds

Pi star bonds, also known as (\pi)*-bonds, are formed when the axial nodes of the molecular orbitals do not lie on the molecular axis. These orbitals exhibit planar symmetry and contribute to secondary bonding interactions between atoms. Pi star bonds can be destabilizing for the molecule due to their higher energy compared to pi bonds.

Molecular Orbital Diagrams

Molecular orbital diagrams provide a visual representation of the interactions between atomic orbitals and the formation of molecular orbitals. The diagrams show the relative energies of bonding and antibonding orbitals, with the bonding orbitals lower in energy due to their stabilizing effect on the molecule. The filling of molecular orbitals follows the same principles as atomic orbitals, following Aufbau's principle, Pauli's exclusion principle, and Hund's rule of maximum multiplicity.