Difference Between Bonding and Antibonding Molecular Orbitals

Main Difference – Bonding vs Antibonding Molecular Orbitals

The molecular orbital theory explains the chemical bonding between atoms in a molecule. It states that two atomic orbitals overlap with each other in order to form a bond. This overlapping causes the mixing of two orbitals, forming a molecular orbital. There are two types of molecular orbitals: bonding molecular orbitals and antibonding molecular orbital. Bonding molecular orbitals are composed of bond electrons. These electrons are paired with each other forming a covalent bond. Antibonding molecular orbitals reside outside the bond since they do not participate in bonding. The main difference between bonding and antibonding molecular orbitals is that bonding molecular orbitals represent the shape of a molecule whereas antibonding molecular orbitals do not contribute to the determination of the shape of a molecule.

Key Areas Covered

1. What are Bonding Molecular Orbitals
      – Definition, Structures, Contribution to Chemical Bonding
2. What are Antibonding Molecular Orbitals
      – Definition, Structures, Contribution to Chemical Bonding
3. What is the Difference Between Bonding and Antibonding Molecular Orbitals
      – Comparison of Key Differences

Key Terms: Antibonding Molecular Orbital, Asterisk Mark, Atomic Orbital, Bond Electron Pair, Bonding Molecular Orbital, Molecular Orbital Theory

What are Bonding Molecular Orbitals

Bonding molecular orbitals are a type of molecular orbitals that are involved in the formation of a chemical bond. These orbitals are formed due to the overlapping of two atomic orbitals of two different atoms. This overlapping results in mixing of two atomic orbitals, forming the molecular orbitals. In order to be mixed like this, the two atomic orbitals should have comparable energies and correct symmetry.

The electron density of bonding molecular orbitals is higher than that of antibonding orbitals. The energy of these bonding molecular orbitals is less than that of the atomic orbitals which were mixed together to form the bonding molecular orbital. These bonding molecular orbitals are more stable as a lower energy level indicates a higher stability.

In addition, bonding molecular orbitals contribute to the determination of the molecular geometry of a certain molecule. The spatial arrangement of these bonding molecular orbitals represents the shape of the molecule since the bond electron pairs reside in these bonding molecular orbitals.

Figure 1: Molecular Orbital Diagram of H₂

The above image shows the molecular orbital diagram of He₂ molecule. The atomic orbitals of two H atoms are shown in the right and left sides. In the middle, the bonding and antibonding orbitals are shown. Here, the bonding orbital is given as σ_1s because it is the bonding orbital of the 1s orbital of H. “E” represents the energy. Therefore, the energy level of bonding molecular orbitals is less than that of antibonding molecular orbitals and the atomic orbitals.

What are Antibonding Molecular Orbitals

Antibonding molecular orbitals are orbitals containing electrons outside the region between two atomic nuclei. Electrons in the antibonding orbitals reduce the stability of a molecule since these electrons spend most of their time outside the atomic nuclei. Therefore, the electron density of the antibonding molecular orbitals is less compared to that of bonding molecular orbitals, and antibonding molecular orbitals indicate the electron density outside the bond.

Antibonding molecular orbitals have a higher energy than that of atomic orbitals and bonding molecular orbitals. This is because the electrons in these orbitals do not contribute to the reduction of repulsion between two atomic nuclei. Therefore, the stability of compounds having electrons in the antibonding molecular orbitals is lower. However, in stable compounds, the presence of electrons in antibonding molecular orbitals is no or less. The spatial arrangement of antibonding molecular orbitals does not determine the shape or the geometry of a molecule.

Figure 2: Molecular Orbital Energy of He₂ Molecule

According to the above image, the electron density in bonding molecular orbital is equal to that of the antibonding molecular orbital. Therefore, it is a very unstable molecule. Hence, He₂ molecule does not exist. The antibonding molecular orbital is given as σ*.

Difference Between Bonding and Antibonding Molecular Orbitals

Definition

Bonding Molecular Orbitals: Bonding molecular orbitals are a type of molecular orbitals that are involved in the formation of a chemical bond.

Antibonding Molecular Orbitals: Antibonding molecular orbitals are orbitals containing electrons outside the region between two atomic nuclei.

Electron Density

Bonding Molecular Orbitals: The electron density in bonding molecular orbitals is higher.

Antibonding Molecular Orbitals: The electron density in antibonding molecular orbitals is low.

Energy

Bonding Molecular Orbitals: The energy of bonding molecular orbital is lower comparatively.

Antibonding Molecular Orbitals: The energy of antibonding molecular orbital is higher comparatively.

Representation

Bonding Molecular Orbitals: The bonding molecular orbitals are represented without using an asterisk mark (*).

Antibonding Molecular Orbitals: The antibonding molecular orbitals are represented using an asterisk mark (*).

Geometry of Molecule

Bonding Molecular Orbitals: The geometry of a molecule is represented by the spatial arrangement of bonding molecular orbitals.

Antibonding Molecular Orbitals: The geometry of a molecule does not depend on the spatial arrangement of antibonding molecular orbitals.

Electrons

Bonding Molecular Orbitals: Electrons in the bonding molecular orbital contribute to the formation of a bond.

Antibonding Molecular Orbitals: The electrons in antibonding molecular orbitals do not contribute to the formation of the bond.

Stability

Bonding Molecular Orbitals:  The stability of bonding molecular orbitals is comparatively higher .

Antibonding Molecular Orbitals: The stability of antibonding molecular orbitals is comparatively lower.

Conclusion

The molecular orbital theory explains the formation of a chemical bond between two atoms through overlapping or mixing of atomic orbitals. This mixing of atomic orbitals form new orbitals that are called molecular orbitals. The molecular orbitals can be found as either bonding molecular orbitals or antibonding molecular orbitals. The main difference between bonding and antibonding molecular orbitals is that bonding molecular orbitals represent the shape of a molecule whereas antibonding molecular orbitals do not contribute to the determination of the shape of a molecule.

References:

1.”Bonding and Antibonding Molecular Orbitals – Boundless Open Textbook.” Boundless. Boundless, 26 May 2016. Web. Available here. 10 Aug. 2017. 
2.”Bonding and antibonding orbitals.” Chemistry LibreTexts. Libretexts, 19 June 2017. Web. Available here. 10 Aug. 2017. 

Image Courtesy:

1. “Dihydrogen-MO-Diagram” By CCoil (talk) – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “He2 antibonding orbital” By Helvet – Own work (GFDL) via Commons Wikimedia