Main Difference – Valence Bond Theory vs Molecular Orbital Theory
An atom is composed of orbitals where electrons reside. These atomic orbitals can be found in different shapes and in different energy levels. When an atom is in a molecule in combination with other atoms, these orbitals are arranged in a different manner. The arrangement of these orbitals will determine the chemical bonding and the shape or the geometry of the molecule. In order to explain the arrangement of these orbitals, we can use either valence bond theory or the molecular orbital theory. The main difference between valence bond theory and the molecular orbital theory is that valence bond theory explains the hybridization of orbitals whereas the molecular orbital theory does not give details about the hybridization of orbitals.
Key Areas Covered
1. What is Valence Bond Theory
– Definition, Theory, Examples
2. What is Molecular Orbital Theory
– Definition, Theory, Examples
3. What is the Difference Between Valence Bond Theory and Molecular Orbital Theory
– Comparison of Key Differences
Key Terms: Antibonding Molecular Orbitals, Bonding Molecular Orbitals, Hybridization, Hybrid Orbitals, Molecular Orbital Theory, Pi Bond, Sigma Bond, sp Orbital, sp2 Orbital, sp3 Orbital, sp3d1 Orbital, Valence Bond Theory
What is Valence Bond Theory
Valence bond theory is a basic theory that is used to explain the chemical bonding of atoms in a molecule. The valence bond theory explains the pairing of electrons through the overlapping of orbitals. Atomic orbitals are mainly found as s orbitals, p orbitals and d orbitals. According to the valence bond theory, overlapping of two s orbitals or head to head overlapping of p orbitals will form a sigma bond. Overlapping of two parallel p orbitals will form a pi bond. Therefore, a single bond will contain only a sigma bond whereas a double bond will contain a sigma bond and a pi bond. A triple bond may contain a sigma bond along with two pi bonds.
Simple molecules such as H2 form a sigma bond just by overlapping of the orbitals since hydrogen (H) atoms are composed only of s orbitals. But for atoms composed of s and p orbitals having unpaired electrons, the valence bond theory has a concept known as “hybridization”.
The hybridization of orbitals results in hybrid orbitals. These hybrid orbitals are arranged in such a way that the repulsion between these orbitals is minimized. Followings are some hybrid orbitals.
sp Orbital
This hybrid orbital is formed when an s orbital is hybridized with a p orbital. Therefore, the sp orbital has 50% of s orbital characteristics and 50% of p orbital characteristics. An atom composed of sp hybrid orbitals has two un-hybridized p orbitals. Therefore, those two p orbitals can be overlapped in a parallel manner forming two pi bonds. The final arrangement of the hybridized orbitals is linear.
sp2 Orbital
This hybrid orbital is formed from the hybridization of an s orbital with two p orbitals. Therefore, this sp2 hybrid orbital comprises about 33% of s orbital properties and about 67% of p orbital properties. Atoms that undergo this type of hybridization are composed of one un-hybridized p orbital. The final arrangement of the hybrid orbital is trigonal planar.
sp3 Orbital
This hybrid orbital is formed from the hybridization of an s orbital with three p orbitals. Therefore, this sp3 hybrid orbital comprises about 25% of s orbital properties and about 75% of p orbital properties. Atoms that undergo this type of hybridization have no un-hybridized p orbital. The final arrangement of the hybrid orbitals is tetrahedral.
sp3d1 Orbital
This hybridization involves an s orbital, three p orbitals and a d orbital.
These hybrid orbitals will determine the final geometry or the shape of the molecule.
Figure 1: Geometry of CH4 is tetrahedral
The above image shows the geometry of CH4 molecule. It is tetrahedral. The ash colored orbitals are sp3 hybridized orbitals of carbon atom whereas the blue colored orbitals are s orbitals of hydrogen atoms that has been overlapped with hybrid orbitals of carbon atom forming covalent bonds.
What is Molecular Orbital Theory
The molecular orbital theory explains the chemical bonding of a molecule using hypothetical molecular orbitals. It also describes how a molecular orbital is formed when atomic orbitals are overlapped (mixed). According to this theory, a molecular orbital can hold a maximum of two electrons. These electrons have opposite spin in order to minimize the repulsion between them. These electrons are called bond electron pair. As explained in this theory, molecular orbitals can be of two types: bonding molecular orbitals and antibonding molecular orbitals.
Bonding Molecular Orbitals
Bonding molecular orbitals have a lower energy than atomic orbitals (atomic orbital that participated in the formation of this molecular orbital). Therefore, bonding orbitals are stable. Bonding molecular orbitals are given the symbol σ.
Antibonding Molecular Orbitals
Antibonding molecular orbitals have a higher energy than atomic orbitals. Therefore, these antibonding orbitals are unstable compared to bonding and atomic orbitals. The antibonding molecular orbitals are given the symbol σ*.
The bonding molecular orbitals cause the formation of a chemical bond. This chemical bond can be either a sigma bond or a pi bond. Antibonding orbitals are not involved in the formation of a chemical bond. They reside outside the bond. A sigma bond is formed when a head-to-head overlapping occurs. A pi bond is formed inside-to-side overlapping of orbitals.
Figure 2: Molecular orbital diagram for the bonding in oxygen molecule
In the above diagram, the atomic orbitals of the two oxygen atoms are shown in the left side and the right side. In the middle, the molecular orbitals of O2 molecule are shown as bonding and antibonding orbitals.
Difference Between Valence Bond Theory and Molecular Orbital Theory
Definition
Valence Bond Theory: Valence bond theory is a basic theory that is used to explain the chemical bonding of atoms in a molecule.
Molecular Orbital Theory: Molecular orbital theory explains the chemical bonding of a molecule using hypothetical molecular orbitals.
Molecular Orbitals
Valence Bond Theory: The valence bond theory does not give details about molecular orbitals. It explains the bonding of atomic orbitals.
Molecular Orbital Theory: The molecular orbital theory is developed based on the molecular orbitals.
Types of Orbitals
Valence Bond Theory: The valence bond theory describes hybrid orbitals.
Molecular Orbital Theory: The molecular orbital theory describes bonding molecular orbitals and antibonding molecular orbitals.
Hybridization
Valence Bond Theory: The valence bond theory explains the hybridization of molecular orbitals.
Molecular Orbital Theory: The molecular orbital theory does not explain about the hybridization of orbitals.
Conclusion
Bothe valence bond theory and molecular orbital theory are used to explain the chemical bonding between atoms in molecules. However, the valance bond theory cannot be used to explain the bonding in complex molecules. It is much suitable for diatomic molecules. But molecular orbital theory can be used to explain the bonding in any molecule. Therefore it has many advanced applications than the valence bond theory. This is the difference between valence bond theory and molecular orbital theory.
References:
1. “Pictorial Molecular Orbital Theory.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. Available here. 09 Aug. 2017.
2. “Valence Bond Theory and Hybrid Atomic Orbitals.” Valence Bond Theory and Hybrid Atomic Orbitals. N.p., n.d. Web. Available here. 09 Aug. 2017.
Image Courtesy:
1. “Ch4 hybridization” By K. Aainsqatsi at English Wikipedia(Original text: K. Aainsqatsi) – Own work (Original text: self-made) (Public Domain) via Commons Wikimedia
2. “Oxygen molecule orbitals diagram” By Anthony.Sebastian – (CC BY-SA 3.0) via Commons Wikimedia
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