The key difference between molecular orbital theory and valence bond theory is that molecular orbital theory describes the molecular orbital formation, whereas valence bond theory describes atomic orbitals.
Different molecules have different chemical and physical properties than individual atoms which joined to form these molecules. To understand these differences between atomic and molecular properties, it is necessary to understand the chemical bond formation between several atoms to make a molecule. At present, we use two quantum mechanical theories to describe the covalent bond and electronic structure of molecules. These are valence bond theory and molecular orbital theory.
CONTENTS
1. Overview and Key Difference
2. What is Molecular Orbital Theory
3. What is Valence Bond Theory
4. Side by Side Comparison – Molecular Orbital Theory vs Valence Bond Theory in Tabular Form
5. Summary
What is Molecular Orbital Theory?
In molecules, electrons reside in molecular orbitals, but their shapes are different, and they are associated with more than one atomic nuclei. The molecular orbital theory is the description of molecules based on molecular orbitals.
We can obtain the wave function describing a molecular orbital by the linear combination of atomic orbitals. A bonding orbital forms when two atomic orbitals interact in the same phase (constructive interaction). When they interact out of phase (destructive interaction), anti-bonding orbitals from. Therefore, there are bonding and anti-bonding orbitals for each suborbital interaction. Bonding orbitals have low energy, and electrons are more likely to reside in those. Anti-bonding orbitals are high in energy, and when all the bonding orbitals are filled, electrons go and fill the anti-bonding orbitals.
What is Valence Bond Theory?
Valence bond theory is based on localized bond approach, which assumes that electrons in a molecule occupy atomic orbitals of the individual atoms. For example, in the formation of the H2 molecule, two hydrogen atoms overlap their 1s orbitals. By overlapping the two orbitals, they share a common region in the space. Initially, when the two atoms are far apart, there is no interaction between them. Therefore, the potential energy is zero.
As the atoms approach each other, each electron is attracted by the nucleus in the other atom, and at the same time, electrons repel each other, as do the nuclei. While the atoms are still separated, the attraction is greater than the repulsion, so the potential energy of the system decreases. The point at which the potential energy reaches the minimum value, the system is at stability. This is what happens when two hydrogen atoms are coming together and forming the molecule.
Figure 01: Formation of a Pi Bond
However, this overlapping concept can only describe simple molecules like H2, F2, HF, etc. This theory fails to explain molecules like CH4. Nevertheless, this problem can be solved by combining this theory with the hybrid orbital theory. Hybridization is the mixing of two nonequivalent atomic orbitals. For example, in CH4, C has four hybridized sp3 orbitals overlapping with the s orbitals of each H.
What is the Difference Between Molecular Orbital Theory and Valence Bond Theory?
At present, we use two quantum mechanical theories to describe the covalent bond and electronic structure of molecules. These are Valence bond theory and molecular orbital theory. The key difference between molecular orbital theory and valence bond theory is that molecular orbital theory describes the molecular orbital formation, whereas valence bond theory describes atomic orbitals. Moreover, valence bond theory can only be applied for diatomic molecules, and not for polyatomic molecules. However, we can apply the molecular orbital theory for any molecule.
Summary – Molecular Orbital Theory vs Valence Bond Theory
Valence bond theory and molecular orbital theory are the two quantum mechanical theories that describe the covalent bond and electronic structure of molecules. The key difference between molecular orbital theory and valence bond theory is that molecular orbital theory describes the molecular orbital formation, whereas valence bond theory describes atomic orbitals.
Reference:
1. Helmenstine, Anne Marie. “Valence Bond (VB) Theory Definition.” ThoughtCo, Aug. 4, 2019, Available here.
Image Courtesy:
1. “Pi-Bond” By Pi-bond.jpg: JoJanderivative work: Vladsinger (talk) – Pi-bond.jpg (CC BY-SA 3.0) via Commons Wikimedia