Molecular Orbital Theory vs Valence Bond Theory
We know that molecules have different chemical and physical properties than the individual atoms which joined to make the molecule. When atoms join to form molecules, how the atomic properties are changing to molecular properties is a question. To understand these differences it is necessary to understand the chemical bonding formation between several atoms in making molecules. Lewis proposed a way to represent the bonding. He represented the valence electrons of an atom with dots and said that when these valence electrons are shared or given to another atom to attain the noble gas configuration, chemical bonds are formed. However, this theory could not account for many observed chemical properties. Therefore, for a proper explanation of the chemical bond formation, we must look for quantum mechanics. At present, two quantum mechanical theories are used to describe the covalent bond and electronic structure of molecules. Those are Valence bond theory and molecular orbital theory which are described below.
Valence Bond Theory
Valence bond theory is based on localized bond approach, in which it assumes that the electrons in a molecule occupy atomic orbitals of the individual atoms. For example, in the formation of 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. So 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. And this is what happens when two hydrogen atoms are coming together and forming the molecule. However, this overlapping concept can only describe simple molecules like H2, F2, HF, etc. But when it comes to molecules like CH4, this theory fails to explain them. However, by combining this theory with the hybrid orbital theory, this problem can be overcome. Hybridization is the mixing of two non equivalent atomic orbitals. For example, in CH4, C has four hybrized sp3 orbitals overlapping with the s orbitals of each H.
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 description of molecules based on molecular orbitals is called molecular orbital theory. The wave function describing a molecular orbital can be obtained by the linear combination of atomic orbitals. The 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. So there are a bonding and anti bonding orbital for each sub orbital interaction. In molecules, bonding and anti bonding orbitals are arranged. 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 the difference between Valence bond theory and molecular orbital theory?
• Molecular orbital theory talks about molecular orbital formation, whereas valence bond theory talks about atomic orbitals.
• Molecular orbital theory says about the mixing of atomic orbitals when forming molecules. But valence theory says that molecules occupy atomic orbitals.
• Valence bond theory can only be applied for diatomic molecules, not for polyatomic molecules.