Asking to make sure I understand: Valence bond vs. Molecular Orbital theory.

[amk0713]

Hello. I am currently a freshman in college and finding the chapter on Quantum Mechanics incredibly interesting. Whether I decide to delve into this field is still up in the air, but I am having trouble fully understanding the concepts behind Molecular Orbital Theory and how it relates to the Valence Bond Theory, and would really like to get a grasp on these theories simply for the sake of understanding.

I understand the VBT basically points out that orbitals overlap or mix to create strong bonds between molecules and they hybridize whenever necessary to make room and accommodate sigma and pi bonds. How do these normal and hybrid orbitals relate to the model shown in MO diagrams? I understand that the MO theory states that bonds become delocalized throughout the entire molecule, but it seems that the lowest state bonding orbital and anti-bonding orbital bond s orbital electrons when it isn't necessary; why is this?

For example: According to Valence Bond, O₂ is made from two Oxygen atoms that overlap with their respective 2p_y and 2p_z orbitals, leaving 2s and 2p_x untouched. But in the MO diagram, each oxygen atom contributes 1 electron from its own 2s orbital to the 2s molecular bonding orbital and the 2s molecular anti-bond. Then once again, each Oxygen contributes 1 electron from its 2p_x orbital to the 2p_x molecular bonding orbital. Why is this? I thought those electrons I mentioned remained separate as lone pairs? Am I just looking at this the completely wrong way?

I understand this is a full question, but I would really appreciate some help. :)

Thank you.

 

[BigJDubs]

The Valence Bond Theory (VBT) explains how atomic orbitals interact to form molecular orbitals. It states that when two atoms approach each other, their atomic orbitals interact and overlap, forming molecular orbitals which can then be used to explain the chemical bonding between the atoms. The VBT is useful for understanding the structure of molecules, but does not provide an explanation of the energies of the orbitals. Molecular Orbital Theory (MOT) extends the VBT by incorporating quantum mechanics in order to explain the energies of the orbitals. It states that the wave functions of the atomic orbitals combine to form molecular orbitals, which are then used to explain the chemical bonding of the atoms. The MOT provides a more detailed explanation of the bonding by showing how the electrons are distributed among the orbitals. In the case of O2, the VBT states that the two oxygen atoms form two sigma bonds by overlapping their 2py and 2pz orbitals. The MOT, on the other hand, states that each oxygen atom contributes one electron from its own 2s orbital to the 2s molecular bonding orbital, and one electron from its 2px orbital to the 2px molecular bonding orbital. This allows the electrons to become delocalized throughout the entire molecule, resulting in a stronger bond. Since the electrons are delocalized, the electrons in the 2s and 2px orbitals can now interact with the electrons in the other oxygen atom's orbitals, resulting in a stronger overall bond.

 

[charng]

Hello, it's great to hear that you are finding the chapter on Quantum Mechanics interesting! To answer your question, Valence Bond Theory (VBT) and Molecular Orbital Theory (MO) are two different approaches used to describe the bonding and structure of molecules. VBT focuses on the overlap of atomic orbitals to form bonds, while MO theory describes the delocalization of electrons in molecular orbitals.

In VBT, atoms come together and their atomic orbitals (such as s, p, d, etc.) overlap to form hybrid orbitals that are used to create bonds. These hybrid orbitals are specific to the molecule and are formed to maximize bonding. In MO theory, the atomic orbitals combine to form molecular orbitals that are spread out over the entire molecule. This allows for the delocalization of electrons, making the molecule more stable.

In the case of O2, VBT would predict that the two oxygen atoms would overlap their 2py and 2pz orbitals to form a sigma bond, while their 2s and 2px orbitals remain untouched. However, in MO theory, the molecular orbitals are spread out over the entire molecule, allowing for the contribution of electrons from all atomic orbitals. This is why each oxygen atom contributes one electron from its 2s and 2px orbitals to the molecular bonding and anti-bonding orbitals.

In MO theory, the lowest energy bonding orbital is always filled first, which is why the 2s electrons are used to fill the 2s molecular bonding orbital. The same applies for the 2px electrons filling the 2px molecular bonding orbital. The electrons from these orbitals are no longer considered lone pairs, but rather part of the molecular bonding.

I hope this helps clarify the differences between VBT and MO theory. Keep exploring the world of Quantum Mechanics and don't hesitate to ask more questions!