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Molecular orbital theory


Arnav
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A basic doubt in molecular orbital theory.

While writing the electronic configuration of molecular orbital, for example say of Fluorine molecule, which forms a single sigma bond between sp3 orbitals, we start the configuration with sigma1s² orbital.

Does this mean molecular orbital theory considers that when atoms combine, all of their orbitals are fused? Even the ones not participating in the bond? 

Or does it consider that the bond between atoms is not just a result of the bond between valence orbitals, but all of the orbitals of combining atoms?

I have only been introduced to this theory qualitatively, so the questions may seem a bit weird.

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22 hours ago, Arnav said:

A basic doubt in molecular orbital theory.

While writing the electronic configuration of molecular orbital, for example say of Fluorine molecule, which forms a single sigma bond between sp3 orbitals, we start the configuration with sigma1s² orbital.

Does this mean molecular orbital theory considers that when atoms combine, all of their orbitals are fused? Even the ones not participating in the bond? 

Or does it consider that the bond between atoms is not just a result of the bond between valence orbitals, but all of the orbitals of combining atoms?

I have only been introduced to this theory qualitatively, so the questions may seem a bit weird.

You seem to be mixing MO theory with Valence Bond (VB) theory. The concept of hybridisation, e.g. sp3, comes from VB theory, not MO theory.

In MO theory, you would represent F2 as shown in this link: https://www.chemtube3d.com/orbitalsfluorine/

Note that each pair of atomic orbitals combines to give a new pair of MOs and that, in most of these pairs, both the bonding and antibonding MOs are populated with electrons, leading to no net bonding. The overall result is equivalent to a single 2-electron sigma bond.

(I only found this website today. I think it is quite cool. You can click the buttons to see the shape of the electron cloud due to each MO.🙂)

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Yes the University of Liverpool has produced a great site in 'Chemtube'.

And it's free.

+1

I second the distinction beteewn Molecular Orbital theory and Atomic Orbital theory.

It is generally too difficult to derive molecular orbitals from scratch in the same way as for atomic orbitals as there are more than one nuclei involved, maybe very many.

One approach is called the LCAO method or Linear Combination of Atomic Orbitals which involves the fabricattion of atomic orbitals from both (all) participating atoms.

Edited by studiot
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On 8/6/2021 at 11:00 PM, exchemist said:

You seem to be mixing MO theory with Valence Bond (VB) theory. The concept of hybridisation, e.g. sp3, comes from VB theory, not MO theory.

In MO theory, you would represent F2 as shown in this link: https://www.chemtube3d.com/orbitalsfluorine/

Note that each pair of atomic orbitals combines to give a new pair of MOs and that, in most of these pairs, both the bonding and antibonding MOs are populated with electrons, leading to no net bonding. The overall result is equivalent to a single 2-electron sigma bond.

(I only found this website today. I think it is quite cool. You can click the buttons to see the shape of the electron cloud due to each MO.🙂)

Yo man you just cleared my head completely! Thanks! I really confused whether to give up the principles of VBT while studying MOT or not, but thanks to you it's clear now

Will definitely check the website

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2 hours ago, Arnav said:

Yo man you just cleared my head completely! Thanks! I really confused whether to give up the principles of VBT while studying MOT or not, but thanks to you it's clear now

Will definitely check the website

Yes, it's best to think of VB and MO as separate alternative models, each with its strengths and weaknesses. For instance VB gives you more spatial information, for example the idea of electron lone pairs, but has the snag that you have to invoke the rather ropey notion of "resonance" in order to explain the behaviour of some molecules. MO theory is far better at predicting the energy levels, hence spectroscopic and magnetic properties (e. g, the paramagnetism of O₂, which VB fails to predict), and things like the  π- bonding in aromatic rings and other conjugated systems, but it gives you less spatial information.

Edited by exchemist
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