Molecular Orbitals: Understanding & Rules

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Discussion Overview

The discussion revolves around the understanding of molecular orbitals (MOs), focusing on their formation, characteristics, and the implications of electron occupancy in bonding and antibonding orbitals. Participants explore theoretical aspects and clarify concepts related to molecular orbital theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes that N atoms form N molecular orbitals, with half being asymmetric and corresponding to antibonding orbitals, and the other half being symmetric for bonding orbitals.
  • Another participant suggests that antibonding orbitals can be occupied by two electrons with antiparallel spins, while bonding orbitals can also be empty.
  • A different participant questions the implications of having opposite spins in antibonding orbitals, suggesting that this may lead to a symmetric overall wave function, which could conflict with the properties of fermions.
  • Another reply indicates that combining identical orbitals results in a symmetric product, necessitating an antisymmetric spin function, hinting at a potential confusion with valence bond theory.
  • A later response acknowledges the confusion regarding the terms symmetric and antisymmetric, proposing that these could be better understood as constructive and destructive interference in the context of molecular orbitals.
  • A participant identifies their background as a retired electrical engineer re-learning semiconductor band theory, indicating a personal interest in the topic despite not being a chemist.

Areas of Agreement / Disagreement

Participants express differing views on the occupancy and characteristics of bonding and antibonding orbitals, particularly regarding the implications of spin and symmetry. The discussion remains unresolved with multiple competing interpretations present.

Contextual Notes

There are limitations in the discussion regarding the definitions of symmetric and antisymmetric states, as well as the implications of electron occupancy in molecular orbitals. Some assumptions about the nature of wave functions and their symmetries remain unaddressed.

nigelscott
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My general understanding of Molecular Orbitals is as follows:

N atoms come together to form N molecular orbitals.

N/2 of the orbitals will be spatially asymmetric with symmetric spins.This corresponds to the antibonding orbital.

N/2 will be spatially symmetric with antisymmetric spins. This corresponds to the bonding orbital..

Each bonding orbital can contain a maximum of 2 electron with opposite spins.

Each antibonding orbital can either be empty or contain 1 electron since parallel spins in the same state are not allowed and with opposite spins the overall orbital would be symmetric which is not allowed for a fermion.

Excited electron can move to both higher energy bonding and antibonding orbitals as long as the Pauli Exclusion principle is not violated.

Is this the correct interpretation (particularly the last paragraph)?
 
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Well, also anti-bonding orbitals can be occupied with 2 electrons whose spin has to be antiparallel then. On the other hand, bonding orbitals can also be empty.
 
But if the spins are opposite in the anti-bonding case doesn't the overall wave function become symmetric which is not allowed for Fermions.
 
If you have a product of two identical orbitals, this is always symmetric and you have to combine it with an antisymmetrric spin function. Could it be that you mixed this up with valence bond theory?
 
Last edited:
Yes, I think you are right. I think the thing that is throwing me is the usage of symmetric and antisymmetric. I understand this for AOs but maybe a better way to look at this for MOs is to replace these terms with constructive and destructive interference. Now what you are saying makes perfect sense. The bonding orbitals get filled first followed by the anti-bonding orbitals with the molecular stability being determined by the Bond Order.

I am not a chemist but a retired EE who is re-learning semiconductor band theory for fun!
 

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