Question about molecular orbital theory

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SUMMARY

The discussion centers on the energy levels of molecular orbitals, specifically the sigma 2p and pi 2p orbitals. It is established that the sigma 2p antibonding orbital is always higher in energy than the pi 2p antibonding orbital due to the mixing of sigma bonding p orbitals with sigma bonding s orbitals, which raises their energy. This energy shift occurs more significantly in bonding orbitals compared to antibonding orbitals, leading to a consistent ordering of these molecular orbitals across different diatomic molecules like C2, N2, and B2.

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  • Understanding of molecular orbital theory
  • Familiarity with bonding and antibonding orbitals
  • Knowledge of energy levels in diatomic molecules
  • Basic concepts of orbital mixing and hybridization
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  • Study the molecular orbital diagrams for diatomic molecules such as C2, N2, and B2
  • Learn about the effects of orbital mixing in molecular orbital theory
  • Explore the concept of hybridization and its impact on molecular geometry
  • Investigate the role of s and p orbitals in determining molecular stability
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reyrey389
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I know that the sigma 2p bonding orbital could be less/higher in energy than the pi 2p bonding (based on if it is C2,N2,B2 etc), but

Why is the sigma 2p antibonding orbital always higher in energy than the pi 2p antibonding one?
 
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When you neglect in a first step the s orbitals, the sigma bonding p orbital would be below the pi bonding p orbitals. However, in the second step the sigma bonding p orbital mixes with the sigma bonding s orbitals which shifts them up in energy. Also the antibonding p orbital can be shifted up by that mechanism, although the effect is weaker due to their larger energetic separation and it would not change the ordering of the orbitals.
 

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