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Molecular Orbital energy levels

  1. Dec 11, 2011 #1
    Hi everyone
    I'm currently studying the MO theory and ran into some trouble understanding orbital energy levels.

    As I have previously read, σ bonds are stronger than ∏ bonds, therefore are lower in orbital energy. However, from the MO theory, the σ2p orbital is actually higher in energy than the ∏2p orbital in molecules like B2 and N2. But in molecules like O2 and F2, this sequence is reversed, with the ∏2p orbital having the higher orbital energy.

    Can anyone tell me why?
  2. jcsd
  3. Dec 12, 2011 #2


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    Science Advisor

    a) I think you are referring to the anti-bonding orbitals, not the bonding ones.
    b) the sigma bonds are not formed from pure p orbitals but from a superposition of 2s and 2p orbitals. The energetic distance of the 2s and 2p orbitals increases from left to right in a period, which explains the interchange of the energetic ordering of the molecular orbitals.
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