I am a bit confused about how molecular bond theory predicts pairings of unbounded electrons. For example, in $\ce{O2}$ there are two pairs of electrons on each oxygen along with the double bond. However, in the molecular orbital diagram, there are two unpaired electrons in degenerate $\pi$ antibonding orbitals. Obviously, bond order predicts the double bond. But how can one "predict" the two pairs of non-bonded electrons in $\ce{O2}$ from this?

enter image description here

Source: https://commons.wikimedia.org/wiki/File:Oxygen_molecule_orbitals_diagram.JPG

Pairs of non bounded electron refer to these -> ++++++++++++ooooo++++++++++ooooo++++ enter image description here

  • $\begingroup$ Hund's first rule applies to MOs as well as AOs. $\endgroup$ Feb 12, 2022 at 23:19
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    $\begingroup$ but in O$_2$ lewis structure, the electrons are $paired$, which is equivalent of two electrons in the AO being in the same orbit (ie, both red arrows are on the left). $\endgroup$ Feb 13, 2022 at 17:22
  • $\begingroup$ Well, okay. So we have here two theories which contradict each other. How do we resolve this? One says that oxygen should be paramagnetic, the other says it should be diamagnetic. And it turns out that oxygen is famously paramagnetic. So one theory correctly predicts this, and the other one is wrong. $\endgroup$ Feb 13, 2022 at 17:53
  • $\begingroup$ What do you mean by "the two pairs of non-bonded electrons in O2" $\endgroup$
    – M.L
    Feb 13, 2022 at 18:43
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    $\begingroup$ @AdamRadekMartinez check this out: chemistry.stackexchange.com/questions/73768/… $\endgroup$
    – M.L
    Feb 14, 2022 at 3:59


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