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    Hi guys Im struggling a bit with understand Molecular orbital diagrams.

    The example of o2

    First question why is the 1s not considered - Is it because that is what is holding it together so you dont consider it

    So then the way you workout valence electrons is just the electronic configuration without the 1s sub-shell

    so oxygen is 1s2 2s2 2p4, so each oxygen will have 6 electrons so o2 will have 12 in valence shell.

    Now the bit I am more confused about is how they fill up. I sort of get the 2s bit where there are 2 electrons in bonding and anti-bonding.
    But I am confused when it gets to the pi2p, sigma 2p etc.

    How come there are 4 electrons in pi2p for O2?

    Edit not to worry think I have sorted it, if anyone can clarify the bit about the 1s and why it isn't considered would be appreciated.


    In regards to your edit, in oxygen the 1s (core) electrons are very low in energy compared to the 2s/2p (valence) electrons, so you don't need to include them in MO diagrams as they're not going to be doing anything anyway.

    As this is a homonuclear diatomic molecule, the inner shell electrons will give equal bonding and anti-bonding MO's. These cancel each other out. These also don't add much to understanding the way the molecule is bonding so are often omitted.
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