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Molecular orbital diagrams

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.

Thanks
(edited 6 years ago)
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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