Why are the 1s bonding and antibonding molecular orbitals excluded while making Molecular Orbital diagram for diatomic homo/heteroatomic molecules?

Do these molecular orbitals not form or are they left out as something general that doesn't require repeated mentioning?
I'm in my first semester of college so please keep the explanation simple:)




When there are orbitals other than 1s, 1s do not participate in bonding because they are too compact. Other orbitals take up much more space (partially also because of radial nodal surfaces), so when the atoms come closer to one another, the bond is formed using the outermost shells.


To form a bond means that the atomic orbitals on more than one atom are to combine in such a way so that the energy of one of the combined orbitals is significantly lower than those of the initial two orbitals. For that, You need the atomic orbitals to be:


* close in energy (1s orbitals fulfill this);
* close spatially.


But You also must reckon in all the other orbitals which are occupied with electrons at the same time. What happens with them when You move the atoms closer together?


If atoms tried to get close enough so that the inner shells get overlapped enough to form bonds, the electrons in outer shells would be repulsing so hard, the energy of *their* combination would overshoot whatever bond is formed by the 1s orbitals **so hard** that the full energy of the new molecule would be much more than that of individual atoms, and even more than that of atoms bonded with their outer shells.


This is why the inner shells (not just 1s) are typically not shown in the MO diagrams.


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