1s orbitals and 2nd period diatomic elements

a general chemistry textbook I am reading says this:



> 
> "In second-period diatomic molecules, the 1s orbitals of the two atoms barely overlap. Because the $\sigma \_{g1s}$ bonding and $\sigma \_{u1s}$\*
>  antibonding orbitals are both doubly occupied, they have little net effect on bonding properties and need not be considered"
> 
> 
> 


i'm confused as to what it means...



> 
> 1. 1s AOs don't and won't form MOs due to weak overlap
> 2. MOs formed from 1s Aos aren't significant for chemical reactivity
> 
> 
> 


I may be missing something, but if anyone can clarify as to what the text meant that will be great. and also can you mention if it extends beyond the 2nd period or even to heteronuclear molecules.




The energy splitting between a bonding and the corresponding anti-bonding orbital is dependent on the overlap: the greater the overlap, the greater the splitting. But take note that the relationship is not really linear and that the split is not perfectly symmetrical: the anti-bonding orbital usually rises more strongly than the bonding orbital lowers its energy. 


This means that the interaction of the 1s orbitals in, say, dinitrogen, is repulsive (in terms of atom-atom distance), but also very small because the 1s orbitals are small (a consequence of the high nuclear charge compared to hydrogen), which leads to poor overlap.


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