Schematic orbital correlation diagram for homonuclear diatomic molecules

Let us read the diagram from right to left. With the two X atoms separated, we imagine a very weak quadrupolar field perpendicular to their line of centers. 

Chapter 3. Diatomic Molecules and their Molecular Orbitals 

As the internuclear distance gets shorter, the distinction between bonding and antibonding orbitals makes itself felt. The familiar criteria for bonding are  

An MO is bonding if the electron density can be large between the nuclei 

The relative extent of the stabilization or destabilization of an MO at any particular internuclear distance depends on the overlap of the AOs centered on and X.  

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