The nuclei may not be concentric as the last unpaired orbit aligns. And since \(n=6\) has higher energy than \(n=3\), energy is required to form \(O_2\) from two \(O\)s. Oxidation, where \(O_2\) breaks into two \(O\)s occurs readily as the process gives off energy. Chlorination on the other hand, where \(Cl_2\) breaks into two \(Cl\)s, involves the separation of a orbit pair, will require energy to proceed.
The process of separating an odd number outer shell into two equal lower shells each with an unpaired orbit is endothermic. The process of separating an even number outer shell into two equal lower shells both with all paired orbits is exothermic.
And that settles that! Hello, Nobel Prize.
