re: thought experiment
You can't fission nitrogen, oxygen, or the other components of the atmosphere, no matter how hard you slam one of the molecules.
Fission chain reactions work in reactors because they are filled with--wait for it--fissionable material.
Thermal energy in a system tends to even out (a rather basic law of nature.) Ten super-energetic molecules among a billion lazy ones are going to end up as one billion and ten molecules a tiny fraction above the lazy point.