Particle spin has always been one of my favorite physics topics. You could always find the question to ask the teacher or later professor and not get the answer (starting with the simplest “what is it” :), “why is it”, “why some particles have it, some not”, “why does it give rise to magnetic momentum, how” ... etc (I may forgot some more interesting, there’s always later)). But, I have more specific one’s. We all know that electrons occupy different orbitals of a given subshell before dubly occupying any one of them. In that way they manage to have the same spins, spins are in the parallel arrangement. Explanation is lowering Coulombic repulsion between electrons. Why is that, I know that probability of finding two electrons with parallel spins in same orbital is 0, but how that helps this lowering of Coulombic replulsion, and net lowering of enrgy ? Than this effect of spin correlation leads to shrinking of atom slightly, and improving nucleus –electron interaction. If that’s so, what’s with the proton – electron spin interaction (correct me if I’m wrong they both have 1/2 spin), then should I conclude that in this case better orientation would be antiparallel one ?