I have looked around for help with this, including on existing threads, but I can't quite find what I'm looking for. I know that in the nuclear shell model we fill the shells in the same way as with electrons, so 2 protons in the first and 6 in the second etc, with the same being true for neutrons. I take this to mean that the 1s shell, for example, can contain 2 protons and 2 neutrons. Do I have the right idea? If so, does this not contradict with the fact that proton-proton bound states are not observed in nature? Theoretically could we not have Helium-2, with pp in the 1s shell?