The atomic nucleus of helium-4 is composed of how many protons and how many neutrons ?
The unusual stability of the helium-4 nucleus is also important cosmologically : it explains the fact that in the first few minutes after the Big Bang , as the " soup " of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible , almost all first compound atomic nuclei to form were helium-4 nuclei .