Free neutrons break up into protons, electrons, and neutrinos. Neutrons are stable when they combine with protons to form nuclei. A particularly stable nucleus is a pair of protons with a pair of neutrons. That is a helium nucleus. Some of the most stable of the lighter nuclei are those that may be built up from several helium nuclei. The most abundant forms of carbon, oxygen, neon, magnesium, silicon, and sulfur have the same number of protons and neutrons as three, four, five, six, seven, or eight helium nuclei respectively.