Big Bang nucleosynthesis amounts of the hydrogen isotope deuterium (H or D), the helium isotope helium-3 (He), and a very small amount of the lithium isotope lithium-7 (Li). In addition to these stable nuclei, two unstable or radioactive isotopes were also produced: the heavy hydrogen isotope tritium (H or T); and the beryllium isotope beryllium-7 (Be); but these unstable isotopes later decayed into He and Li, as above. Essentially all of the elements that are heavier than lithium were created much later, by stellar nucleosynthesis in evolving and exploding stars. There are several important characteristics of Big Bang nucleosynthesis (BBN): The key parameter
Isotope (counting as stable), and 26 elements have only a single stable isotope (of these, 19 are so-called mononuclidic elements, having a single primordial stable isotope that dominates and fixes the atomic weight of the natural element to high precision; 3 radioactive mononuclidic elements occur as well). In total, there are 253 nuclides that have not been observed to decay. For the 80 elements that have one or more stable isotopes, the average number of stable isotopes is 253/80 = 3.1625 isotopes per element. The proton:neutron ratio is not the only factor affecting nuclear stability. It depends also on evenness or