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 persisted down to the present because their rate of decay is so slow (e.g. uranium-238 and potassium-40). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g. uranium to radium). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom. As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope. Thus, about two-thirds