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
Kinetic isotope effect effects to be easily measured and interpreted. In contrast, secondary effects are generally very small for heavier elements and close in magnitude to the experimental uncertainty, which complicates their interpretation and limits their utility. In the context of isotope effects, "hydrogen" is frequently used to refer to the light isotope, protium (H), specifically. In the rest of this article, reference to "hydrogen" and "deuterium" in parallel grammatical constructions or direct comparisons between them should be interpreted as referring to protium and deuterium. The theory of kinetic isotope effects was first formulated by Jacob Bigeleisen in 1949. Bigeleisen's general formula for