The chemical elements came into being in two phases. The first commenced shortly after the
Big Bang. From ten seconds to 20 minutes after the beginning of the universe the
earliest condensation of light atoms was responsible for the manufacture of the four lightest elements. The vast majority of this primordial production consisted of the three lightest isotopes of
hydrogen—
protium,
deuterium and
tritium—and two of the nine known isotopes of
helium—
helium-3 and
helium-4. Trace amounts of
lithium-7 and
beryllium-7 were likely also produced. So far as is known, all heavier elements came into being starting around 100 million years later, in a second phase of
nucleosynthesis that commenced with the birth of the
first stars. The nuclear furnaces that power stellar evolution were necessary to create large quantities of all elements heavier than helium, and the
r- and
s-processes of neutron capture that occur in stellar cores are thought to have created all such elements up to
iron and
nickel (atomic numbers 26 and 28). The
extreme conditions that attend
supernovae explosions are capable of creating the elements between
oxygen and
rubidium (i.e., atomic numbers 8 through 37). The creation of heavier elements, including those without stable isotopes—all elements with atomic numbers greater than lead's, 82—appears to rely on r-process nucleosynthesis operating amid the immense concentrations of free neutrons released during
neutron star mergers. Most of the isotopes of each chemical element present in the Earth today were formed by such processes no later than the time of
our planet's condensation from the solar
protoplanetary disc, around 4.5 billion years ago. The exceptions to these so-called
primordial elements are those that have resulted from the radioactive disintegration of unstable parent nuclei as they progress down one of several decay chains, each of which terminates with the production of one of the 251 stable isotopes known to exist. Aside from cosmic or stellar nucleosynthesis, and decay chains the only other ways of producing a chemical element rely on
atomic weapons, nuclear reactors (
natural or
manmade) or the laborious atom-by-atom
assembly of nuclei with
particle accelerators. Unstable isotopes decay to their daughter products (which may sometimes be even more unstable) at a given rate; eventually, often after a series of decays, a stable isotope is reached: there are 251 stable isotopes in the universe. In stable isotopes, light elements typically have a lower ratio of neutrons to protons in their nucleus than heavier elements. Light elements such as
helium-4 have close to a 1:1 neutron:proton ratio. The heaviest elements such as uranium have close to 1.5 neutrons per proton (e.g. 1.587 in
uranium-238). No nuclide heavier than lead-208 is stable; these heavier elements have to shed mass to achieve stability, mostly by
alpha decay. The other common way for isotopes with a high neutron to proton ratio (n/p) to decay is
beta decay, in which the nuclide changes elemental identity while keeping the same mass number and lowering its n/p ratio. For some isotopes with a relatively low n/p ratio, there is an
inverse beta decay, by which a proton is transformed into a neutron, thus moving towards a stable isotope; however, since fission almost always produces products which are neutron heavy,
positron emission or
electron capture are rare compared to electron emission. There are many relatively short beta decay chains, at least two (a heavy, beta decay and a light,
positron decay) for every discrete weight up to around 207 and some beyond, but for the higher mass elements (isotopes heavier than lead) there are only four pathways which encompass all decay chains. This is because there are just two main decay methods:
alpha radiation, which reduces the mass number by 4, and beta, which leaves it unchanged. The four paths are termed 4n, 4n + 1, 4n + 2, and 4n + 3; the remainder from dividing the atomic mass by four gives the chain the isotope will follow in its decay. There are other decay modes, but they invariably occur at a lower probability than alpha or beta decay. (It should not be supposed that these chains have no branches: the diagram below shows a few branches of chains, and in reality there are many more, because there are many more isotopes possible than are shown in the diagram.) For example, the third atom of
nihonium-278 synthesised underwent six alpha decays down to
mendelevium-254, followed by an
electron capture (a form of beta decay) to
fermium-254, and then a seventh alpha to
californium-250, it is the last step in the chain before stable thallium-205. Because this bottleneck is so long-lived, very small quantities of the final decay product have been produced, and for most practical purposes bismuth-209 is the final decay product. In the past, during the first few million years of the history of the Solar System, there were more unstable high-mass nuclides in existence, and the four chains were longer, as they included nuclides that have since decayed away. Notably, 244Pu, 237Np, and 247Cm have half-lives over a million years and would have then been bottlenecks higher in the 4n, 4n+1, and 4n+3 chains respectively - 244Pu and 247Cm have been identified as having been present. (There is no nuclide with a half-life over a million years above 238U in the 4n+2 chain.) Today some of these formerly extinct isotopes are again in existence as they have been manufactured. Thus they again take their places in the chain: plutonium-239, used in nuclear weapons, is the major example, decaying to uranium-235 via alpha emission with a half-life 24,500 years. There has also been large-scale production of neptunium-237, resurrecting the extinct fourth chain. The tables below hence start the four decay chains at isotopes of
californium with mass numbers from 249 to 252. These four chains are summarised in the chart in the following section. == Types of decay ==