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Unstable atoms (radionuclides) do not have to necessarily decay directly into stable elements. They rather frequently decay into other radionuclides. Between the original radionuclide and the final stable atom, there is a whole series of changes known as the decay series.
There are four different decay series, namely the thorium, neptunium, uranium (also known as uranium—radium), and the actinium (also known as uranium—actinium) series. The name is derived from the radionuclide having the longest half-life they contain. Except for the neptunium series, they also start with this radionuclide. The decay of the elements along the decay series always happens by emitting alpha particles (helium nuclides) or beta particles (electrons). During alpha decay, the mass number is decreased by 4 and during beta decay it remains unchanged. So, two series starting with atoms whose mass numbers differ by 1 will always decay in a different way and the series will never meet or merge.
The decay series contain radionuclides with various half-lives and they describe their transformation. A series may branch out since some atoms may decay in two different ways. Thanks to our understanding of the decay series, we can determine the composition of ancient rocks, even though their original elements already decayed a long time ago.
Neptunium is a man-made element and the neptunium decay series is not found in nature.