Editing Promethium (section)

===Isotopes===
{{Main|Isotopes of promethium}}
Promethium is the only [[lanthanide]] and one of only two elements among the first 82 with no stable or long-lived ([[primordial isotope|primordial]]) isotopes. This is a result of a [[Isotopes of technetium#Stability of technetium isotopes|rarely occurring effect]] of the [[liquid drop model]] and stabilities of neighbor element isotopes; it is also the least stable element of the first 84.{{NUBASE2020|ref}} The primary decay products are [[neodymium]] and [[samarium]] isotopes (promethium-146 decays to both, the lighter isotopes generally to neodymium via [[positron decay]] and [[electron capture]], and the heavier isotopes to samarium via beta decay). Promethium [[nuclear isomer]]s may decay to other promethium isotopes and one isotope (<sup>145</sup>Pm) has a very rare alpha decay mode to stable [[praseodymium]]-141.{{NUBASE2020|ref}}

The most stable isotope of the element is promethium-145, which has a specific activity of {{convert|139|Ci/g|TBq/g|abbr=on|lk=on}} and a half-life of 17.7&nbsp;years via [[electron capture]].{{NUBASE2020|ref}}<ref name="CRCel" /> Because it has 84 neutrons (two more than 82, which is a [[magic number (physics)|magic number]] corresponding to a stable neutron configuration), it may emit an [[alpha particle]] (which has 2 neutrons) to form praseodymium-141 with 82 neutrons. Thus it is the only promethium isotope with an experimentally observed [[alpha decay]].{{sfn|Lavrukhina|Pozdnyakov|1966|p=114}} Its [[partial half-life]] for alpha decay is about 6.3{{e|9}}&nbsp;years, and the relative probability for a <sup>145</sup>Pm nucleus to decay in this way is 2.8{{e|-7}}&nbsp;%. Several other promethium isotopes such as <sup>144</sup>Pm, <sup>146</sup>Pm, and <sup>147</sup>Pm also have a positive energy release for alpha decay; their alpha decays are predicted to occur but have not been observed. In total, 41 isotopes of promethium are known, ranging from <sup>126</sup>Pm to <sup>166</sup>Pm.{{NUBASE2020|ref}}<ref name=Ln922>{{cite journal |last1=Kiss |first1=G. G. |last2=Vitéz-Sveiczer |first2=A. |last3=Saito |first3=Y. |display-authors=et al. |title=Measuring the β-decay properties of neutron-rich exotic Pm, Sm, Eu, and Gd isotopes to constrain the nucleosynthesis yields in the rare-earth region |journal=The Astrophysical Journal |volume=936 |issue=107 |date=2022 |page=107 |doi=10.3847/1538-4357/ac80fc|bibcode=2022ApJ...936..107K |s2cid=252108123 |hdl=2117/375253 |hdl-access=free |doi-access=free }}</ref>

The element also has 18 nuclear isomers, with [[mass number]]s of 133 to 142, 144, 148, 149, 152, and 154 (some mass numbers have more than one isomer). The most stable of them is promethium-148m, with a half-life of 43.1&nbsp;days; this is longer than the half-lives of the ground states of all promethium isotopes, except for promethium-143 to 147. In fact, promethium-148m has a longer half-life than its ground state, promethium-148.{{NUBASE2020|ref}}