Isotopes of promethium
Promethium (Pm) has no stable isotopes, and does not exist in nature, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu. It is a synthetic element, first produced in 1945. Thirty-eight radioisotopes have been characterized, with the most stable being 145Pm with a half-life of 17.7 years, 146Pm with a half-life of 5.53 years, and 147Pm with a half-life of 2.6234 years. All of the remaining radioactive isotopes have half-lives that are less than 365 days, and the majority of these have half-lives that are less than 30 seconds. This element also has 18 meta states with the most stable being 148mPm (t1/2 41.29 days), 152m2Pm (t1/2 13.8 minutes) and 152mPm (t1/2 7.52 minutes).
The isotopes of promethium range in atomic weight from 125.95752 u (126Pm) to 162.95368 u (163Pm). The primary decay mode before the longest-lived isotope, 145Pm, is electron capture, and the primary mode after is beta decay. The primary decay products before 145Pm are isotopes of neodymium and the primary products after are isotopes of samarium.
Since promethium does not exist in nature, a relative atomic mass cannot be given.
Promethium is one of the two elements of the first 82 elements that have no stable isotopes; the other is technetium (Z = 43). This is a rarely occurring effect of the liquid drop model.
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