Promethium-145

Main isotopes of promethium

Iso­tope Decay abun­dance half-life mode energy (MeV) pro­duct 145Pm syn 17.7 y ε 0.163 145Nd 146Pm syn 5.53 y ε 1.472 146Nd β− 1.542 146Sm 147Pm trace 2.6234 y β− 0.224 147Sm

Promethium (₆₁Pm) 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 (t_1/2 41.29 days), ^152m2Pm (t_1/2 13.8 minutes) and ^152mPm (t_1/2 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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