Promethium (61Pm) is an artificial element, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was first synthesized in 1945. Like technetium, it is preceded and followed by elements with stable isotopes.
The known isotopes run from 128Pm to 166Pm, 39 in all; the most stable are 145Pm with a half-life of 17.7 years, 146Pm with a half-life of 5.53 years, and 147Pm (the common isotope) with a half-life of 2.6234 years. 143Pm and 144Pm also have lengthy if poorly-known lives on the order of a year, but all the others have half-lives that are less than six days, with the majority less than a few minutes. There are also 24 known meta states with the most stable being 148mPm at a half-life of 41.29 days.
The primary decay mode for isotopes lighter than 146Pm is electron capture resulting in isotopes of neodymium, and the primary decay mode heavier than 146Pm is beta decay giving isotopes of samarium; promethium-146 itself decays both ways.
List of isotopes
|-id=Promethium-125 | 125Pm | style="text-align:right" | 61 | style="text-align:right" | 64 | | style="text-align:center" | 2025 | >310 ns
|-id=Promethium-126 | 126Pm | style="text-align:right" | 61 | style="text-align:right" | 65 | 125.95733(54)# | style="text-align:center" | 2025 | 500# ms[>310 ns]
|-id=Promethium-127 | 127Pm | style="text-align:right" | 61 | style="text-align:right" | 66 | 126.95136(43)# | style="text-align:center" | 2025 | 1# s[>310 ns]
| 3/2+# | |-id=Promethium-128 | rowspan=2|128Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 67 | rowspan=2|127.94823(32)# | rowspan=2 style="text-align:center" | 1999 | rowspan=2|1.0(3) s | β+ (?%) | 128Nd | rowspan=2|4+# | rowspan=2| |- | β+, p (?%) | 127Pr |-id=Promethium-129 | 129Pm | style="text-align:right" | 61 | style="text-align:right" | 68 | 128.94291(32)# | style="text-align:center" | 2004 | 2.4(9) s | β+ | 129Nd | 5/2+# | |-id=Promethium-130 | rowspan=2|130Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 69 | rowspan=2|129.94045(22)# | rowspan=2 style="text-align:center" | 1985 | rowspan=2|2.6(2) s | β+ (?%) | 130Nd | rowspan=2|(5+, 6+, 4+) | rowspan=2| |- | β+, p (?%) | 129Pr |-id=Promethium-131 | 131Pm | style="text-align:right" | 61 | style="text-align:right" | 70 | 130.93583(22)# | style="text-align:center" | 1998 | 6.3(8) s | β+ | 131Nd | (11/2−) | |-id=Promethium-132 | rowspan=2|132Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 71 | rowspan=2|131.93384(16)# | rowspan=2 style="text-align:center" | 1977 | rowspan=2|6.2(6) s | β+ | 132Nd | rowspan=2|(3+) | rowspan=2| |- | β+, p (5×10−5%) | 131Pr |-id=Promethium-133 | 133Pm | style="text-align:right" | 61 | style="text-align:right" | 72 | 132.929782(54) | style="text-align:center" | 1977 | 13.5(21) s | β+ | 133Nd | (3/2+) | |-id=Promethium-133m | style="text-indent:1em" | 133mPm | colspan="3" style="text-indent:2em" | 129.7(7) keV | style="text-align:center" | (1996)Half-life not measured, not included in discovery database | 8# s
| (11/2−) | |-id=Promethium-134 | 134Pm | style="text-align:right" | 61 | style="text-align:right" | 73 | 133.928326(45) | style="text-align:center" | 1977 | 22(1) s | β+ | 134Nd | (5+) | |-id=Promethium-134m1 | style="text-indent:1em" | 134m1Pm | colspan="3" style="text-indent:2em" | 50(50)# keVOrder of ground state and isomer is uncertain. | style="text-align:center" | 1989 | ~5 s | β+ | 134Nd | (2+) | |-id=Promethium-134m2 | style="text-indent:1em" | 134m2Pm | colspan="3" style="text-indent:2em" | 120(50)# keV | style="text-align:center" | 2009 | 20(1) μs | IT | 134Pm | (7−) | |-id=Promethium-135 | 135Pm | style="text-align:right" | 61 | style="text-align:right" | 74 | 134.924785(89) | style="text-align:center" | 1975 | 49(3) s | β+ | 135Nd | (3/2+, 5/2+) | |-id=Promethium-135m | style="text-indent:1em" | 135mPm | colspan="3" style="text-indent:2em" | 240(100)# keV | style="text-align:center" | 1989 | 40(3) s | β+ | 135Nd | (11/2−) | |-id=Promethium-136 | 136Pm | style="text-align:right" | 61 | style="text-align:right" | 75 | 135.923596(74) | style="text-align:center" | 1982 | 107(6) s | β+ | 136Nd | 7+# | |-id=Promethium-136m1 | style="text-indent:1em" | 136m1Pm | colspan="3" style="text-indent:2em" | 100(120) keV | style="text-align:center" | 1988 | 90(35) s | β+ | 136Nd | 2+# | |-id=Promethium-136m2 | style="text-indent:1em" | 136m2Pm | colspan="3" style="text-indent:2em" | 42.7(2) keV | style="text-align:center" | 2008 | 1.5(1) μs | IT | 136Pm | 7−# | |-id=Promethium-137 | 137Pm | style="text-align:right" | 61 | style="text-align:right" | 76 | 136.920480(14) | style="text-align:center" | 1975 | 2# min
| 5/2−# | |-id=Promethium-137m | style="text-indent:1em" | 137mPm | colspan="3" style="text-indent:2em" | 160(50) keV | style="text-align:center" | (1973)Not clear that this is a different state from the ground state | 2.4(1) min | β+ | 137Nd | 11/2− | |-id=Promethium-138 | 138Pm | style="text-align:right" | 61 | style="text-align:right" | 77 | 137.919576(12) | style="text-align:center" | 1981 | 3.24(5) min | β+ | 138Nd | 3−# | |-id=Promethium-139 | 139Pm | style="text-align:right" | 61 | style="text-align:right" | 78 | 138.916799(15) | style="text-align:center" | 1967 | 4.15(5) min | β+ | 139Nd | (5/2)+ | |-id=Promethium-139m | style="text-indent:1em" | 139mPm | colspan="3" style="text-indent:2em" | 188.7(3) keV | style="text-align:center" | 1975 | 180(20) ms | IT | 139Pm | (11/2)− | |-id=Promethium-140 | 140Pm | style="text-align:right" | 61 | style="text-align:right" | 79 | 139.916036(26) | style="text-align:center" | 1966 | 9.2(2) s | β+ | 140Nd | 1+ | |-id=Promethium-140m | style="text-indent:1em" | 140mPm | colspan="3" style="text-indent:2em" | 429(28) keV | style="text-align:center" | 1968 | 5.95(5) min | β+ | 140Nd | 8− | |-id=Promethium-141 | 141Pm | style="text-align:right" | 61 | style="text-align:right" | 80 | 140.913555(15) | style="text-align:center" | 1952 | 20.90(5) min | β+ | 141Nd | 5/2+ | |-id=Promethium-141m1 | style="text-indent:1em" | 141m1Pm | colspan="3" style="text-indent:2em" | 628.62(7) keV | style="text-align:center" | 1975 | 630(20) ns | IT | 141Pm | 11/2− | |-id=Promethium-141m2 | style="text-indent:1em" | 141m2Pm | colspan="3" style="text-indent:2em" | 2530.75(17) keV | style="text-align:center" | 1985 | >2 μs | IT | 141Pm | (23/2+) | |-id=Promethium-142 | rowspan=2|142Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 81 | rowspan=2|141.912891(25) | rowspan=2 style="text-align:center" | 1959 | rowspan=2|40.5(5) s | β+ (77.1%) | rowspan=2|142Nd | rowspan=2|1+ | rowspan=2| |- | EC (22.9%) |-id=Promethium-142m1 | style="text-indent:1em" | 142m1Pm | colspan="3" style="text-indent:2em" | 883.17(16) keV | style="text-align:center" | 1971 | 2.0(2) ms | IT | 142Pm | (8)− | |-id=Promethium-142m2 | style="text-indent:1em" | 142m2Pm | colspan="3" style="text-indent:2em" | 2828.7(6) keV | style="text-align:center" | 2004 | 67(5) μs | IT | 142Pm | (13−) | |-id=Promethium-143 | rowspan=2|143Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 82 | rowspan=2 | 142.9109381(32) | rowspan=2 style="text-align:center" | 1952 | rowspan=2 | 265(7) d | EC | rowspan=2 | 143Nd | rowspan=2 | 5/2+ | rowspan=2 | |- | β+ (<5.7×10−6%) |-id=Promethium-144 | rowspan=2|144Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 83 | rowspan=2 | 143.9125962(31) | rowspan=2 style="text-align:center" | 1952 | rowspan=2 | 363(14) d | EC | rowspan=2 | 144Nd | rowspan=2 | 5− | rowspan=2 | |- | β+ (<8×10−5%) |-id=Promethium-144m1 | style="text-indent:1em" | 144m1Pm | colspan="3" style="text-indent:2em" | 840.90(5) keV | style="text-align:center" | 1976 | 780(200) ns | IT | 144Pm | (9)+ | |-id=Promethium-144m2 | style="text-indent:1em" | 144m2Pm | colspan="3" style="text-indent:2em" | 8595.8(22) keV | style="text-align:center" | 1993 | ~2.7 μs | IT | 144Pm | (27+) | |-id=Promethium-145 | rowspan=2|145Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 84 | rowspan=2|144.9127557(30) | rowspan=2 style="text-align:center" | 1951 | rowspan=2|17.7(4) y | EC | 145Nd | rowspan=2|5/2+ | rowspan=2| |- | α (2.8×10−7%) | 141Pr |-id=Promethium-146 | rowspan=2|146Pm | rowspan=2 style="text-align:right" | 61 | rowspan=2 style="text-align:right" | 85 | rowspan=2|145.9147022(46) | rowspan=2 style="text-align:center" | 1960 | rowspan=2|5.53(5) y | EC (66.0%) | 146Nd | rowspan=2|3− | rowspan=2| |- | β− (34.0%) | 146Sm |-id=Promethium-147 | 147PmFission product | style="text-align:right" | 61 | style="text-align:right" | 86 | 146.9151449(14) | style="text-align:center" | 1947 | 2.6234(2) y | β− | 147Sm | 7/2+ | TraceSpontaneous fission product of 232Th, 235U, 238U and alpha decay daughter of primordial 151Eu |-id=Promethium-148 | 148Pm | style="text-align:right" | 61 | style="text-align:right" | 87 | 147.9174811(61) | style="text-align:center" | 1947 | 5.368(7) d | β− | 148Sm | 1− | |-id=Promethium-148m | rowspan=2 style="text-indent:1em" | 148mPm | rowspan=2 colspan="3" style="text-indent:2em" | 137.9(3) keV | rowspan=2 style="text-align:center" | 1952 | rowspan=2|41.29(11) d | β− (95.8%) | 148Sm | rowspan=2|5−, 6− | rowspan=2| |- | IT (4.2%) | 148Pm |-id=Promethium-149 | 149Pm | style="text-align:right" | 61 | style="text-align:right" | 88 | 148.9183415(23) | style="text-align:center" | 1947 | 53.08(5) h | β− | 149Sm | 7/2+ | |-id=Promethium-149m | style="text-indent:1em" | 149mPm | colspan="3" style="text-indent:2em" | 240.214(7) keV | style="text-align:center" | 1966 | 35(3) μs | IT | 149Pm | 11/2− | |-id=Promethium-150 | 150Pm | style="text-align:right" | 61 | style="text-align:right" | 89 | 149.920990(22) | style="text-align:center" | 1952 | 2.698(15) h | β− | 150Sm | (1−) | |-id=Promethium-151 | 151Pm | style="text-align:right" | 61 | style="text-align:right" | 90 | 150.9212166(49) | style="text-align:center" | 1952 | 28.40(4) h | β− | 151Sm | 5/2+ | |-id=Promethium-152 | 152Pm | style="text-align:right" | 61 | style="text-align:right" | 91 | 151.923505(28) | style="text-align:center" | 1958 | 4.12(8) min | β− | 152Sm | 1+ | |-id=Promethium-152m1 | style="text-indent:1em" | 152m1Pm | colspan="3" style="text-indent:2em" | 140(90) keV | style="text-align:center" | 1969 | 7.52(8) min | β− | 152Sm | 4(−) | |-id=Promethium-152m2 | rowspan=2 style="text-indent:1em" | 152m2Pm | rowspan=2 colspan="3" style="text-indent:2em" | 150+x keV | rowspan=2 style="text-align:center" | 1971 | rowspan=2|13.8(2) min | β− (≤100%) | 152Sm | rowspan=2|(8) | rowspan=2| |- | IT (≥0%) | 152Pm |-id=Promethium-153 | 153Pm | style="text-align:right" | 61 | style="text-align:right" | 92 | 152.9241563(97) | style="text-align:center" | 1962 | 5.25(2) min | β− | 153Sm | 5/2− | |-id=Promethium-154 | 154Pm | style="text-align:right" | 61 | style="text-align:right" | 93 | 153.926713(27) | style="text-align:center" | 1958 | 2.68(7) min | β− | 154Sm | (4+) | |-id=Promethium-154m | style="text-indent:1em" | 154mPm | colspan="3" style="text-indent:2em" | −230(50) keV | style="text-align:center" | 1971 | 1.73(10) min | β− | 154Sm | (1−) | |-id=Promethium-155 | 155Pm | style="text-align:right" | 61 | style="text-align:right" | 94 | 154.9281370(51) | style="text-align:center" | 1982 | 41.5(2) s | β− | 155Sm | (5/2−) | |-id=Promethium-156 | 156Pm | style="text-align:right" | 61 | style="text-align:right" | 95 | 155.9311141(13) | style="text-align:center" | 1986 | 27.4(5) s | β− | 156Sm | 4+ | |-id=Promethium-156m | rowspan=2 style="text-indent:1em" | 156mPm | rowspan=2 colspan="3" style="text-indent:2em" | 150.30(10) keV | rowspan=2 style="text-align:center" | 2007 | rowspan=2|2.3(20) s | IT (98%) | 156Pm | rowspan=2|1+# | rowspan=2| |-