# Isotopes of mendelevium

Mendelevium ($_{101}$Md) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was $^{256}$Md (which was also the first isotope of any element produced one atom at a time) in 1955. There are 17 known radioisotopes, ranging in atomic mass from $^{244}$Md to $^{260}$Md, and 5 isomers. The longest-lived isotope is $^{258}$Md with a half-life of 51.3 days, and the longest-lived isomer is $^{258m}$Md with a half-life of 57 minutes.

## List of isotopes

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity |
| Spin and parity |||
| $^{244}$Md | 101 | 143 | 244.08116(40)# | 0.36(14) s | α | $^{240}$Es | 3+# |
| $^{244}$Md | 101 | 143 | 244.08116(40)# | 0.36(14) s | β$^+$, SF (<14%) | (various) | 3+# |
| $^{244m}$Md | 200(150)# keV | 200(150)# keV | 200(150)# keV | ~9 μs | IT | $^{244}$Md | 7+# |
| $^{245}$Md | 101 | 144 | 245.08086(28)# | 0.38(10) s | α | $^{241}$Es | (7/2−) |
| $^{245m}$Md | 100(100)# keV | 100(100)# keV | 100(100)# keV | 0.90(25) ms | SF | (various) | 1/2−# |
| $^{246}$Md | 101 | 145 | 246.08171(28)# | 0.92(18) s | α | $^{242}$Es | 1−# |
| $^{246m}$Md | 60(60) keV | 60(60) keV | 60(60) keV | 4.4(8) s | β$^+$ (~67%) | $^{246}$Fm | 4−# |
| $^{246m}$Md | 60(60) keV | 60(60) keV | 60(60) keV | 4.4(8) s | α (<23%) | $^{242}$Es | 4−# |
| $^{246m}$Md | 60(60) keV | 60(60) keV | 60(60) keV | 4.4(8) s | β$^+$, SF (>10%) | (various) | 4−# |
| $^{247}$Md | 101 | 146 | 247.08152(22)# | 1.20(12) s | α (99.14%) | $^{243}$Es | (7/2−) |
| $^{247}$Md | 101 | 146 | 247.08152(22)# | 1.20(12) s | SF (0.86%) | (various) | (7/2−) |
| $^{247m}$Md | 153 keV | 153 keV | 153 keV | 0.23(3) s | α (80%) | $^{243}$Es | (1/2−) |
| $^{247m}$Md | 153 keV | 153 keV | 153 keV | 0.23(3) s | SF (20%) | (various) | (1/2−) |
| $^{248}$Md | 101 | 147 | 248.08261(20)# | 7(3) s | β$^+$ (80%) | $^{248}$Fm | |
| $^{248}$Md | 101 | 147 | 248.08261(20)# | 7(3) s | α (20%) | $^{244}$Es | |
| $^{248}$Md | 101 | 147 | 248.08261(20)# | 7(3) s | β$^+$, SF (<0.05%) | (various) | |
| $^{249}$Md | 101 | 148 | 249.08286(18) | 25.6(9) s | α (75%) | $^{245}$Es | (7/2−) |
| $^{249}$Md | 101 | 148 | 249.08286(18) | 25.6(9) s | β$^+$ (25%) | $^{249}$Fm | (7/2−) |
| $^{249m}$Md | 100(100)# keV | 100(100)# keV | 100(100)# keV | 1.9(9) s | α | $^{245}$Es | (1/2−) |
| $^{250}$Md | 101 | 149 | 250.084165(98) | 54(4) s | β$^+$ (93.0%) | $^{250}$Fm | 2−# |
| $^{250}$Md | 101 | 149 | 250.084165(98) | 54(4) s | α (7.0%) | $^{246}$Es | 2−# |
| $^{250}$Md | 101 | 149 | 250.084165(98) | 54(4) s | β$^+$, SF (0.026%) | (various) | 2−# |
| $^{250m}$Md | 120(40) keV | 120(40) keV | 120(40) keV | 42.4(45) s | α | $^{246}$Es | 7+# |
| $^{251}$Md | 101 | 150 | 251.084774(20) | 4.21(23) min | β$^+$ (90%) | $^{251}$Fm | (7/2−) |
| $^{251}$Md | 101 | 150 | 251.084774(20) | 4.21(23) min | α (10%) | $^{247}$Es | (7/2−) |
| $^{251m}$Md | 53(8) keV | 53(8) keV | 53(8) keV | 20# s | | | (1/2−) |
| $^{252}$Md | 101 | 151 | 252.086385(98) | 2.3(8) min | β$^+$ | $^{252}$Fm | 1+# |
| $^{253}$Md | 101 | 152 | 253.087143(34)# | 12(8) min | β$^+$ (~99.3%) | $^{253}$Fm | (7/2−) |
| $^{253}$Md | 101 | 152 | 253.087143(34)# | 12(8) min | α (~0.7%) | $^{249}$Es | (7/2−) |
| $^{253m}$Md | 60(30) keV | 60(30) keV | 60(30) keV | 1# min | | | (1/2−) |
| $^{254}$Md | 101 | 153 | 254.08959(11)# | 10(3) min | β$^+$ | $^{254}$Fm | 0−# |
| $^{254m}$Md | 50(100)# keV | 50(100)# keV | 50(100)# keV | 28(8) min | β$^+$ | $^{254}$Fm | 3−# |
| $^{255}$Md | 101 | 154 | 255.0910817(60) | 27(2) min | β$^+$ (93%) | $^{255}$Fm | 7/2− |
| $^{255}$Md | 101 | 154 | 255.0910817(60) | 27(2) min | α (7%) | $^{251}$Es | 7/2− |
| $^{256}$Md | 101 | 155 | 256.09389(13)# | 77.7(18) min | β$^+$ (90.8%) | $^{256}$Fm | (1−) |
| $^{256}$Md | 101 | 155 | 256.09389(13)# | 77.7(18) min | α (9.2%) | $^{252}$Es | (1−) |
| $^{256}$Md | 101 | 155 | 256.09389(13)# | 77.7(18) min | SF (<3%) | (various) | (1−) |
| $^{257}$Md | 101 | 156 | 257.0955373(17) | 5.52(5) h | EC (85%) | $^{257}$Fm | (7/2−) |
| $^{257}$Md | 101 | 156 | 257.0955373(17) | 5.52(5) h | α (15%) | $^{253}$Es | (7/2−) |
| $^{258}$Md | 101 | 157 | 258.0984336(37) | 51.59(29) d | α | $^{254}$Es | (8−)# |
| $^{258}$Md | 101 | 157 | 258.0984336(37) | 51.59(29) d | β$^−$ (<0.0015%) | $^{258}$No | (8−)# |
| $^{258}$Md | 101 | 157 | 258.0984336(37) | 51.59(29) d | β$^+$ (<0.0015%) | $^{258}$Fm | (8−)# |
| $^{258m}$Md | 0(200)# keV | 0(200)# keV | 0(200)# keV | 57.0(9) min | EC (85%) | $^{258}$Fm | 1−# |
| $^{258m}$Md | 0(200)# keV | 0(200)# keV | 0(200)# keV | 57.0(9) min | SF (<15%) | (various) | 1−# |
| $^{258m}$Md | 0(200)# keV | 0(200)# keV | 0(200)# keV | 57.0(9) min | α (<1.2%) | $^{254}$Es | 1−# |
| $^{259}$Md | 101 | 158 | 259.10045(11)# | 1.60(6) h | SF | (various) | 7/2−# |
| $^{260}$Md | 101 | 159 | 260.10365(34)# | 27.8(8) d | SF | (various) | |
| $^{260}$Md | 101 | 159 | 260.10365(34)# | 27.8(8) d | α (<5%) | $^{256}$Es | |
| $^{260}$Md | 101 | 159 | 260.10365(34)# | 27.8(8) d | EC (<5%) | $^{260}$Fm | |
| $^{260}$Md | 101 | 159 | 260.10365(34)# | 27.8(8) d | β$^−$ (<3.5%) | $^{260}$No | |
| This table header & footer: view ||||||||
* ↑ $^m$Md – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* ↑ Modes of decay: EC:Electron capture SF:Spontaneous fission
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* 1 2 Discovery of this isotope is disputed due to conflicting data.
* 1 2 3 4 5 Order of ground state and isomer is uncertain.
* ↑ Not directly synthesized, occurs as decay product of $^{259}$No

## Chronology of isotope discovery

| Isotope | Discovered | Reaction |
| $^{244}$Md | 2020 | $^{209}$Bi($^{40}$Ar,5n) |
| $^{245}$Md | 1996 | $^{209}$Bi($^{40}$Ar,4n) |
| $^{246}$Md | 1996 | $^{209}$Bi($^{40}$Ar,3n) |
| $^{247}$Md | 1981 | $^{209}$Bi($^{40}$Ar,2n) |
| $^{248}$Md | 1973 | $^{241}$Am($^{12}$C,5n) |
| $^{249}$Md | 1973 | $^{241}$Am($^{12}$C,4n) |
| $^{250}$Md | 1973 | $^{243}$Am($^{12}$C,5n), $^{243}$Am($^{13}$C,6n) |
| $^{251}$Md | 1973 | $^{243}$Am($^{12}$C,4n), $^{243}$Am($^{13}$C,5n) |
| $^{252}$Md | 1973 | $^{243}$Am($^{13}$C,4n) |
| $^{253}$Md | 1992 | $^{243}$Am($^{13}$C,3n) |
| $^{254}$Md | 1970 | $^{253}$Es(α,3n) |
| $^{255}$Md | 1958 | $^{253}$Es(α,2n) |
| $^{256}$Md | 1955 | $^{253}$Es(α,n) |
| $^{257}$Md | 1964 | $^{252}$Cf($^{11}$B,α2n) |
| $^{258}$Md | 1970 | $^{255}$Es(α,n) |
| $^{259}$Md | 1982 | $^{248}$Cm($^{18}$O,α3n) |
| $^{260}$Md | 1989 | $^{254}$Es+$^{18}$O, $^{22}$Ne — transfer |