Mendelevium (101Md) 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 256Md (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 244Md to 260Md, and 10 isomers. The longest-lived isotope is 258Md with a half-life of 51.6 days, and the longest-lived isomer is 258mMd with a half-life of 57 minutes.
List of isotopes
|-id=Mendelevium-244 | rowspan=2|244Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 143 | rowspan=2|244.08116(40)# | rowspan=2 style="text-align:center" | 2020 | rowspan=2|[] | α | 240Es | rowspan=2|3+# |- | β+, SF (<14%) | (various) |-id=Mendelevium-244m | style="text-indent:1em" | 244mMd | colspan="3" style="text-indent:2em" | 200(150)# keV | style="text-align:center" | (2020) Half-life only estimated, not included in discovery database | ~9 μs | IT | 244Md | 7+# |-id=Mendelevium-245 | 245Md | style="text-align:right" | 101 | style="text-align:right" | 144 | 245.08086(28)# | style="text-align:center" | 1996 | 0.38(10) s | α | 241Es | (7/2−) |-id=Mendelevium-245m | style="text-indent:1em" | 245mMdOrder of ground state and isomer is uncertain. | colspan="3" style="text-indent:2em" | 100(100)# keV | style="text-align:center" | 1996 | 0.90(25) ms | SF | (various) | 1/2−# |-id=Mendelevium-246 | 246Md | style="text-align:right" | 101 | style="text-align:right" | 145 | 246.08171(28)# | style="text-align:center" | 1996 | 0.92(18) s | α | 242Es | 1−# |-id=Mendelevium-246m | rowspan=3 style="text-indent:1em" | 246mMd | rowspan=3 colspan="3" style="text-indent:2em" | 60(60) keV | rowspan=3 style="text-align:center" | 2010 | rowspan=3| | β+ (~67%) | 246Fm | rowspan=3|4−# |- | α (<23%) | 242Es |- | β+, SF (>10%) | (various) |-id=Mendelevium-247 | rowspan=2|247Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 146 | rowspan=2|247.08152(22)# | rowspan=2 style="text-align:center" | 1981 | rowspan=2| | α (99.14%) | 243Es | rowspan=2|(7/2−) |- | SF (0.86%) | (various) |-id=Mendelevium-247m | rowspan=2 style="text-indent:1em" | 247mMd | rowspan=2 colspan="3" style="text-indent:2em" | 153 keV | rowspan=2 style="text-align:center" | 2010 | rowspan=2| | α (80%) | 243Es | rowspan=2|(1/2−) |- | SF (20%) | (various) |-id=Mendelevium-248 | rowspan=3|248Md | rowspan=3 style="text-align:right" | 101 | rowspan=3 style="text-align:right" | 147 | rowspan=3|248.08261(20)# | rowspan=3 style="text-align:center" | 1973 | rowspan=3| | β+ (80%) | 248Fm | rowspan=3| |- | α (20%) | 244Es |- | β+, SF (<0.05%) | (various) |-id=Mendelevium-249 | rowspan=2|249Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 148 | rowspan=2|249.08286(18) | rowspan=2 style="text-align:center" | 1973 | rowspan=2| | α (75%) | 245Es | rowspan=2|(7/2−) |- | β+ (25%) | 249Fm |-id=Mendelevium-249m1 | style="text-indent:1em" | 249m1Md | colspan="3" style="text-indent:2em" | 100(100)# keV | style="text-align:center" | 2001 | [] | α | 245Es | (1/2−) |-id=Mendelevium-249m2 | style="text-indent:1em" | 249m2Md | colspan="3" style="text-indent:2em" | ≥910# keV | style="text-align:center" | 2021 | 2.8(5) ms
| (19/2−) |-id=Mendelevium-250 | rowspan=3|250Md | rowspan=3 style="text-align:right" | 101 | rowspan=3 style="text-align:right" | 149 | rowspan=3|250.084165(98) | rowspan=3 style="text-align:center" | 1973 | rowspan=3|54(4) s | β+ (93.0%) | 250Fm | rowspan=3|2−# |- | α (7.0%) | 246Es |- | β+, SF (0.026%) | (various) |-id=Mendelevium-250m | style="text-indent:1em" | 250mMd | colspan="3" style="text-indent:2em" | 120(40) keV | style="text-align:center" | 2019 | 42.4(45) s | α | 246Es | 7+# |-id=Mendelevium-251 | rowspan=2|251Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 150 | rowspan=2|251.084774(20) | rowspan=2 style="text-align:center" | 1973 | rowspan=2| | β+ (90%) | 251Fm | rowspan=2|(7/2−) |- | α (10%) | 247Es |-id=Mendelevium-251m | style="text-indent:1em" | 251mMd | colspan="3" style="text-indent:2em" | 53(8) keV | style="text-align:center" | 2021 | 20# s
| (1/2−) |-id=Mendelevium-252 | 252Md | style="text-align:right" | 101 | style="text-align:right" | 151 | 252.086385(98) | style="text-align:center" | 1973 | | β+ | 252Fm | 1+# |-id=Mendelevium-253 | rowspan=2|253Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 152 | rowspan=2|253.087143(34)# | rowspan=2 style="text-align:center" | 1992 | rowspan=2|[] | β+ (~99.3%) | 253Fm | rowspan=2|(7/2−) |- | α (~0.7%) | 249Es |-id=Mendelevium-253m | style="text-indent:1em" | 253mMd | colspan="3" style="text-indent:2em" | 60(30) keV | style="text-align:center" | (1971) Half-life not measured, not included in discovery database | 1# min
| 1/2−# |-id=Mendelevium-254 | 254Md | style="text-align:right" | 101 | style="text-align:right" | 153 | 254.08959(11)# | style="text-align:center" | 1970 | | β+ | 254Fm | 0−# |-id=Mendelevium-254m | style="text-indent:1em" | 254mMd | colspan="3" style="text-indent:2em" | 50(100)# keV | style="text-align:center" | 1970 | | β+ | 254Fm | 3−# |-id=Mendelevium-255 | rowspan=2|255Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 154 | rowspan=2|255.0910817(60) | rowspan=2 style="text-align:center" | 1958 | rowspan=2| | β+ (93%) | 255Fm | rowspan=2|7/2− |- | α (7%) | 251Es |-id=Mendelevium-256 | rowspan=3|256Md | rowspan=3 style="text-align:right" | 101 | rowspan=3 style="text-align:right" | 155 | rowspan=3|256.09389(13)# | rowspan=3 style="text-align:center" | 1955 | rowspan=3| | β+ (90.8%) | 256Fm | rowspan=3|(1−) |- | α (9.2%) | 252Es |- | SF (<3%) | (various) |-id=Mendelevium-257 | rowspan=2|257Md | rowspan=2 style="text-align:right" | 101 | rowspan=2 style="text-align:right" | 156 | rowspan=2|257.0955373(17) | rowspan=2 style="text-align:center" | 1965 | rowspan=2| | EC (85%) | 257Fm | rowspan=2|(7/2−) |- | α (15%) | 253Es |-id=Mendelevium-258 | rowspan=3|258Md | rowspan=3 style="text-align:right" | 101 | rowspan=3 style="text-align:right" | 157 | rowspan=3|258.0984336(37) | rowspan=3 style="text-align:center" | 1970 | rowspan=3| | α | 254Es | rowspan=3|8−# |- | β− (<0.0015%) | 258No |- | β+ (<0.0015%) | 258Fm |-id=Mendelevium-258m | rowspan=3 style="text-indent:1em" | 258mMd | rowspan=3 colspan="3" style="text-indent:2em" | 0(200)# keV | rowspan=3 style="text-align:center" | 1980 | rowspan=3| | EC (85%) | 258Fm | rowspan=3|1−# |- | SF (<15%) | (various) |- | α (<1.2%) | 254Es |-id=Mendelevium-259 | 259MdNot directly synthesized, occurs as decay product of 259No | style="text-align:right" | 101 | style="text-align:right" | 158 | 259.10045(11)# | style="text-align:center" | 1982 | | SF | (various) | 7/2−# |-id=Mendelevium-260 | rowspan=4|260Md | rowspan=4 style="text-align:right" | 101 | rowspan=4 style="text-align:right" | 159 | rowspan=4|260.10365(34)# | rowspan=4 style="text-align:center" | 1989 | rowspan=4| | SF | (various) | rowspan=4| |- | α (<5%) | 256Es |- | EC (<5%) | 260Fm |- | β− (<3.5%) | 260No
References
Isotope masses from:
Half-life, spin, and isomer data selected from the following sources.