Isotopes of mendelevium

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 5 isomers. The longest-lived isotope is 258Md with a half-life of 51.3 days, and the longest-lived isomer is 258mMd with a half-life of 57 minutes.

Isotopes of mendelevium (101Md)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
256Md synth 1.17 h ε 256Fm
257Md synth 5.52 h ε 257Fm
α 253Es
SF –
258Md synth 51.5 d α 254Es
ε 258Fm
β− 258No
259Md synth 1.60 h SF –
α 255Es
260Md synth 31.8 d SF –
α 256Es
ε 260Fm
β− 260No

List of isotopes
Nuclide
[n 1]		 Z N Isotopic mass (Da)
[n 2][n 3]		 Half-life
 Decay
mode
[n 4]		 Daughter
isotope
 Spin and
parity
[n 5][n 6]
Excitation energy[n 6]
244Md[2] 101 143 0.30+0.19
−0.09 s		 α 240Es
245Md[2] 101 144 245.08081(33)# 0.33+0.15
−0.08 s		 α 241Es 7/2−#
β+ (rare) 245Fm
245mMd 200(100)# keV 0.90(25) ms SF (various) 1/2−#
246Md[3] 101 145 246.08171(28)# 0.9(2) s α 242Es
β+? 246Fm
246mMd 4.4(8) s β+ (>67%) 246Fm
α (<23%) 242Es
β+, SF (>10%) (various)
247Md[4] 101 146 247.08152(22)# 1.20(12) s α (99.14%) 243Es (7/2−)
SF (0.86%) (various)
β+? 247Fm
247mMd 153 keV 0.23(3) s α (80%) 243Es (1/2−)
SF (20%) (various)
248Md 101 147 248.08282(26)# 7(3) s β+ (80%) 248Fm
α (20%) 244Es
β+, SF (.05%) (various)
249Md[5] 101 148 249.08291(22)# 26(1) s α (75%) 245Es (7/2−)
β+ (25%) 249Fm
249mMd[6] 100(100)# keV 1.5+1.2
−0.5 s		 α 245Es (1/2−)
250Md[7] 101 149 250.08442(32)# 54(4) s β+ (93.0%) 250Fm
α (7.0%) 246Es
β+, SF (0.026%) (various)
251Md[8] 101 150 251.084774(20) 4.27(26) min β+ (90%) 251Fm (7/2−)
α (10%) 247Es
252Md 101 151 252.08643(14)# 2.3(8) min β+ 252Fm
α (rare)[9] 248Es
253Md 101 152 253.08714(3)# 12(8) min
[6.4+11.6
−3.6 min]		 β+ (~99%)[10] 253Fm (7/2−)
α (~1%) 249Es
254Md 101 153 254.08959(11)# 10(3) min β+ 254Fm (0−)
α (rare) 250Es
254mMd 50(100)# keV 28(8) min β+ 254Fm (3−)
α (rare) 250Es
255Md 101 154 255.091084(7) 27(2) min β+ (92%) 255Fm (7/2−)
α (8%) 251Es
SF (.15%) (various)
256Md 101 155 256.09389(13)# 77(2) min β+ (89%) 256Fm (1−)
α (11%) 252Es
257Md 101 156 257.0955424(29) 5.52(5) h EC (84.8%) 257Fm (7/2−)
α (15.2%) 253Es
SF (rare) (various)
258Md[11] 101 157 258.098431(5) 51.59(29) d α 254Es (8−)#
β− (<0.0015%) 258No
β+ (<0.0015%) 258Fm
258mMd 0(200)# keV 57.0(9) min EC 258Fm 1−#
SF (<15%) (various)
α (<1.2%) 254Es
β− (rare) 258No
259Md[n 7][12] 101 158 259.10051(22)# 1.60(6) h SF (various) 7/2−#
α (rare) 255Es
260Md[13] 101 159 260.10365(34)# 27.8(8) d SF (various)
α (<5%) 256Es
EC (<5%) 260Fm
β− (<3.5%) 260No
This table header & footer:
  1. mMd – Excited nuclear isomer.
  2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. Modes of decay:
    EC:Electron capture
    SF:Spontaneous fission
  5. ( ) spin value – Indicates spin with weak assignment arguments.
  6. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  7. Not directly synthesized, occurs as decay product of 259No

Chronology of isotope discovery
IsotopeDiscoveredReaction
244Md2020209Bi(40Ar,5n)
245Md1996209Bi(40Ar,4n)
246Md1996209Bi(40Ar,3n)
247Md1981209Bi(40Ar,2n)
248Md1973241Am(12C,5n)
249Md1973241Am(12C,4n)
250Md1973243Am(12C,5n), 243Am(13C,6n)
251Md1973243Am(12C,4n), 243Am(13C,5n)
252Md1973243Am(13C,4n)
253Md1992243Am(13C,3n)
254Md1970253Es(α,3n)
255Md1958253Es(α,2n)
256Md1955253Es(α,n)
257Md1964252Cf(11B,α2n)
258Md1970255Es(α,n)
259Md1982248Cm(18O,α3n)[14]
260Md1989254Es+18O, 22Ne — transfer

References
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  14. see nobelium
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