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 77.7 min ε 256Fm α 252Es 257Md synth 5.52 h ε 257Fm α 253Es SF – 258Md synth 51.59 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

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List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[2] [n 2][n 3]	Half-life[1]	Decay mode[1] [n 4]	Daughter isotope	Spin and parity[1] [n 5][n 6]
	Excitation energy[n 6]
244Md[n 7]	101	143	244.08116(40)#	0.36(14) s	α	240Es	3+#
					β+, SF (<14%)	(various)
244mMd[n 7]	200(150)# keV			~9 μs	IT	244Md	7+#
245Md	101	144	245.08086(28)#	0.38(10) s	α	241Es	(7/2−)
245mMd[n 8]	100(100)# keV			0.90(25) ms	SF	(various)	1/2−#
246Md	101	145	246.08171(28)#	0.92(18) s	α	242Es	1−#
246mMd[n 8]	60(60) keV			4.4(8) s	β+ (~67%)	246Fm	4−#
					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)
247mMd	153 keV			0.23(3) s	α (80%)	243Es	(1/2−)
					SF (20%)	(various)
248Md	101	147	248.08261(20)#	7(3) s	β+ (80%)	248Fm
					α (20%)	244Es
					β+, SF (<0.05%)	(various)
249Md	101	148	249.08286(18)	25.6(9) s	α (75%)	245Es	(7/2−)
					β+ (25%)	249Fm
249mMd[n 8]	100(100)# keV			1.9(9) s	α	245Es	(1/2−)
250Md	101	149	250.084165(98)	54(4) s	β+ (93.0%)	250Fm	2−#
					α (7.0%)	246Es
					β+, SF (0.026%)	(various)
250mMd	120(40) keV			42.4(45) s	α	246Es	7+#
251Md	101	150	251.084774(20)	4.21(23) min	β+ (90%)	251Fm	(7/2−)
					α (10%)	247Es
251mMd	53(8) keV			20# s			(1/2−)
252Md	101	151	252.086385(98)	2.3(8) min	β+	252Fm	1+#
253Md	101	152	253.087143(34)#	12(8) min	β+ (~99.3%)	253Fm	(7/2−)
					α (~0.7%)	249Es
253mMd	60(30) keV			1# min			(1/2−)
254Md	101	153	254.08959(11)#	10(3) min	β+	254Fm	0−#
254mMd[n 8]	50(100)# keV			28(8) min	β+	254Fm	3−#
255Md	101	154	255.0910817(60)	27(2) min	β+ (93%)	255Fm	7/2−
					α (7%)	251Es
256Md	101	155	256.09389(13)#	77.7(18) min	β+ (90.8%)	256Fm	(1−)
					α (9.2%)	252Es
					SF (<3%)	(various)
257Md	101	156	257.0955373(17)	5.52(5) h	EC (85%)	257Fm	(7/2−)
					α (15%)	253Es
258Md	101	157	258.0984336(37)	51.59(29) d	α	254Es	(8−)#
					β− (<0.0015%)	258No
					β+ (<0.0015%)	258Fm
258mMd[n 8]	0(200)# keV			57.0(9) min	EC (85%)	258Fm	1−#
					SF (<15%)	(various)
					α (<1.2%)	254Es
259Md[n 9]	101	158	259.10045(11)#	1.60(6) h	SF	(various)	7/2−#
260Md	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: view

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. Discovery of this isotope is disputed due to conflicting data.^[3]
8. Order of ground state and isomer is uncertain.
9. Not directly synthesized, occurs as decay product of ²⁵⁹No

Chronology of isotope discovery

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