Isotopes of einsteinium

Einsteinium (₉₉Es) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be discovered (in nuclear fallout from the Ivy Mike H-bomb test) was ²⁵³Es in 1952. There are 18 known radioisotopes from ²⁴⁰Es to ²⁵⁷Es, and 8 nuclear isomers. The longest-lived isotope is ²⁵²Es with a half-life of 471.7 days, or around 1.293 years.

Isotopes of einsteinium (₉₉Es)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 252Es synth 471.7 d α 248Bk ε 252Cf 253Es synth 20.47 d α 249Bk SF – 254Es synth 275.7 d α 250Bk β− 254Fm 255Es synth 39.8 d β− 255Fm α 251Bk SF –

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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]
240Es	99	141	240.06895(39)#	6.0(17) s	α (70%)	236Bk	4−#
					β+ (30%)	240Cf
					β+, SF (0.16%)	(various)
241Es	99	142	241.06859(25)#	4.3+2.4 −1.2 s[3]	α	237Bk	3/2−#
242Es[4]	99	143	242.06957(28)#	16.9(8) s	β+	242Cf	2+#
					α (41%)	238Bk
					β+, SF (1.5%)	(various)
243Es	99	144	243.06951(22)#	22.1(14) s	α (61%)	239Bk	(7/2+)
					β+? (39%)	243Cf
243mEs[n 7]	50(50)# keV			>50# μs	IT?	243Es	3/2−#
					α?	239Bk
					β+?	243Cf
244Es	99	145	244.07088(20)#	37(4) s	β+ (95%)	244Cf	6+#
					α (5%)	240Bk
					β+, SF (0.011%)	(various)
245Es	99	146	245.07119(18)#	1.11(6) min	β+ (51%)	245Cf	(3/2−)
					α (49%)	241Bk
245mEs[n 7]	30(15)# keV			>50# μs	IT?	245Es	7/2+#
					α?	241Bk
					β+?	245Cf
246Es	99	147	246.07281(10)	7.5(5) min	β+ (90.1%)	246Cf	4−#
					α (9.9%)	242Bk
					β+, SF (0.003%)	(various)
247Es	99	148	247.073622(21)	4.55(26) min	β+ (93%)	247Cf	(7/2+)
					α (7%)	243Bk
247mEs[n 7]	50(50)# keV			>20# μs	IT?	247Es	(3/2−)
					α?	243Bk
					β+	247Cf
248Es	99	149	248.07547(6)#	24(3) min	β+ (99.75%)	248Cf	2−#
					α (0.25%)	244Bk
					β+, SF (3.5×10−4%)	(various)
249Es	99	150	249.07641(3)#	102.2(6) min	β+ (99.43%)	249Cf	7/2+
					α (0.57%)	245Bk
250Es	99	151	250.07861(11)#	8.6(1) h	β+	250Cf	6(+)
250mEs[n 7]	200(150)# keV			2.22(5) h	β+	250Cf	1(−)
251Es	99	152	251.079991(6)	33(1) h	EC (99.5%)	251Cf	3/2−
					α (0.5%)	247Bk
251mEs	8.4(10) keV			>200# μs	IT?	251Es	(7/2+)
					EC?	251Cf
252Es	99	153	252.08298(5)	471.7(19) d	α (78%)	248Bk	(4+)
					EC (22%)	252Cf
253Es[n 8]	99	154	253.0848212(13)	20.47(3) d	α	249Bk	7/2+
					SF (8.7×10−6%)	(various)
253mEs	106(4) keV			>10# μs	IT?	253Es	3/2−#
254Es	99	155	254.088024(3)	275.7(5) d	α[n 9]	250Bk	7+
					β− (1.74×10−4%)	254Fm
					SF (<3×10−6%)	(various)
254mEs	80.4(11) keV			39.3(2) h	β− (98%)	254Fm	2+
					IT (<3%)	254Es
					α (0.32%)	250Bk
					EC (0.076%)	254Cf
					SF (<0.045%)	(various)
255Es	99	156	255.090274(12)	39.8(12) d	β− (92.0%)	255Fm	(7/2+)
					α (8.0%)	251Bk
					SF (0.0041%)	(various)
256Es	99	157	256.09360(11)#	7.6 h	β−	256Fm	7+#
					β−, SF (0.002%)	(various)
256mEs[n 7]	0(100)# keV			25.4(12) min	β−	256Fm	0+#
257Es	99	158	257.09598(44)#	7.7(2) d	β−	257Fm	7/2+#
This table header & footer: view

1. ^mEs – 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
   IT:	Isomeric transition
   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. Order of ground state and isomer is uncertain.
8. Most common isotope
9. Theoretically capable of electron capture to ²⁵⁴Cf^[1]