Einsteinium (₉₉Es) is a synthetic element, and thus a

standard atomic weight The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, is ...

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 3 nuclear isomers (^250mEs, ^254mEs, and ^256mEs). The longest-lived isotope is ²⁵²Es with a half-life of 471.7 days, or around 1.293 years.

List of isotopes

, - , rowspan=2, ²⁴⁰Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 141 , rowspan=2, 240.06892(43)# , rowspan=2, 6(2) s , α , ²³⁶Bk , rowspan=2, , - , β⁺ (rare) , ²⁴⁰Cf , - , rowspan=2, ²⁴¹Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 142 , rowspan=2, 241.06854(24)# , rowspan=2, 10(5) s
(+6−5) s, α , ²³⁷Bk , rowspan=2, (3/2−) , - , β⁺ (rare) , ²⁴¹Cf , - , rowspan=3, ²⁴²Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 143 , rowspan=3, 242.06975(35)# , rowspan=3, 13.5(25) s , α (99.94%) , ²³⁸Bk , rowspan=3, , - , β⁺, SF (.6%) , (various) , - , β⁺ (rare) , ²⁴²Cf , - , rowspan=2, ²⁴³Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 144 , rowspan=2, 243.06955(25)# , rowspan=2, 21(2) s , β⁺ (70%) , ²⁴³Cf , rowspan=2, 3/2−# , - , α (30%) , ²³⁹Bk , - , rowspan=3, ²⁴⁴Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 145 , rowspan=3, 244.07088(20)# , rowspan=3, 37(4) s , β⁺ (94.69%) , ²⁴⁴Cf , rowspan=3, , - , α (5.3%) , ²⁴⁰Bk , - , β⁺, SF (.01%) , (various) , - , rowspan=2, ²⁴⁵Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 146 , rowspan=2, 245.07132(22)# , rowspan=2, 1.1(1) min , β⁺ (60%) , ²⁴⁵Cf , rowspan=2, (3/2−) , - , α (40%) , ²⁴¹Bk , - , rowspan=3, ²⁴⁶Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 147 , rowspan=3, 246.07290(24)# , rowspan=3, 7.7(5) min , β⁺ (90.1%) , ²⁴⁶Cf , rowspan=3, 4−# , - , α (9.9%) , ²⁴²Bk , - , β⁺, SF (.003%) , (various) , - , rowspan=3, ²⁴⁷Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 148 , rowspan=3, 247.07366(3)# , rowspan=3, 4.55(26) min , β⁺ (93%) , ²⁴⁷Cf , rowspan=3, 7/2+# , - , α (7%) , ²⁴³Bk , - , SF (9×10⁻⁵%) , (various) , - , rowspan=3, ²⁴⁸Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 149 , rowspan=3, 248.07547(6)# , rowspan=3, 27(5) min , β⁺ (99.75%) , ²⁴⁸Cf , rowspan=3, 2−#, 0+# , - , α (.25%) , ²⁴⁴Bk , - , β⁺, SF (3×10⁻⁵%) , (various) , - , rowspan=2, ²⁴⁹Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 150 , rowspan=2, 249.07641(3)# , rowspan=2, 102.2(6) min , β⁺ (99.43%) , ²⁴⁹Cf , rowspan=2, 7/2+ , - , α (.57%) , ²⁴⁵Bk , - , rowspan=2, ²⁵⁰Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 151 , rowspan=2, 250.07861(11)# , rowspan=2, 8.6(1) h , β⁺ (97%) , ²⁵⁰Cf , rowspan=2, (6+) , - , α (3%) , ²⁴⁶Bk , - , rowspan=2 style="text-indent:1em" , ^250mEs , rowspan=2 colspan="3" style="text-indent:2em" , 200(150)# keV , rowspan=2, 2.22(5) h , EC (99%) , ²⁵⁰Cf , rowspan=2, 1(−) , - , α (1%) , ²⁴⁶Bk , - , rowspan=2, ²⁵¹Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 152 , rowspan=2, 251.079992(7) , rowspan=2, 33(1) h , EC (99.51%) , ²⁵¹Cf , rowspan=2, (3/2−) , - , α (.49%) , ²⁴⁷Bk , - , rowspan=3, ²⁵²Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 153 , rowspan=3, 252.08298(5) , rowspan=3, 471.7(19) d , α (76%) , ²⁴⁸Bk , rowspan=3, (5−) , - , EC (24%) , ²⁵²Cf , - , β⁻ (.01%) , ²⁵²Fm , - , rowspan=2, ²⁵³EsMost common isotope , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 154 , rowspan=2, 253.0848247(28) , rowspan=2, 20.47(3) d , α , ²⁴⁹Bk , rowspan=2, 7/2+ , - , SF (8.7×10⁻⁶%) , (various) , - , rowspan=4, ²⁵⁴Es , rowspan=4 style="text-align:right" , 99 , rowspan=4 style="text-align:right" , 155 , rowspan=4, 254.088022(5) , rowspan=4, 275.7(5) d , α , ²⁵⁰Bk , rowspan=4, (7+) , - , EC (10⁻⁴%) , ²⁵⁴Cf , - , SF (3×10⁻⁶%) , (various) , - , β⁻ (1.74×10⁻⁶%) , ²⁵⁴Fm , - , rowspan=5 style="text-indent:1em" , ^254mEs , rowspan=5 colspan="3" style="text-indent:2em" , 84.2(25) keV , rowspan=5, 39.3(2) h , β⁻ (98%) , ²⁵⁴Fm , rowspan=5, 2+ , - , IT (3%) , ²⁵⁴Es , - , α (.33%) , ²⁵⁰Bk , - , EC (.078%) , ²⁵⁴Cf , - , SF (.0045%) , (various) , - , rowspan=3, ²⁵⁵Es , rowspan=3 style="text-align:right" , 99 , rowspan=3 style="text-align:right" , 156 , rowspan=3, 255.090273(12) , rowspan=3, 39.8(12) d , β⁻ (92%) , ²⁵⁵Fm , rowspan=3, (7/2+) , - , α (8%) , ²⁵¹Bk , - , SF (.0041%) , (various) , - , ²⁵⁶Es , style="text-align:right" , 99 , style="text-align:right" , 157 , 256.09360(11)# , 25.4(24) min , β⁻ , ²⁵⁶Fm , (1+, 0−) , - , rowspan=2 style="text-indent:1em" , ^256mEs , rowspan=2 colspan="3" style="text-indent:2em" , 0(100)# keV , rowspan=2, 7.6 h , β⁻ (99.99%) , ²⁵⁶Fm , rowspan=2, (8+) , - , β⁻, SF (.002%) , (various) , - , rowspan=2, ²⁵⁷Es , rowspan=2 style="text-align:right" , 99 , rowspan=2 style="text-align:right" , 158 , rowspan=2, 257.09598(44)# , rowspan=2, 7.7(2) d , β⁻ , ²⁵⁷Fm , rowspan=2, 7/2+# , - , α , ²⁵³Bk

References

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Navbox element isotopes Einsteinium Einsteinium