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Astatine (85At) has 41 known isotopes, all of which are radioactive; their mass numbers range from 188 to 229 (though 189At is undiscovered). [2] There are also 24 known metastable excited states. The longest-lived isotope is 210At, which has a half-life of 8.1 hours; the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of 56 seconds.
Nuclide [n 1] | Z | N | Isotopic mass (Da) [n 2] [n 3] | Half-life | Decay mode [n 4] | Daughter isotope [n 5] | Spin and parity [n 6] [n 7] | Isotopic abundance | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Excitation energy [n 7] | |||||||||||||||||||
188At [2] | 85 | 103 | 190+350 −80 μs | α (~50%) | 184Bi | ||||||||||||||
p (~50%) | 187Po | ||||||||||||||||||
190At [2] | 85 | 105 | 1.0+14 −4 ms | α | 186Bi | (10−) | |||||||||||||
191At [3] | 85 | 106 | 1.7+11 −5 ms | α | 187Bi | (1/2+) | |||||||||||||
191mAt | 50(30) keV | 2.1+4 −3 ms | α | 187Bi | (7/2−) | ||||||||||||||
192At [4] | 85 | 107 | 192.00314(28) | 11.5(6) ms | α | 188Bi | 3+# | ||||||||||||
β+ (rare) | 192Po | ||||||||||||||||||
β+, SF (0.42%) | (various) | ||||||||||||||||||
192mAt | 0(40) keV | 88(6) ms | α | 188mBi | (9−, 10−) | ||||||||||||||
β+ (rare) | 192Po | ||||||||||||||||||
β+, SF (0.42%) | (various) | ||||||||||||||||||
193At [4] | 85 | 108 | 192.99984(6) | 28+5 −4 ms | α | 189Bi | (1/2+) | ||||||||||||
193m1At | 8(9) keV | 21(5) ms | α | 189m1Bi | (7/2−) | ||||||||||||||
193m2At | 42(9) keV | 27+4 −3 ms | IT (76%) | 193At | (13/2+) | ||||||||||||||
α (24%) | 189m2Bi | ||||||||||||||||||
194At [4] | 85 | 109 | 193.99873(20) | 286(7) ms | α (91.7%#) | 190Bi | (5-) | ||||||||||||
β+ (8.3%#) | 194Po | ||||||||||||||||||
β+, SF (0.032%#) | (various) | ||||||||||||||||||
194mAt | -20(40) keV | 323(7) ms | α (91.7%#) | 190Bi | (10-) | ||||||||||||||
β+ (8.3%#) | 194Po | ||||||||||||||||||
β+, SF (0.032%#) | (various) | ||||||||||||||||||
195At [4] | 85 | 110 | 194.996268(10) | 290(20) ms | α | 191mBi | (1/2+) | ||||||||||||
β+? | 195Po | ||||||||||||||||||
195mAt | 29(7) keV | 143(3) ms | α (88%) | 191Bi | (7/2-) | ||||||||||||||
IT (12%) | 195At | ||||||||||||||||||
β+? | 195Po | ||||||||||||||||||
196At [4] | 85 | 111 | 195.99579(6) | 377(4) ms | α (97.5%) | 192Bi | (3+) | ||||||||||||
β+ (2.5%) | 196Po | ||||||||||||||||||
196m1At | −40(40) keV | 20# ms | α | 192mBi | (10−) | ||||||||||||||
196m2At | 157.9(1) keV | 11(2) μs | IT | 196At | (5+) | ||||||||||||||
197At [4] | 85 | 112 | 196.99319(5) | 388.2(5.6) ms | α (96.1%) | 193Bi | (9/2−) | ||||||||||||
β+ (3.9%) | 197Po | ||||||||||||||||||
197m1At | 45(8) keV | 2.0(2) s | α | 193m1Bi | (1/2+) | ||||||||||||||
IT (<0.004%) | 197At | ||||||||||||||||||
β+? | 197Po | ||||||||||||||||||
197m2At | 310.7(2) keV | 1.3(2) μs | IT | 197At | (13/2+) | ||||||||||||||
198At | 85 | 113 | 197.99284(5) | 4.2(3) s | α (94%) | 194Bi | (3+) | ||||||||||||
β+ (6%) | 198Po | ||||||||||||||||||
198mAt | 330(90)# keV | 1.0(2) s | (10−) | ||||||||||||||||
199At | 85 | 114 | 198.99053(5) | 6.92(13) s | α (89%) | 195Bi | (9/2−) | ||||||||||||
β+ (11%) | 199Po | ||||||||||||||||||
200At | 85 | 115 | 199.990351(26) | 43.2(9) s | α (57%) | 196Bi | (3+) | ||||||||||||
β+ (43%) | 200Po | ||||||||||||||||||
200m1At | 112.7(30) keV | 47(1) s | α (43%) | 196Bi | (7+) | ||||||||||||||
IT | 200At | ||||||||||||||||||
β+ | 200Po | ||||||||||||||||||
200m2At | 344(3) keV | 3.5(2) s | (10−) | ||||||||||||||||
201At | 85 | 116 | 200.988417(9) | 85(3) s | α (71%) | 197Bi | (9/2−) | ||||||||||||
β+ (29%) | 201Po | ||||||||||||||||||
202At | 85 | 117 | 201.98863(3) | 184(1) s | β+ (88%) | 202Po | (2, 3)+ | ||||||||||||
α (12%) | 198Bi | ||||||||||||||||||
202m1At | 190(40) keV | 182(2) s | (7+) | ||||||||||||||||
202m2At | 580(40) keV | 460(50) ms | (10−) | ||||||||||||||||
203At | 85 | 118 | 202.986942(13) | 7.37(13) min | β+ (69%) | 203Po | 9/2− | ||||||||||||
α (31%) | 199Bi | ||||||||||||||||||
204At | 85 | 119 | 203.987251(26) | 9.2(2) min | β+ (96%) | 204Po | 7+ | ||||||||||||
α (3.8%) | 200Bi | ||||||||||||||||||
204mAt | 587.30(20) keV | 108(10) ms | IT | 204At | (10−) | ||||||||||||||
205At | 85 | 120 | 204.986074(16) | 26.2(5) min | β+ (90%) | 205Po | 9/2− | ||||||||||||
α (10%) | 201Bi | ||||||||||||||||||
205mAt | 2339.65(23) keV | 7.76(14) μs | 29/2+ | ||||||||||||||||
206At | 85 | 121 | 205.986667(22) | 30.6(13) min | β+ (99.11%) | 206Po | (5)+ | ||||||||||||
α (0.9%) | 202Bi | ||||||||||||||||||
206mAt | 807(3) keV | 410(80) ns | (10)− | ||||||||||||||||
207At | 85 | 122 | 206.985784(23) | 1.80(4) h | β+ (91%) | 207Po | 9/2− | ||||||||||||
α (8.6%) | 203Bi | ||||||||||||||||||
208At | 85 | 123 | 207.986590(28) | 1.63(3) h | β+ (99.5%) | 208Po | 6+ | ||||||||||||
α (0.55%) | 204Bi | ||||||||||||||||||
209At | 85 | 124 | 208.986173(8) | 5.41(5) h | β+ (96%) | 209Po | 9/2− | ||||||||||||
α (4.0%) | 205Bi | ||||||||||||||||||
210At | 85 | 125 | 209.987148(8) | 8.1(4) h | β+ (99.8%) | 210Po | (5)+ | ||||||||||||
α (0.18%) | 206Bi | ||||||||||||||||||
210m1At | 2549.6(2) keV | 482(6) μs | (15)− | ||||||||||||||||
210m2At | 4027.7(2) keV | 5.66(7) μs | (19)+ | ||||||||||||||||
211At | 85 | 126 | 210.9874963(30) | 7.214(7) h | EC (58.2%) | 211Po | 9/2− | ||||||||||||
α (42%) | 207Bi | ||||||||||||||||||
212At | 85 | 127 | 211.990745(8) | 314(2) ms | α [n 8] | 208Bi | (1−) | ||||||||||||
212m1At | 223(7) keV | 119(3) ms | α | 208Bi | (9−) | ||||||||||||||
212m2At | 4771.6(11) keV | 152(5) μs | IT | 212At | (25−) | ||||||||||||||
213At | 85 | 128 | 212.992937(5) | 125(6) ns | α | 209Bi | 9/2− | ||||||||||||
214At | 85 | 129 | 213.996372(5) | 558(10) ns | α | 210Bi | 1− | ||||||||||||
214m1At | 59(9) keV | 265(30) ns | α | 210Bi | |||||||||||||||
214m2At | 231(6) keV | 760(15) ns | α | 210Bi | 9− | ||||||||||||||
215At | 85 | 130 | 214.998653(7) | 0.10(2) ms | α | 211Bi | 9/2− | Trace [n 9] | |||||||||||
216At | 85 | 131 | 216.002423(4) | 0.30(3) ms | α [n 10] | 212Bi | 1− | ||||||||||||
216mAt | 161(11) keV [1] | 100# μs | α | 212Bi | 9−# | ||||||||||||||
217At | 85 | 132 | 217.004719(5) | 32.3(4) ms | α (99.98%) | 213Bi | 9/2− | Trace [n 11] | |||||||||||
β− (.012%) | 217Rn | ||||||||||||||||||
218At | 85 | 133 | 218.008694(12) | 1.27(6) s [6] | α (99.9%) | 214Bi | (2−,3−) | Trace [n 12] | |||||||||||
β− (0.1%) [5] | 218Rn | ||||||||||||||||||
219At | 85 | 134 | 219.011162(4) | 56(3) s | α (97%) | 215Bi | (9/2−) | Trace [n 9] | |||||||||||
β− (3.0%) | 219Rn | ||||||||||||||||||
220At | 85 | 135 | 220.015433(15) | 3.71(4) min | β− (92%) | 220Rn | 3(−#) | ||||||||||||
α (8.0%) | 216Bi | ||||||||||||||||||
221At | 85 | 136 | 221.018017(15) | 2.3(2) min | β− | 221Rn | 3/2−# | ||||||||||||
222At | 85 | 137 | 222.022494(17) | 54(10) s | β− | 222Rn | |||||||||||||
223At | 85 | 138 | 223.025151(15) | 50(7) s | β− | 223Rn | 3/2−# | ||||||||||||
224At | 85 | 139 | 224.029749(24) | 2.5(1.5) min | β− | 224Rn | 2+# | ||||||||||||
225At | 85 | 140 | 225.03253(32)# | 3# s | β− | 225Rn | 1/2+# | ||||||||||||
226At | 85 | 141 | 226.03721(32)# | 7# min | β− | 226Rn | 2+# | ||||||||||||
227At | 85 | 142 | 227.04018(32)# | 5# s | β− | 227Rn | 1/2+# | ||||||||||||
228At | 85 | 143 | 228.04496(43)# | 1# min | β− | 228Rn | 3+# | ||||||||||||
229At | 85 | 144 | 229.04819(43)# | 1# s | β− | 229Rn | 1/2+# | ||||||||||||
This table header & footer: |
EC: | Electron capture |
IT: | Isomeric transition |
Mass number | Mass excess [5] | Mass excess of daughter [5] | Average energy of alpha decay | Half-life [5] | Probability of alpha decay [5] | Alpha decay half-life |
---|---|---|---|---|---|---|
207 | −13.243 MeV | −19.116 MeV | 5.873 MeV | 1.80 h | 8.6% | 20.9 h |
208 | −12.491 MeV | −18.243 MeV | 5.752 MeV | 1.63 h | 0.55% | 12.3 d |
209 | −12.880 MeV | −18.638 MeV | 5.758 MeV | 5.41 h | 4.1% | 5.5 d |
210 | −11.972 MeV | −17.604 MeV | 5.632 MeV | 8.1 h | 0.175% | 193 d |
211 | −11.647 MeV | −17.630 MeV | 5.983 MeV | 7.21 h | 41.8% | 17.2 h |
212 | −8.621 MeV | −16.436 MeV | 7.825 MeV | 0.31 s | ≈100% | 0.31 s |
213 | −6.579 MeV | −15.834 MeV | 9.255 MeV | 125 ns | 100% | 125 ns |
214 | −3.380 MeV | −12.366 MeV | 8.986 MeV | 558 ns | 100% | 558 ns |
219 | 10.397 MeV | 4.073 MeV | 6.324 MeV | 56 s | 97% | 58 s |
220 | 14.350 MeV | 8.298 MeV | 6.052 MeV | 3.71 min | 8% | 46.4 min |
221 [lower-alpha 2] | 16.810 MeV | 11.244 MeV | 5.566 MeV | 2.3 min | experimentally alpha stable | ∞ |
Astatine has 23 nuclear isomers (nuclei with one or more nucleons – protons or neutrons – in an excited state). A nuclear isomer may also be called a "meta-state"; this means the system has more internal energy than the "ground state" (the state with the lowest possible internal energy), making the former likely to decay into the latter. There may be more than one isomer for each isotope. The most stable of them is astatine-202m1, [lower-alpha 3] which has a half-life of about 3 minutes; this is longer than those of all ground states except those of isotopes 203–211 and 220. The least stable one is astatine-214m1; its half-life of 265 ns is shorter than those of all ground states except that of astatine-213. [5]
Alpha decay energy follows the same trend as for other heavy elements. [7] Lighter astatine isotopes have quite high energies of alpha decay, which become lower as the nuclei become heavier. However, astatine-211 has a significantly higher energy than the previous isotope; it has a nucleus with 126 neutrons, and 126 is a magic number (corresponding to a filled neutron shell). Despite having a similar half-life time as the previous isotope (8.1 hours for astatine-210 and 7.2 hours for astatine-211), the alpha decay probability is much higher for the latter: 41.8 percent versus just 0.18 percent. [5] [lower-alpha 4] The two following isotopes release even more energy, with astatine-213 releasing the highest amount of energy of all astatine isotopes. For this reason, it is the shortest-lived astatine isotope. [7] Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists; this happens due to the increasing role of beta decay. [7] This decay mode is especially important for astatine: as early as 1950, it was postulated that the element has no beta-stable isotopes (i.e. ones that do not undergo beta decay at all), [8] though nuclear mass measurements reveal that 215At is in fact beta-stable, as it has the lowest mass of all isobars with A = 215. [9] A beta decay mode has been found for all other astatine isotopes except for 212At, 213At, 214At, 216At and 216mAt. [5] [1] Among other isotopes: astatine-210 and the lighter isotopes decay by positron emission; astatine-217 and the heavier isotopes undergo beta decay; and astatine-211 decays by electron capture instead. [5] Astatine-212 and astatine-216 are expected to decay either way.
The most stable isotope of astatine is astatine-210, which has a half-life of about 8.1 hours. This isotope's primary decay mode is positron emission to the relatively long-lived alpha emitter, polonium-210. In total, only five isotopes of astatine have half-lives exceeding one hour: those between 207 and 211. The least stable ground state isotope is astatine-213, with a half-life of about 125 nanoseconds. It undergoes alpha decay to the extremely long-lived (in practice, stable) isotope bismuth-209. [5]
Actinium (89Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 34 known isotopes, from 203Ac to 236Ac, and 7 isomers. Three isotopes are found in nature, 225Ac, 227Ac and 228Ac, as intermediate decay products of, respectively, 237Np, 235U, and 232Th. 228Ac and 225Ac are extremely rare, so almost all natural actinium is 227Ac.
Francium (87Fr) has no stable isotopes. A standard atomic weight cannot be given. Its most stable isotope is 223Fr with a half-life of 22 minutes, occurring in trace quantities in nature as an intermediate decay product of 235U.
There are 42 isotopes of polonium (84Po). They range in size from 186 to 227 nucleons. They are all radioactive. 210Po with a half-life of 138.376 days has the longest half-life of any naturally-occurring isotope of polonium and is the most common isotope of polonium. It is also the most easily synthesized polonium isotope. 209Po, which does not occur naturally, has the longest half-life of all isotopes of polonium at 124 years. 209Po can be made by using a cyclotron to bombard bismuth with protons, as can 208Po.
Thallium (81Tl) has 41 isotopes with atomic masses that range from 176 to 216. 203Tl and 205Tl are the only stable isotopes and 204Tl is the most stable radioisotope with a half-life of 3.78 years. 207Tl, with a half-life of 4.77 minutes, has the longest half-life of naturally occurring Tl radioisotopes. All isotopes of thallium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.
There are seven stable isotopes of mercury (80Hg) with 202Hg being the most abundant (29.86%). The longest-lived radioisotopes are 194Hg with a half-life of 444 years, and 203Hg with a half-life of 46.612 days. Most of the remaining 40 radioisotopes have half-lives that are less than a day. 199Hg and 201Hg are the most often studied NMR-active nuclei, having spin quantum numbers of 1/2 and 3/2 respectively. All isotopes of mercury are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed. These isotopes are predicted to undergo either alpha decay or double beta decay.
Naturally occurring platinum (78Pt) consists of five stable isotopes (192Pt, 194Pt, 195Pt, 196Pt, 198Pt) and one very long-lived (half-life 4.83×1011 years) radioisotope (190Pt). There are also 34 known synthetic radioisotopes, the longest-lived of which is 193Pt with a half-life of 50 years. All other isotopes have half-lives under a year, most under a day. All isotopes of platinum are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed. Platinum-195 is the most abundant isotope.
Naturally occurring thulium (69Tm) is composed of one stable isotope, 169Tm. Thirty-nine radioisotopes have been characterized, with the most stable being 171Tm with a half-life of 1.92 years, 170Tm with a half-life of 128.6 days, 168Tm with a half-life of 93.1 days, and 167Tm with a half-life of 9.25 days. All of the remaining radioactive isotopes have half-lives that are less than 64 hours, and the majority of these have half-lives that are less than 2 minutes. This element also has 26 meta states, with the most stable being 164mTm, 160mTm and 155mTm.
Natural holmium (67Ho) contains one observationally stable isotope, 165Ho. The below table lists 36 isotopes spanning 140Ho through 175Ho as well as 33 nuclear isomers. Among the known synthetic radioactive isotopes; the most stable one is 163Ho, with a half-life of 4,570 years. All other radioisotopes have half-lives not greater than 1.117 days in their ground states, and most have half-lives under 3 hours.
Naturally occurring terbium (65Tb) is composed of one stable isotope, 159Tb. Thirty-seven radioisotopes have been characterized, with the most stable being 158Tb with a half-life of 180 years, 157Tb with a half-life of 71 years, and 160Tb with a half-life of 72.3 days. All of the remaining radioactive isotopes have half-lives that are less than 6.907 days, and the majority of these have half-lives that are less than 24 seconds. This element also has 27 meta states, with the most stable being 156m1Tb, 154m2Tb and 154m1Tb.
Naturally occurring europium (63Eu) is composed of two isotopes, 151Eu and 153Eu, with 153Eu being the most abundant (52.2% natural abundance). While 153Eu is observationally stable (theoretically can undergo alpha decay with half-life over 5.5×1017 years), 151Eu was found in 2007 to be unstable and undergo alpha decay. The half-life is measured to be (4.62 ± 0.95(stat.) ± 0.68(syst.)) × 1018 years which corresponds to 1 alpha decay per two minutes in every kilogram of natural europium. Besides the natural radioisotope 151Eu, 36 artificial radioisotopes have been characterized, with the most stable being 150Eu with a half-life of 36.9 years, 152Eu with a half-life of 13.516 years, 154Eu with a half-life of 8.593 years, and 155Eu with a half-life of 4.7612 years. The majority of the remaining radioactive isotopes, which range from 130Eu to 170Eu, have half-lives that are less than 12.2 seconds. This element also has 18 meta states, with the most stable being 150mEu (t1/2 12.8 hours), 152m1Eu (t1/2 9.3116 hours) and 152m2Eu (t1/2 96 minutes).
Promethium (61Pm) is an artificial element, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was first synthesized in 1945.
Naturally occurring praseodymium (59Pr) is composed of one stable isotope, 141Pr. Thirty-eight radioisotopes have been characterized with the most stable being 143Pr, with a half-life of 13.57 days and 142Pr, with a half-life of 19.12 hours. All of the remaining radioactive isotopes have half-lives that are less than 5.985 hours and the majority of these have half-lives that are less than 33 seconds. This element also has 15 meta states with the most stable being 138mPr, 142mPr and 134mPr.
Naturally occurring cerium (58Ce) is composed of 4 stable isotopes: 136Ce, 138Ce, 140Ce, and 142Ce, with 140Ce being the most abundant and the only one theoretically stable; 136Ce, 138Ce, and 142Ce are predicted to undergo double beta decay but this process has never been observed. There are 35 radioisotopes that have been characterized, with the most stable being 144Ce, with a half-life of 284.893 days; 139Ce, with a half-life of 137.640 days and 141Ce, with a half-life of 32.501 days. All of the remaining radioactive isotopes have half-lives that are less than 4 days and the majority of these have half-lives that are less than 10 minutes. This element also has 10 meta states.
Naturally occurring lanthanum (57La) is composed of one stable (139La) and one radioactive (138La) isotope, with the stable isotope, 139La, being the most abundant (99.91% natural abundance). There are 39 radioisotopes that have been characterized, with the most stable being 138La, with a half-life of 1.02×1011 years; 137La, with a half-life of 60,000 years and 140La, with a half-life of 1.6781 days. The remaining radioactive isotopes have half-lives that are less than a day and the majority of these have half-lives that are less than 1 minute. This element also has 12 nuclear isomers, the longest-lived of which is 132mLa, with a half-life of 24.3 minutes. Lighter isotopes mostly decay to isotopes of barium and heavy ones mostly decay to isotopes of cerium. 138La can decay to both.
Antimony (51Sb) occurs in two stable isotopes, 121Sb and 123Sb. There are 35 artificial radioactive isotopes, the longest-lived of which are 125Sb, with a half-life of 2.75856 years; 124Sb, with a half-life of 60.2 days; and 126Sb, with a half-life of 12.35 days. All other isotopes have half-lives less than 4 days, most less than an hour.
Indium (49In) consists of two primordial nuclides, with the most common (~ 95.7%) nuclide (115In) being measurably though weakly radioactive. Its spin-forbidden decay has a half-life of 4.41×1014 years, much longer than the currently accepted age of the Universe.
Naturally occurring rhodium (45Rh) is composed of only one stable isotope, 103Rh. The most stable radioisotopes are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, and 99Rh with a half-life of 16.1 days. Thirty other radioisotopes have been characterized with atomic weights ranging from 88.949 u (89Rh) to 121.943 u (122Rh). Most of these have half-lives that are less than an hour except 100Rh and 105Rh. There are also numerous meta states with the most stable being 102mRh (0.141 MeV) with a half-life of about 3.7 years and 101mRh (0.157 MeV) with a half-life of 4.34 days.
Naturally occurring vanadium (23V) is composed of one stable isotope 51V and one radioactive isotope 50V with a half-life of 2.71×1017 years. 24 artificial radioisotopes have been characterized (in the range of mass number between 40 and 65) with the most stable being 49V with a half-life of 330 days, and 48V with a half-life of 15.9735 days. All of the remaining radioactive isotopes have half-lives shorter than an hour, the majority of them below 10 seconds, the least stable being 42V with a half-life shorter than 55 nanoseconds, with all of the isotopes lighter than it, and none of the heavier, have unknown half-lives. In 4 isotopes, metastable excited states were found (including 2 metastable states for 60V), which adds up to 5 meta states.
Naturally occurring scandium (21Sc) is composed of one stable isotope, 45Sc. Twenty-five radioisotopes have been characterized, with the most stable being 46Sc with a half-life of 83.8 days, 47Sc with a half-life of 3.35 days, and 48Sc with a half-life of 43.7 hours and 44Sc with a half-life of 3.97 hours. All the remaining isotopes have half-lives that are less than four hours, and the majority of these have half-lives that are less than two minutes, the least stable being proton unbound 39Sc with a half-life shorter than 300 nanoseconds. This element also has 13 meta states with the most stable being 44m2Sc.
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 was 253Es in 1952. There are 18 known radioisotopes from 240Es to 257Es, and 5 nuclear isomers. The longest-lived isotope is 252Es with a half-life of 471.7 days, or around 1.293 years.