The only stable isotopes of thallium (81Tl) are 203Tl and 205Tl, which make up all natural thallium. The five short-lived isotopes 206Tl through 210Tl also occur in nature, but only as part of the natural decay chains of heavier elements. Synthetic radioisotopes are known from 176Tl to 217Tl; the most stable is 204Tl with a half-life of 3.78 years, followed by 202Tl (half-life 12.31 days) and 201Tl (half-life 3.0421 days). The naturally-occurring radioisotopes live minutes only, with the longest being 207Tl, with a half-life of 4.77 minutes. 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.
The isotope 204Tl is made by the neutron activation of stable thallium in a nuclear reactor.Manual for reactor produced radioisotopes from the International Atomic Energy Agency while 202Tl can be made in a cyclotron as can 201Tl (see section below).
In the fully ionized state, the isotope 205Tl81+ becomes unstable, undergoing bound-state β− decay to 205Pb81+ with a half-life of days, but 203Tl remains stable.
205Tl is the decay product of bismuth-209, an isotope that was once thought to be stable but is now known to undergo alpha decay with an extremely long half-life of 2.01×1019 y. Thus 205Tl is now placed at the end of the neptunium decay chain.
List of isotopes
|-id=Thallium-176 | rowspan=2|176Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 95 | rowspan=2|176.000628(89) | rowspan=2 style="text-align:center" | 2004 | rowspan=2| | p (50%) | 175Hg | rowspan=2|(3−,4−) | rowspan=2| | rowspan=2| |- | α (50%) | 172Au |-id=Thallium-176m | rowspan=2 style="text-indent:1em" | 176mTl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 671 keV | rowspan=2 style="text-align:center" | (2020)Only published in a thesis and not a refereed journal | rowspan=2| | p (50%) | 175Hg | rowspan=2| | rowspan=2| | rowspan=2| |- | α (50%) | 172mAu |-id=Thallium-177 | rowspan=2|177Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 96 | rowspan=2|176.996414(23) | rowspan=2 style="text-align:center" | 1999 | rowspan=2|18(5) ms | α (73%) | 173Au | rowspan=2|(1/2+) | rowspan=2| | rowspan=2| |- | p (27%) | 176Hg |-id=Thallium-177m | rowspan=2 style="text-indent:1em" | 177mTl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 807(18) keV | rowspan=2 style="text-align:center" | 1999 | rowspan=2|230(40) μs | p (51%) | 176Hg | rowspan=2|(11/2−) | rowspan=2| | rowspan=2| |- | α (49%) | 173Au |-id=Thallium-178 | rowspan=3|178Tl | rowspan=3| | rowspan=3 style="text-align:right" | 81 | rowspan=3 style="text-align:right" | 97 | rowspan=3|177.99505(11)# | rowspan=3 style="text-align:center" | 1997 | rowspan=3|255(9) ms | α (62%) | 174Au | rowspan=3| (4-,5-) | rowspan=3| | rowspan=3| |- | β+ (38%) | 178Hg |- | β+, SF (0.15%) | (various) |-id=Thallium-179 | rowspan=2|179Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 98 | rowspan=2|178.991122(41) | rowspan=2 style="text-align:center" | 1983 | rowspan=2|437(9) ms | α (60%) | 175Au | rowspan=2|1/2+ | rowspan=2| | rowspan=2| |- | β+ (40%) | 179Hg |-id=Thallium-179m1 | style="text-indent:1em" | 179m1Tl | | colspan="3" style="text-indent:2em" | 825(10)# keV | style="text-align:center" | 1983 | 1.41(2) ms | α | 175Au | (11/2−)
|-id=Thallium-179m2 | style="text-indent:1em" | 179m2Tl | | colspan="3" style="text-indent:2em" | 904.5(9) keV | style="text-align:center" | 2017 | 119(14) ns | IT | 179Tl | (9/2−)
|-id=Thallium-180 | rowspan=3|180Tl | rowspan=3| | rowspan=3 style="text-align:right" | 81 | rowspan=3 style="text-align:right" | 99 | rowspan=3|179.989919(75) | rowspan=3 style="text-align:center" | 1987 | rowspan=3|1.09(1) s | β+ (93%) | 180Hg | rowspan=3|(4-) | rowspan=3| | rowspan=3| |- | α (7%) | 176Au |- | β+, SF (0.0032%) | (various) |-id=Thallium-181 | rowspan=2|181Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 100 | rowspan=2|180.9862600(98) | rowspan=2 style="text-align:center" | 1996 | rowspan=2|2.9(1) s | β+ (91.4%) | 181Hg | rowspan=2|1/2+ | rowspan=2| | rowspan=2| |- | α (8.6%) | 177Au |-id=Thallium-181m | rowspan=2 style="text-indent:1em" | 181mTl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 835.9(4) keV | rowspan=2 style="text-align:center" | 1998 | rowspan=2|1.40(3) ms | IT (99.60%) | 181Tl | rowspan=2|(9/2−) | rowspan=2| | rowspan=2| |- | α (0.40%) | 177Au |-id=Thallium-182 | rowspan=3|182Tl | rowspan=3| | rowspan=3 style="text-align:right" | 81 | rowspan=3 style="text-align:right" | 101 | rowspan=3|181.985693(13) | rowspan=3 style="text-align:center" | 1991 | rowspan=3|1.9(1) s | β+ (<99.41%) | 182Hg | rowspan=3|(4−) | rowspan=3| | rowspan=3| |- | α (>0.49%) | 178Au |- | β+, SF (<3.4%) | 182Hg |-id=Thallium-182m | rowspan=2 style="text-indent:1em" | 182mTlOrder of ground state and isomer is uncertain. | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 50(50)# keV | rowspan=2 style="text-align:center" | 2003 | rowspan=2|3.1(10) s | β+ (97.5%) | 182Hg | rowspan=2|(7+) | rowspan=2| | rowspan=2| |- | α (2.5%) | 178Au |-id=Thallium-183 | rowspan=2|183Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 102 | rowspan=2|182.982193(10) | rowspan=2 style="text-align:center" | 1980 | rowspan=2|6.9(7) s | β+ (?%) | 183Hg | rowspan=2|1/2+ | rowspan=2| | rowspan=2| |- | α (?%) | 179Au |-id=Thallium-183m1 | rowspan=3 style="text-indent:1em" | 183m1Tl | rowspan=3| | rowspan=3 colspan="3" style="text-indent:2em" | 628.7(5) keV | rowspan=3 style="text-align:center" | 1999 | rowspan=3|53.3(3) ms | IT (?%) | 183Tl | rowspan=3|(9/2−) | rowspan=3| | rowspan=3| |- | α (1.5%) | 179Au |- | β+ (?%) | 183Hg |-id=Thallium-183m2 | style="text-indent:1em" | 183m2Tl | | colspan="3" style="text-indent:2em" | 975.3(6) keV | style="text-align:center" | 2001 | 1.48(10) μs | IT | 183Tl | (13/2+)
|-id=Thallium-184 | rowspan=2|184Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 103 | rowspan=2|183.981875(11) | rowspan=2 style="text-align:center" | 1976 | rowspan=2|9.5(2) s | β+ (98.78%) | 184Hg | rowspan=2|2− | rowspan=2| | rowspan=2| |- | α (1.22%) | 180Au |-id=Thallium-184m1 | rowspan=2 style="text-indent:1em" | 184m1Tl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | −50(30) keV | rowspan=2 style="text-align:center" | 2016 | rowspan=2|10.6(5) s | β+ (99.53%) | 184Hg | rowspan=2|(7+) | rowspan=2| | rowspan=2| |- | α (0.47%) | 180Au |-id=Thallium-184m2 | rowspan=2 style="text-indent:1em" | 184m2Tl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 450(30) keV | rowspan=2 style="text-align:center" | 2015 | rowspan=2| 47.1(7) ms | IT (99.91%) | 184Tl | rowspan=2|(10−) | rowspan=2| | rowspan=2| |- | α (0.089%) | 180Au |-id=Thallium-185 | 185Tl | | style="text-align:right" | 81 | style="text-align:right" | 104 | 184.978789(22) | style="text-align:center" | 1976 | 19.5(5) s | β+ | 185Hg | 1/2+
|-id=Thallium-185m | rowspan=2 style="text-indent:1em" | 185mTl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 454.8(15) keV | rowspan=2 style="text-align:center" | 1977 | rowspan=2|1.93(8) s | IT | 185Tl | rowspan=2|9/2− | rowspan=2| | rowspan=2| |- | α (?%) | 181Au |-id=Thallium-186 | rowspan=2|186Tl | rowspan=2| | rowspan=2 style="text-align:right" | 81 | rowspan=2 style="text-align:right" | 105 | rowspan=2|185.978655(22) | rowspan=2 style="text-align:center" | 1974 | rowspan=2|3.5(5) s | β+ (?%) | 186Hg | rowspan=2|(2−) | rowspan=2| | rowspan=2| |- | α (?%) | 182Au |-id=Thallium-186m1 | rowspan=2 style="text-indent:1em" | 186m1Tl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 20(40) keV | rowspan=2 style="text-align:center" | 1975 | rowspan=2|27.5(10) s | β+ (99.99%) | 186Hg | rowspan=2|7+ | rowspan=2| | rowspan=2| |- | α (0.006%) | 182Au |-id=Thallium-186m2 | rowspan=2 style="text-indent:1em" | 186m2Tl | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 390(40) keV | rowspan=2 style="text-align:center" | 1975 | rowspan=2|3.40(9) s | IT (<94.1%) | 186Tl | rowspan=2|10− | rowspan=2| | rowspan=2| |- | β+ (>5.9%) | 186Hg |-id=Thallium-187 | 187Tl | | style="text-align:right" | 81 | style="text-align:right" | 106 | 186.9759047(86) | style="text-align:center" | 1976 | ~51 s | β+ | 187Hg | 1/2+
|-id=Thallium-187m1 | rowspan=3 style="text-indent:1em" | 187m1Tl | rowspan=3| | rowspan=3 colspan="3" style="text-indent:2em" | 334(3) keV | rowspan=3 style="text-align:center" | 1977 | rowspan=3|15.60(12) s | β+ (?%) | 187Hg | rowspan=3|9/2− | rowspan=3| | rowspan=3| |- | IT (?%) | 187Tl |- | α (0.15%) | 183Au |-id=Thallium-187m2 | style="text-indent:1em" | 187m2Tl | | colspan="3" style="text-indent:2em" | 1875(50)# keV | style="text-align:center" | 2000 | 1.11(7) μs | IT | 187Tl
|-id=Thallium-187m3 | style="text-indent:1em" | 187m3Tl | | colspan="3" style="text-indent:2em" | 2582.5(3) keV | style="text-align:center" | 2000 | 693(38) ns | IT | 187Tl | 29/2+#
|-id=Thallium-188 | 188Tl | | style="text-align:right" | 81 | style="text-align:right" | 107 | 187.976021(32) | style="text-align:center" | 1970 | 71(2) s | β+ | 188Hg | 2−#
|-id=Thallium-188m1 | style="text-indent:1em" | 188m1Tl | | colspan="3" style="text-indent:2em" | 30(30) keV | style="text-align:center" | 1974 | 71.5(15) s | β+ | 188Hg | 7+
|-id=Thallium-188m2 | style="text-indent:1em" | 188m2Tl | | colspan="3" style="text-indent:2em" | 299(30) keV | style="text-align:center" | 1981 | 41(4) ms | IT | 188Tl | 9−
|-id=Thallium-189 | 189Tl | | style="text-align:right" | 81 | style="text-align:right" | 108 | 188.9735735(90) | style="text-align:center" | 1972 | 2.3(2) min | β+ | 189Hg | 1/2+
|-id=Thallium-189m | style="text-indent:1em" | 189mTl | | colspan="3" style="text-indent:2em" | 285(6) keV | style="text-align:center" | 1974 | 1.4(1) min | β+ | 189Hg | 9/2−
|-id=Thallium-190 | 190Tl | | style="text-align:right" | 81 | style="text-align:right" | 109 | 189.9738418(78) | style="text-align:center" | 1970 | 2.6(3) min | β+ | 190Hg | 2−
|-id=Thallium-190m1 | style="text-indent:1em" | 190m1Tl | | colspan="3" style="text-indent:2em" | 70(7) keV | style="text-align:center" | 1970 | 3.6(3) min | β+ | 190Hg | 7+
|-id=Thallium-190m2 | style="text-indent:1em" | 190m2Tl | | colspan="3" style="text-indent:2em" | 306(10) keV | style="text-align:center" | 1991 | 60# ms | IT | 190Tl | (9−)
|-id=Thallium-191 | 191Tl | | style="text-align:right" | 81 | style="text-align:right" | 110 | 190.9717841(79) | style="text-align:center" | 1974 | 20# min | β+ | 191Hg | 1/2+
|-id=Thallium-191m | style="text-indent:1em" | 191mTl | | colspan="3" style="text-indent:2em" | 297(7) keV | style="text-align:center" | 1981 | 5.22(16) min | β+ | 191Hg | 9/2−
|-id=Thallium-192 | 192Tl | | style="text-align:right" | 81 | style="text-align:right" | 111 | 191.972225(34) | style="text-align:center" | 1961 | 9.6(4) min | β+ | 192Hg | 2−
|-id=Thallium-192m1 | style="text-indent:1em" | 192m1Tl | | colspan="3" style="text-indent:2em" | 196(7) keV | style="text-align:center" | 1961 | 10.8(2) min | β+ | 192Hg | 7+
|-id=Thallium-192m2 | style="text-indent:1em" | 192m2Tl | | colspan="3" style="text-indent:2em" | 447(7) keV | style="text-align:center" | 1980 | 296(5) ns | IT | 192Tl | (8−)
|-id=Thallium-192m3 | style="text-indent:1em" | 192m3Tl | | colspan="3" style="text-indent:2em" | 180(40) keV | style="text-align:center" | (1991)Half-life not measured, not included in discovery database | | α | 188Au | (3+)
|-id=Thallium-193 | 193Tl | | style="text-align:right" | 81 | style="text-align:right" | 112 | 192.9705020(72) | style="text-align:center" | 1960 | 21.6(8) min | β+ | 193Hg | 1/2+
|-id=Thallium-193m | rowspan=2 style="text-indent:1em" | 193mTl | rowspan=2|