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 9 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.

The most stable isotopes are 227Ac with a half-life of 21.772 years, 225Ac with a half-life of 9.919 days, and 226Ac with a half-life of 29.37 hours. All other isotopes have half-lives under seven hours, and most under a minute. The shortest-lived known isotope is 217Ac with a half-life of 69 ns.

Purified 227Ac comes into equilibrium with its decay products (mainly 227Th and 223Ra) after 185 days.

List of isotopes

|-id=Actinium-203 | 203Ac | | style="text-align:right" | 89 | style="text-align:right" | 114 | | style="text-align:center" | 2024 | | α | 199Fr | (1/2+) | |-id=Actinium-204 |204Ac | | style="text-align:right" | 89 | style="text-align:right" | 115 | | style="text-align:center" | 2022 | | α | 200Fr

|-id=Actinium-205 |205Ac | | style="text-align:right" | 89 | style="text-align:right" | 116 | 205.01514(6) | style="text-align:center" | 2014 | | α | 201Fr | 9/2− | |-id=Actinium-206 | 206Ac | | style="text-align:right" | 89 | style="text-align:right" | 117 | 206.01448(7) | style="text-align:center" | 1998 | 25(7) ms | α | 202Fr | (3+) | |-id=Actinium-206m | style="text-indent:1em" | 206mAc | | colspan="3" style="text-indent:2em" | 200(70) keV | style="text-align:center" | 1998 | 41(16) ms | α | 202mFr | (10−) | |-id=Actinium-207 | 207Ac | | style="text-align:right" | 89 | style="text-align:right" | 118 | 207.01197(6) | style="text-align:center" | 1994 | 31(8) ms | α | 203Fr | 9/2−# | |-id=Actinium-207m | style="text-indent:1em" | 207mAc | | colspan="3" style="text-indent:2em" | 374(2) keV | style="text-align:center" | 2026 | | IT | 207Ac | (13/2+) | |-id=Actinium-208 | 208Ac | | style="text-align:right" | 89 | style="text-align:right" | 119 | 208.01155(7) | style="text-align:center" | 1994 | 97(15) ms | α | 204Fr | (3+) | |-id=Actinium-208m | rowspan=2 style="text-indent:1em" | 208mAc | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 500(60) keV | rowspan=2 style="text-align:center" | 1994 | rowspan=2|28(7) ms | α | 204Fr | rowspan=2|(10−) | rowspan=2| |- | IT ? | 208Ac |-id=Actinium-209 | 209Ac | | style="text-align:right" | 89 | style="text-align:right" | 120 | 209.00950(6) | style="text-align:center" | 1968 | 94(10) ms | α | 205Fr | (9/2−) | |-id=Actinium-210 | 210Ac | | style="text-align:right" | 89 | style="text-align:right" | 121 | 210.00941(7) | style="text-align:center" | 1968 | 350(40) ms | α | 206Fr | 7+# | |-id=Actinium-211 | 211Ac | | style="text-align:right" | 89 | style="text-align:right" | 122 | 211.00767(6) | style="text-align:center" | 1968 | 213(25) ms | α | 207Fr | 9/2− | |-id=Actinium-212 | 212Ac | | style="text-align:right" | 89 | style="text-align:right" | 123 | 212.007836(23) | style="text-align:center" | 1968 | 895(28) ms | α | 208Fr | 7+ | |-id=Actinium-213 |213Ac | | style="text-align:right" | 89 | style="text-align:right" | 124 | 213.006593(13) | style="text-align:center" | 1968 | 738(16) ms | α | 209Fr | 9/2− | |-id=Actinium-214 | rowspan=2|214Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 125 | rowspan=2|214.006906(15) | rowspan=2 style="text-align:center" | 1968 | rowspan=2|8.2(2) s | α (93%) | 210Fr | rowspan=2|5+ | rowspan=2| |- | β+ (7%) | 214Ra |-id=Actinium-215 | rowspan=2|215Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 126 | rowspan=2|215.006474(13) | rowspan=2 style="text-align:center" | 1968 | rowspan=2|171(10) ms | α (99.91%) | 211Fr | rowspan=2|9/2− | rowspan=2| |- | β+ (0.09%) | 215Ra |-id=Actinium-215m1 | style="text-indent:1em" | 215m1Ac | | colspan="3" style="text-indent:2em" | 1796.0(9) keV | style="text-align:center" | 1983 | 185(30) ns | IT | 215Ac | (21/2−) | |-id=Actinium-215m2 | style="text-indent:1em" | 215m2Ac | | colspan="3" style="text-indent:2em" | 2488(50)# keV | style="text-align:center" | 1983 | 335(10) ns | IT | 215Ac | (29/2+) | |-id=Actinium-216 | 216Ac | | style="text-align:right" | 89 | style="text-align:right" | 127 | 216.008749(10) | style="text-align:center" | 1966 | 440(16) μs | α | 212Fr | (1−) | |-id=Actinium-216m1 | style="text-indent:1em" | 216m1Ac | | colspan="3" style="text-indent:2em" | 38(5) keV | style="text-align:center" | 1970 | 441(7) μs | α | 212Fr | (9−) | |-id=Actinium-216m2 | style="text-indent:1em" | 216m2Ac | | colspan="3" style="text-indent:2em" | 422(100)# keV | style="text-align:center" | (2006)Only published in a conference proceeding and not a refereed journal | ~300 ns | IT | 216Ac

|-id=Actinium-217 | 217Ac | | style="text-align:right" | 89 | style="text-align:right" | 128 | 217.009342(12) | style="text-align:center" | 1972 | 69(4) ns | α | 213Fr | 9/2− | |-id=Actinium-217m | style="text-indent:1em" | 217mAc | | colspan="3" style="text-indent:2em" | 2012(20) keV | style="text-align:center" | 1973 | 740(40) ns

| 29/2+ | |-id=Actinium-218 | 218Ac | | style="text-align:right" | 89 | style="text-align:right" | 129 | 218.01165(6) | style="text-align:center" | 1970 | 1.00(4) μs | α | 214Fr | (1−) | |-id=Actinium-218m | style="text-indent:1em" | 218mAc | | colspan="3" style="text-indent:2em" | 607(86)# keV | style="text-align:center" | 1994 | 103(11) ns | IT | 218Ac | (11+) | |-id=Actinium-219 | 219Ac | | style="text-align:right" | 89 | style="text-align:right" | 130 | 219.01242(6) | style="text-align:center" | 1970 | 9.4(10) μs | α | 215Fr | 9/2− | |-id=Actinium-220 | 220Ac | | style="text-align:right" | 89 | style="text-align:right" | 131 | 220.014755(7) | style="text-align:center" | 1970 | 26.36(19) ms | α | 216Fr | (3−) | |-id=Actinium-221 | 221Ac | | style="text-align:right" | 89 | style="text-align:right" | 132 | 221.01560(6) | style="text-align:center" | 1968 | 52(2) ms | α | 217Fr | 9/2−# | |-id=Actinium-222 | rowspan=2|222Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 133 | rowspan=2|222.017844(5) | rowspan=2 style="text-align:center" | 1949 | rowspan=2|5.0(5) s | α | 218Fr | rowspan=2|1− | rowspan=2| |- | β+ (<2%) | 222Ra |-id=Actinium-222m | rowspan=3 style="text-indent:1em" | 222mAc | rowspan=3| | rowspan=3 colspan="3" style="text-indent:2em" | 78(21) keV | rowspan=3 style="text-align:center" | 1972 | rowspan=3|1.05(5) min | α (98.6%) | 218Fr | rowspan=3|5+# | rowspan=3| |- | β+ (1.4%) | 222Ra |- | IT? | 222Ac |-id=Actinium-223 | rowspan=3|223Ac | rowspan=3| | rowspan=3 style="text-align:right" | 89 | rowspan=3 style="text-align:right" | 134 | rowspan=3|223.019136(7) | rowspan=3 style="text-align:center" | 1948 | rowspan=3|2.10(5) min | α | 219Fr | rowspan=3|(5/2−) | rowspan=3| |- | EC ? | 223Ra |- | CD (3.2×10−9%) | 209Bi + 14C |-id=Actinium-224 | rowspan=3|224Ac | rowspan=3| | rowspan=3 style="text-align:right" | 89 | rowspan=3 style="text-align:right" | 135 | rowspan=3|224.021722(4) | rowspan=3 style="text-align:center" | 1948 | rowspan=3|2.78(16) h | β+ (90.5%) | 224Ra | rowspan=3|(0−) | rowspan=3| |- | α (9.5%) | 220Fr |- | β− ? | 224Th |-id=Actinium-225 | rowspan=2|225AcHas medical uses | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 136 | rowspan=2|225.023229(5) | rowspan=2 style="text-align:center" | 1947 | rowspan=2|9.9190(21) d | α | 221Fr | rowspan=2|3/2− | rowspan=2|TraceIntermediate decay product of 237Np |- | CD (5.3×10−10%) | 211Bi + 14C |-id=Actinium-226 | rowspan=3|226Ac | rowspan=3| | rowspan=3 style="text-align:right" | 89 | rowspan=3 style="text-align:right" | 137 | rowspan=3|226.026097(3) | rowspan=3 style="text-align:center" | 1950 | rowspan=3|29.37(12) h | β− (83%) | 226Th | rowspan=3|(1−) | rowspan=3| |- | EC (17%) | 226Ra |- | α (0.006%) | 222Fr |-id=Actinium-227 | rowspan=2|227Ac | rowspan=2|ActiniumSource of element's name | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 138 | rowspan=2|227.0277506(21) | rowspan=2 style="text-align:center" | 1911 | rowspan=2|21.772(3) y | β− (98.62%) | 227Th | rowspan=2|3/2− | rowspan=2|TraceIntermediate decay product of 235U |- | α (1.38%) | 223Fr |-id=Actinium-228 | 228Ac | Mesothorium 2 | style="text-align:right" | 89 | style="text-align:right" | 139 | 228.0310197(22) | style="text-align:center" | 1908 | 6.15(2) h | β− | 228Th | 3+ | TraceIntermediate decay product of 232Th |-id=Actinium-228m1 | style="text-indent:1em" | 228m1Ac | | colspan="3" style="text-indent:2em" | 6.11(9) keV | style="text-align:center" | 2021 | 299(11) ns | IT | 228Ac | (1−) | |-id=Actinium-228m2 | style="text-indent:1em" | 228m2Ac | | colspan="3" style="text-indent:2em" | 20.6(5) keV | style="text-align:center" | 2021 | 115(25) ns | IT | 228Ac | (1−) | |-id=Actinium-229 | 229Ac | | style="text-align:right" | 89 | style="text-align:right" | 140 | 229.032947(13) | style="text-align:center" | 1952 | 62.7(5) min | β− | 229Th | 3/2+ | |-id=Actinium-230 | 230Ac | | style="text-align:right" | 89 | style="text-align:right" | 141 | 230.036327(17) | style="text-align:center" | 1973 | 122(3) s | β− | 230Th | (1+) | |-id=Actinium-231 | 231Ac | | style="text-align:right" | 89 | style="text-align:right" | 142 | 231.038393(14) | style="text-align:center" | 1973 | 7.5(1) min | β− | 231Th | 1/2+ | |-id=Actinium-232 | 232Ac | | style="text-align:right" | 89 | style="text-align:right" | 143 | 232.042034(14) | style="text-align:center" | 1986 | 1.98(8) min | β− | 232Th | (1+) | |-id=Actinium-233 | 233Ac | | style="text-align:right" | 89 | style="text-align:right" | 144 | 233.044346(14) | style="text-align:center" | 1983 | 143(10) s | β− | 233Th | (1/2+) | |-id=Actinium-234 | 234Ac | | style="text-align:right" | 89 | style="text-align:right" | 145 | 234.048139(15) | style="text-align:center" | 1986 | 45(2) s | β− | 234Th

|-id=Actinium-235 | 235Ac | | style="text-align:right" | 89 | style="text-align:right" | 146 | 235.050840(15) | style="text-align:center" | 2003 | 62(4) s | β− | 235Th | 1/2+# | |-id=Actinium-236 | 236Ac | | style="text-align:right" | 89 | style="text-align:right" | 147 | 236.05499(4) | style="text-align:center" | 2010 | | β− | 236Th

Actinides vs fission products

Notable isotopes Actinium-225

Actinium-225 is a highly radioactive isotope with 136 neutrons. It is almost exclusively an alpha emitter and has a half-life of 9.919 days. As of 2024, it is being researched as a possible alpha source in targeted alpha therapy. Actinium-225 undergoes a series of three alpha decays – via the short-lived francium-221 and astatine-217 – to 213Bi, which itself is used as an alpha source. Another benefit is that the decay chain of 225Ac ends in the nuclide 209Bi, which has a considerably shorter biological half-life than lead. However, a major factor limiting its usage is the difficulty in producing the short-lived isotope, as it is most commonly isolated from aging parent nuclides (such as 233U); it may also be produced in cyclotrons, linear accelerators, or fast breeder reactors.

Actinium-226 Actinium-226 is an isotope of actinium with a half-life of 29.37 hours. It mainly (83%) undergoes beta decay, sometimes (17%) undergo electron capture, and rarely (0.006%) undergo alpha decay. There are researches on 226Ac to use it in SPECT.

Actinium-227 Actinium-227 is the most stable isotope of actinium, with a half-life of 21.772 years. It mainly (98.62%) undergoes beta decay, but sometimes (1.38%) it will undergo alpha decay instead. 227Ac is a member of the actinium series. It is found only in traces in uranium ores – one tonne of uranium in ore contains about 0.2 milligrams of 227Ac. 227Ac is prepared, in milligram amounts, by the neutron irradiation of in a nuclear reactor.