Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is relatively stable, with a half-life of 1.40×1010 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of 230Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.
Thirty-one radioisotopes have been characterized, with the most stable being 232Th, 230Th with a half-life of 75,400 years, 229Th with a half-life of 7,916 years, and 228Th with a half-life of 1.91 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be It has been proposed to perform laser spectroscopy of the 229Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.
The known isotopes of thorium range in mass number from 207 to 238.
List of isotopes
|-id=Thorium-207 | 207Th | | style="text-align:right" | 90 | style="text-align:right" | 117 | | style="text-align:center" | 2022 | | α | 203Ra
|-id=Thorium-208 | 208Th | | style="text-align:right" | 90 | style="text-align:right" | 118 | 208.017915(34) | style="text-align:center" | 2010 | 2.4(12) ms | α | 204Ra | 0+
|-id=Thorium-209 | 209Th | | style="text-align:right" | 90 | style="text-align:right" | 119 | 209.017998(27) | style="text-align:center" | 1996 | 3.1(12) ms | α | 205Ra | 13/2+
|-id=Thorium-210 | 210Th | | style="text-align:right" | 90 | style="text-align:right" | 120 | 210.015094(20) | style="text-align:center" | 1995 | 16.0(36) ms | α | 206Ra | 0+
|-id=Thorium-211 | 211Th | | style="text-align:right" | 90 | style="text-align:right" | 121 | 211.014897(92) | style="text-align:center" | 1995 | 48(20) ms | α | 207Ra | 5/2−#
|-id=Thorium-211m | style="text-indent:1em" | 211mTh | | colspan="3" style="text-indent:2em" | 778(8) keV | style="text-align:center" | 2021 | 83(8) μs | IT | 211Th | 13/2+
|-id=Thorium-212 | 212Th | | style="text-align:right" | 90 | style="text-align:right" | 122 | 212.013002(11) | style="text-align:center" | 1980 | 31.7(13) ms | α | 208Ra | 0+
|-id=Thorium-213 | 213Th | | style="text-align:right" | 90 | style="text-align:right" | 123 | 213.0130115(99) | style="text-align:center" | 1968 | 144(21) ms | α | 209Ra | 5/2−
|-id=Thorium-213m | style="text-indent:1em" | 213mTh | | colspan="3" style="text-indent:2em" | 1180.0(14) keV | style="text-align:center" | 2007 | 1.4(4) μs | IT | 213Th | (13/2)+
|-id=Thorium-214 | 214Th | | style="text-align:right" | 90 | style="text-align:right" | 124 | 214.011481(11) | style="text-align:center" | 1968 | 87(10) ms | α | 210Ra | 0+
|-id=Thorium-214m | style="text-indent:1em" | 214mTh | | colspan="3" style="text-indent:2em" | 2181.0(27) keV | style="text-align:center" | 2007 | 1.24(12) μs | IT | 214Th | 8+#
|-id=Thorium-215 | 215Th | | style="text-align:right" | 90 | style="text-align:right" | 125 | 215.0117246(68) | style="text-align:center" | 1968 | 1.35(14) s | α | 211Ra | (1/2−)
|-id=Thorium-215m | style="text-indent:1em" | 215mTh | | colspan="3" style="text-indent:2em" | 1471(50)# keV | style="text-align:center" | 2005 | 770(60) ns | IT | 215Th | 9/2+#
|-id=Thorium-216 | 216Th | | style="text-align:right" | 90 | style="text-align:right" | 126 | 216.011056(12) | style="text-align:center" | 1968 | 26.28(16) ms | α | 212Ra | 0+
|-id=Thorium-216m1 | rowspan=2 style="text-indent:1em" | 216m1Th | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 2041(8) keV | rowspan=2 style="text-align:center" | 1983 | rowspan=2|135.4(29) μs | IT (97.2%) | 216Th | rowspan=2|8+ | rowspan=2| | rowspan=2| |- | α (2.8%) | 212Ra |-id=Thorium-216m2 | style="text-indent:1em" | 216m2Th | | colspan="3" style="text-indent:2em" | 2648(8) keV | style="text-align:center" | 2001 | 580(26) ns | IT | 216Th | (11−)
|-id=Thorium-216m3 | style="text-indent:1em" | 216m3Th | | colspan="3" style="text-indent:2em" | 3682(8) keV | style="text-align:center" | 2001 | 740(70) ns | IT | 216Th | (14+)
|-id=Thorium-217 | 217Th | | style="text-align:right" | 90 | style="text-align:right" | 127 | 217.013103(11) | style="text-align:center" | 1968 | 248(4) μs | α | 213Ra | 9/2+#
|-id=Thorium-217m1 | style="text-indent:1em" | 217m1Th | | colspan="3" style="text-indent:2em" | 673.3(1) keV | style="text-align:center" | 1989 | 141(50) ns | IT | 217Th | (15/2−)
|-id=Thorium-217m2 | style="text-indent:1em" | 217m2Th | | colspan="3" style="text-indent:2em" | 2307(32) keV | style="text-align:center" | 2005 | 71(14) μs | IT | 217Th | (25/2+)
|-id=Thorium-218 | 218Th | | style="text-align:right" | 90 | style="text-align:right" | 128 | 218.013276(11) | style="text-align:center" | 1973 | 122(5) ns | α | 214Ra | 0+
|-id=Thorium-219 | 219Th | | style="text-align:right" | 90 | style="text-align:right" | 129 | 219.015526(61) | style="text-align:center" | 1973 | 1.023(18) μs | α | 215Ra | 9/2+#
|-id=Thorium-220 | 220Th | | style="text-align:right" | 90 | style="text-align:right" | 130 | 220.015770(15) | style="text-align:center" | 1973 | 10.2(3) μs | α | 216Ra | 0+
|-id=Thorium-221 | 221Th | | style="text-align:right" | 90 | style="text-align:right" | 131 | 221.0181858(86) | style="text-align:center" | 1970 | 1.75(2) ms | α | 217Ra | 7/2+#
|-id=Thorium-222 | 222Th | | style="text-align:right" | 90 | style="text-align:right" | 132 | 222.018468(11) | style="text-align:center" | 1970 | 2.24(3) ms | α | 218Ra | 0+
|-id=Thorium-223 | 223Th | | style="text-align:right" | 90 | style="text-align:right" | 133 | 223.0208111(85) | style="text-align:center" | 1952 | 0.60(2) s | α | 219Ra | (5/2)+
|-id=Thorium-224 | 224Th | | style="text-align:right" | 90 | style="text-align:right" | 134 | 224.021466(10) | style="text-align:center" | 1949 | 1.04(2) s | αTheorized to also undergo β+β+ decay to 224Ra | 220Ra | 0+
|-id=Thorium-225 | rowspan=2|225Th | rowspan=2| | rowspan=2 style="text-align:right" | 90 | rowspan=2 style="text-align:right" | 135 | rowspan=2|225.0239510(55) | rowspan=2 style="text-align:center" | 1949 | rowspan=2|8.75(4) min | α (~90%) | 221Ra | rowspan=2|3/2+ | rowspan=2| | rowspan=2| |- | EC (~10%) | 225Ac |-id=Thorium-226 | rowspan=2|226Th | rowspan=2| | rowspan=2 style="text-align:right" | 90 | rowspan=2 style="text-align:right" | 136 | rowspan=2|226.0249037(48) | rowspan=2 style="text-align:center" | 1948 | rowspan=2|30.70(3) min | α | 222Ra | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | CD (<%) | 208Pb + 18O |-id=Thorium-227 | 227Th | Radioactinium | style="text-align:right" | 90 | style="text-align:right" | 137 | 227.0277025(22) | style="text-align:center" | 1906 | 18.693(4) d | α | 223Ra | (1/2+) | TraceIntermediate decay product of 235U | |-id=Thorium-228 | rowspan=2|228Th | rowspan=2|Radiothorium | rowspan=2 style="text-align:right" | 90 | rowspan=2 style="text-align:right" | 138 | rowspan=2|228.0287397(19) | rowspan=2 style="text-align:center" | 1905 | rowspan=2|1.9125(7) y | α | 224Ra | rowspan=2|0+ | rowspan=2|TraceIntermediate decay product of 232Th | rowspan=2| |- | CD (1.13×10−11%) | 208Pb + 20O |-id=Thorium-229 | 229Th | | style="text-align:right" | 90 | style="text-align:right" | 139 | 229.0317614(26) | style="text-align:center" | 1947 | 7916(17) y | α | 225Ra | 5/2+ | TraceIntermediate decay product of 237Np | |-id=Thorium-229m | style="text-indent:1em" | 229mTh | | colspan="3" style="text-indent:2em" | 8.355733554021(8) eV | style="text-align:center" | 1990 | 7(1) μs | ITNeutral 229mTh decays rapidly by internal conversion, ejecting an electron. There is not enough energy to eject a second electron, so 229mTh+ ions live much longer, decaying by gamma emission. See . | 229Th+ | 3/2+
|-id=Thorium-229m | style="text-indent:1em" | 229mTh+ | | colspan="3" style="text-indent:2em" | 8.355733554021(8) eV | style="text-align:center" | (1990)Same isomeric state as above | 29(1) min | γ | 229Th+ | 3/2+
|-id=Thorium-230 | rowspan=3|230ThUsed in uranium–thorium dating | rowspan=3|Ionium | rowspan=3 style="text-align:right" | 90 | rowspan=3 style="text-align:right" | 140 | rowspan=3|230.0331323(13) | rowspan=3 style="text-align:center" | 1907 | rowspan=3|7.54(3)×104 y | α | 226Ra | rowspan=3|0+ | rowspan=3|0.0002(2)Intermediate decay product of 238U | rowspan=3| |- | CD (5.8×10−11%) | 206Hg + 24Ne |- | SF (<4×10−12%) | (various) |-id=Thorium-231 | 231Th | Uranium Y | style="text-align:right" | 90 | style="text-align:right" | 141 | 231.0363028(13) | style="text-align:center" | 1911 | 25.52(1) h | β− | 231Pa | 5/2+ | Trace | |-id=Thorium-232 | rowspan=3|232ThPrimordial radionuclide | rowspan=3|Thorium | rowspan=3 style="text-align:right" | 90 | rowspan=3 style="text-align:right" | 142 | rowspan=3|232.0380536(15) | rowspan=3 style="text-align:center" | 1898 | rowspan=3|1.40(1)×1010 y | αTheorized to also undergo β−β− decay to 232U | 228Ra | rowspan=3|0+ | rowspan=3|0.9998(2) | rowspan=3| |- | SF (1.1 × 10−9%) | (various) |- | CD (<2.78×10−10%) | 208Hg + 24Ne206Hg + 26Ne |-id=Thorium-233 | 233Th | | style="text-align:right" | 90 | style="text-align:right" | 143 | 233.0415801(15) | style="text-align:center" | 1935 | 21.83(4) min | β− | 233Pa | 1/2+ | TraceProduced in neutron capture by 232Th | |-id=Thorium-234 | 234Th | Uranium X1 | style="text-align:right" | 90 | style="text-align:right" | 144 | 234.0435998(28) | style="text-align:center" | 1900 | 24.107(24) d | β− | 234mPaENSDF analysis available at | 0+ | Trace | |-id=Thorium-235 | 235Th | | style="text-align:right" | 90 | style="text-align:right" | 145 | 235.047255(14) | style="text-align:center" | 1969 | 7.2(1) min | β− | 235Pa | 1/2+#
|-id=Thorium-236 | 236Th | | style="text-align:right" | 90 | style="text-align:right" | 146 | 236.049657(15) | style="text-align:center" | 1973 | 37.3(15) min | β− | 236Pa | 0+
|-id=Thorium-237 | 237Th | | style="text-align:right" | 90 | style="text-align:right" | 147 | 237.053629(17) | style="text-align:center" | 1993 | 4.8(5) min | β− | 237Pa | 5/2+#
|-id=Thorium-238 | 238Th | | style="text-align:right" | 90 | style="text-align:right" | 148 | 238.05639(30)# | style="text-align:center" | 1999 | 9.4(20) min | β− | 238Pa | 0+
Uses Thorium has been suggested for use in thorium-based nuclear power.
In many countries the use of thorium in consumer products is banned or discouraged because it is radioactive.
It is currently used in cathodes of vacuum tubes, for a combination of physical stability at high temperature and a low work energy required to remove an electron from its surface.
It has, for about a century, been used in mantles of gas and vapor lamps such as gas lights and camping lanterns.
Low dispersion lenses Thorium was also used in certain glass elements of Aero-Ektar lenses made by Kodak during World War II. Thus they are mildly radioactive.f2.5 Aero Ektar Lenses Some images. Two of the glass elements in the f/2.5 Aero-Ektar lenses are 11% and 13% thorium by weight. The thorium-containing glasses were used because they have a high refractive index with a low dispersion (variation of index with wavelength), a highly desirable property. Many surviving Aero-Ektar lenses have a tea colored tint, possibly due to radiation damage to the glass.
These lenses were used for aerial reconnaissance because the radiation level is not high enough to fog film over a short period. This would indicate the radiation level is reasonably safe. However, when not in use, it would be prudent to store these lenses as far as possible from normally inhabited areas; allowing the inverse square relationship to attenuate the radiation.