Uranium (U) is a naturally occurring radioactive element (radioelement) with no stable isotopes. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from U to U (except for U). The standard atomic weight of natural uranium is .

Natural uranium consists of three main isotopes, U (99.2739–99.2752% natural abundance), U (0.7198–0.7202%), and U (0.0050–0.0059%). All three isotopes are radioactive (i.e., they are radioisotopes), and the most abundant and stable is uranium-238, with a half-life of (about the age of the Earth).

Uranium-238 is an alpha emitter, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members and ends in lead-207. The constant rates of decay in these series makes comparison of the ratios of parent-to-daughter elements useful in radiometric dating. Uranium-233 is made from thorium-232 by neutron bombardment.

Uranium-235 is important for both nuclear reactors (energy production) and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile in response to thermal neutrons, i.e., thermal neutron capture has a high probability of inducing fission. A chain reaction can be sustained with a large enough (critical) mass of uranium-235. Uranium-238 is also important because it is fertile: it absorbs neutrons to produce a radioactive isotope that decays into plutonium-239, which also is fissile.

List of isotopes

|-id=Uranium-214 | U | | style="text-align:right" | 92 | style="text-align:right" | 122 | | style="text-align:center" | 2021 | | α | Th | 0+

|-id=Uranium-215 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 123 | rowspan=2|215.026720(11) | rowspan=2 style="text-align:center" | 2015 | rowspan=2| | α | Th | rowspan=2|5/2−# | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-216 | U | | style="text-align:right" | 92 | style="text-align:right" | 124 | 216.024760(30) | style="text-align:center" | 2015 | | α | Th | 0+

|-id=Uranium-216m | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 2015 | | α | Th | 8+

|-id=Uranium-217 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 125 | rowspan=2|217.024660(86)# | rowspan=2 style="text-align:center" | 2000 | rowspan=2| | α | Th | rowspan=2|(1/2−) | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-218 | U | | style="text-align:right" | 92 | style="text-align:right" | 126 | 218.023505(15) | style="text-align:center" | 1992 | | α | Th | 0+

|-id=Uranium-218m | rowspan=2 style="text-indent:1em" | U | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | 2007 | rowspan=2| | α | Th | rowspan=2|8+ | rowspan=2| | rowspan=2| |- | IT? | U |-id=Uranium-219 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 127 | rowspan=2|219.025009(14) | rowspan=2 style="text-align:center" | 1993 | rowspan=2| | α | Th | rowspan=2|(9/2+) | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-221 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 129 | rowspan=2|221.026323(77) | rowspan=2 style="text-align:center" | 2015 | rowspan=2| | α | Th | rowspan=2|(9/2+) | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-222 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 130 | rowspan=2|222.026058(56) | rowspan=2 style="text-align:center" | 1983 | rowspan=2| | α | Th | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-223 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 131 | rowspan=2|223.027961(63) | rowspan=2 style="text-align:center" | 1991 | rowspan=2| | α | Th | rowspan=2|7/2+# | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-224 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 132 | rowspan=2|224.027636(16) | rowspan=2 style="text-align:center" | 1991 | rowspan=2| | α | Th | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-225 | U | | style="text-align:right" | 92 | style="text-align:right" | 133 | 225.029385(11) | style="text-align:center" | 1989 | | α | Th | 5/2+#

|-id=Uranium-226 | U | | style="text-align:right" | 92 | style="text-align:right" | 134 | 226.029339(12) | style="text-align:center" | 1973 | | α | Th | 0+

|-id=Uranium-227 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 135 | rowspan=2|227.0311811(91) | rowspan=2 style="text-align:center" | 1952 | rowspan=2| | α | Th | rowspan=2|(3/2+) | rowspan=2| | rowspan=2| |- | β? | Pa |-id=Uranium-228 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 136 | rowspan=2|228.031369(14) | rowspan=2 style="text-align:center" | 1949 | rowspan=2| | α (97.5%) | Th | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | EC (2.5%) | Pa |-id=Uranium-229 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 137 | rowspan=2|229.0335060(64) | rowspan=2 style="text-align:center" | 1949 | rowspan=2| | β (80%) | Pa | rowspan=2|(3/2+) | rowspan=2| | rowspan=2| |- | α (20%) | Th |-id=Uranium-230 | rowspan=3|U | rowspan=3| | rowspan=3 style="text-align:right" | 92 | rowspan=3 style="text-align:right" | 138 | rowspan=3|230.0339401(48) | rowspan=3 style="text-align:center" | 1948 | rowspan=3| | α | Th | rowspan=3|0+ | rowspan=3| | rowspan=3| |- | SF ? | (various) |- | CD (4.8×10%) | Pb + Ne |-id=Uranium-231 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 139 | rowspan=2|231.0362922(29) | rowspan=2 style="text-align:center" | 1949 | rowspan=2| | EC | Pa | rowspan=2|5/2+# | rowspan=2| | rowspan=2| |- | α (.004%) | Th |-id=Uranium-232 | rowspan=4|U | rowspan=4| | rowspan=4 style="text-align:right" | 92 | rowspan=4 style="text-align:right" | 140 | rowspan=4|232.0371548(19) | rowspan=4 style="text-align:center" | 1949 | rowspan=4| | α | Th | rowspan=4|0+ | rowspan=4| | rowspan=4| |- | CD (8.9×10%) | Pb + Ne |- | SF (2.7x10%) | (various) |- | CD ? | Hg + Mg |-id=Uranium-233 | rowspan=4|U | rowspan=4| | rowspan=4 style="text-align:right" | 92 | rowspan=4 style="text-align:right" | 141 | rowspan=4|233.0396343(24) | rowspan=4 style="text-align:center" | 1947 | rowspan=4| | α | Th | rowspan=4|5/2+ | rowspan=4|TraceIntermediate decay product of Np | rowspan=4| |- | CD (7.2×10%) | Pb + Ne |- | SF ? | (various) |- | CD ? | Hg + Mg |-id=Uranium-234m | rowspan=4|UUsed in uranium–thorium datingUsed in uranium–uranium dating | rowspan=4|Uranium II | rowspan=4 style="text-align:right" | 92 | rowspan=4 style="text-align:right" | 142 | rowspan=4|234.0409503(12) | rowspan=4 style="text-align:center" | 1912 | rowspan=4| | α | Th | rowspan=4|0+ | rowspan=4|[0.000054(5)]Intermediate decay product of U | rowspan=4|0.000050–0.000059 |- | SF (1.64×10%) | (various) |- | CD (1.4×10%) | Hg + Mg |- | CD (9×10%) | Pb + NePb + Ne |-id=Uranium-234m | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1962 | | IT | U | 6−

|-id=Uranium-235 | rowspan=4|UPrimordial radionuclideUsed in Uranium–lead datingImportant in nuclear reactors | rowspan=4|Actino-UraniumActin-Uranium | rowspan=4 style="text-align:right" | 92 | rowspan=4 style="text-align:right" | 143 | rowspan=4|235.0439281(12) | rowspan=4 style="text-align:center" | 1935 | rowspan=4| | α | Th | rowspan=4|7/2− | rowspan=4|[0.007204(6)] | rowspan=4|0.007198–0.007207 |- | SF (7×10%) | (various) |- | CD (8×10%) | Pb + NePb + Ne |- | CD (<1.8×10%) | Hg + MgHg + Mg |-id=Uranium-235m1 | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1957 | | IT | U | 1/2+

|-id=Uranium-235m2 | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 2007 | | SF | (various)

|-id=Uranium-236 | rowspan=3|U | rowspan=3|Thoruranium | rowspan=3 style="text-align:right" | 92 | rowspan=3 style="text-align:right" | 144 | rowspan=3|236.0455661(12) | rowspan=3 style="text-align:center" | 1951 | rowspan=3| | α | Th | rowspan=3|0+ | rowspan=3|10−11 to 10−10Natural and anthropogenic 236U ... uranium and uranium ore samplesIntermediate decay product of Pu, also produced by neutron capture of U | rowspan=3| |- | SF (9.6×10%) | (various) |- | CD (2.0×10%) | Hg + MgHg + Mg |-id=Uranium-236m1 | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1973 | | IT | U | 4−

|-id=Uranium-236m2 | rowspan=2 style="text-indent:1em" | U | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | 1969 | rowspan=2| | IT (87%) | U | rowspan=2|(0+) | rowspan=2| | rowspan=2| |- | SF (13%) | (various) |-id=Uranium-237m | U | | style="text-align:right" | 92 | style="text-align:right" | 145 | 237.0487283(13) | style="text-align:center" | 1940 | | β | Np | 1/2+ | TraceNeutron capture product, parent of trace quantities of Np | |-id=Uranium-237m | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1968 | | IT | U | 7/2−

|-id=Uranium-238 | rowspan=3|U | rowspan=3|Uranium I | rowspan=3 style="text-align:right" | 92 | rowspan=3 style="text-align:right" | 146 | rowspan=3|238.050787618(15) | rowspan=3 style="text-align:center" | 1896 | rowspan=3| | α | Th | rowspan=3|0+ | rowspan=3|[0.992742(10)] | rowspan=3|0.992739–0.992752 |- | SF (5.44×10%) | (various) |- | ββ (2.2×10%) | Pu |-id=Uranium-238m | rowspan=2 style="text-indent:1em" | U | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | 1970 | rowspan=2| | IT (97.4%) | U | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | SF (2.6%) | (various) |-id=Uranium-239 | U | | style="text-align:right" | 92 | style="text-align:right" | 147 | 239.0542920(16) | style="text-align:center" | 1937 | | β | Np | 5/2+ | TraceNeutron capture product; parent of trace quantities of Pu | |-id=Uranium-239m1 | style="text-indent:1em" | U | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1975 | | IT | U | 1/2+

|-id=Uranium-239m2 | rowspan=2 style="text-indent:1em" | U | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | 1994 | rowspan=2| | SF? | (various) | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | IT? | U |-id=Uranium-240 | rowspan=2|U | rowspan=2| | rowspan=2 style="text-align:right" | 92 | rowspan=2 style="text-align:right" | 148 | rowspan=2|240.0565924(27) | rowspan=2 style="text-align:center" | 1953 | rowspan=2| | β | NpENSDF analysis available at | rowspan=2|0+ | rowspan=2|TraceIntermediate decay product of Pu | rowspan=2| |- | α? | Th |-id=Uranium-241 | U | | style="text-align:right" | 92 | style="text-align:right" | 149 | 241.06031(5) | style="text-align:center" | 2023 | | β | Np | 7/2+# | |--> |-id=Uranium-242 | U | | style="text-align:right" | 92 | style="text-align:right" | 150 | 242.06296(10) | style="text-align:center" | 1979 | | β | Np | 0+

Actinides vs fission products

Uranium-230