Uranium-240

Main isotopes of uranium

iso NA half-life DM DE (MeV) DP 232U syn 68.9 y SF – – α 5.414 228Th 233U trace 1.592×105 y SF 197.93 – α 4.909 229Th 234U 0.005% 2.455×105 y SF 197.78 – α 4.859 230Th 235U 0.720% 7.04×108 y SF 202.48 – α 4.679 231Th 236U trace 2.342×107 y SF 201.82 – α 4.572 232Th 238U 99.274% 4.468×109 y α 4.270 234Th SF 205.87 – β−β− – 238Pu
Standard atomic weight (Ar)	7002238028910000000♠238.02891(3)

Uranium (₉₂U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes (uranium-238 and uranium-235) that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The standard atomic weight of natural uranium is 7002238028910000000♠238.02891(3). Other isotopes such as uranium-232 have been produced in breeder reactors.

Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.2739–99.2752% natural abundance), uranium-235 (0.7198–0.7202%), and uranium-234 (0.0050–0.0059%). All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683×10⁹ years (close to the age of the Earth).

Uranium-238 is an α emitter, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members that 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.

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