Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nature as a primordial nuclide.

Uranium-235, 235U
Uranium metal highly enriched in uranium-235
Namesuranium-235, U-235
Protons (Z)92
Neutrons (N)143
Nuclide data
Natural abundance0.72%
Half-life (t1/2)703800000 years
Isotope mass235.0439299 Da
Excess energy40914.062±1.970 keV
Binding energy1783870.285±1.996 keV
Parent isotopes235Pa
Decay products231Th
Decay modes
Decay modeDecay energy (MeV)
Isotopes of uranium
Complete table of nuclides
Diagram of a U-235 atom, with 92 protons, 143 neutrons and 92 electrons.

Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its fission cross section for slow thermal neutrons is about 584.3±1 barns.[1] For fast neutrons it is on the order of 1 barn.[2] Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.[citation needed]

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