U-235, any of the different forms of an isotope of the element uranium with a mass number of 235 (the number of protons and neutrons in the atom's nucleus). When found in minerals, U-235 is always combined with the isotopes U-234 and U-238. Separating the natural U-235 from the other isotopes produces a more potent form of U-235, which is used in atomic bombs and nuclear reactors because it easily undergoes fission. The use of U-235 in nuclear reactors produces extremely toxic isotopes of other elements, such as strontium 90. U-235 was used in the atomic bombs that the United States dropped on Hiroshima and Nagasaki, Japan, in 1945.