Uranium-235 has a mass number of 235 and also has 92 protons. The mass number of an atom is the total number of protons and neutrons (particles) in the nucleus of an atom. Therefore, to find out the number of neutrons in the nucleus you must subtract 235 from 92 which equals -143 (the number of neutrons in the nucleus of the atom). By contrast uranium-238 has a mass number of 238 so to find out number of neutrons you must perform the following calculation: 92 -238 = -146. Therefore, we can conclude that the difference between the two atoms of uranium is that there is three more neutrons in the nucleus of uranium-235.