Final answer:
U-238 and U-235 differ by the number of neutrons in their nuclei; U-238 has three more neutrons than U-235. Both isotopes have 92 protons, but their different neutron counts affect their nuclear properties, particularly in fission reactions. The correct answer is option c. three more neutrons
Step-by-step explanation:
The reason that U-238 is different from U-235 is due to the number of neutrons in their atomic nuclei. Uranium-238 has three more neutrons than Uranium-235.
Both isotopes have the same number of protons since they are both isotopes of uranium, which has an atomic number of 92, indicating the presence of 92 protons. The mass number of an isotope equals the sum of its protons and neutrons. Therefore, for U-235, with a mass number of 235, the calculation to find the number of neutrons is: mass number (235) - atomic number (92) = 143 neutrons. For U-238, with a mass number of 238, the calculation is: mass number (238) - atomic number (92) = 146 neutrons. This extra presence of neutrons in U-238 as compared to U-235 makes them distinct, notably impacting their nuclear properties and reactivity, such as their potential to undergo fission.
The higher neutron count in U-238 makes it less likely to fission as compared to U-235, which has an odd number of neutrons (143). When an additional neutron is introduced to U-235, the result is an even number of neutrons, making the nuclear force more attractive and increasing the likelihood of fission, thus making U-235 a superior fission fuel. In nature, U-238 accounts for 99.27% of natural uranium, while U-235 is only about 0.72%.