Competency #5
Answer and Explanation
U-235 has three fewer neutrons than U-238.
Since both are isotopes of uranium, they must each have 92 protons (otherwise, they would not be uranium).
We know that the mass number is equal to the sum of the protons and neutrons:
Mass number = # protons + # neutrons
We can rearrange the above equation to:
# neutrons = Mass number - # protons
# neutrons (U-238) = 238 – 92 = 146 neutrons
# neutrons (U-235) = 235 – 92 = 143 neutrons
U-238 has 92 protons and 146 neutrons, while U-235 has 92 protons and 143 neutrons.
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