Question

Structures of ${}^{235}U$ and ${}^{238}U$ differ by three :

• A. isotopes
• B. neutrons
• C. electrons
• D. protons

Answer 1

The correct option is B neutrons

Structures of U-235 and U-238 differ by three neutrons.
The atomic number of U is 92. U has 92 protons and 92 electrons. U-235 contains $235-92=143$ neutrons. U-238 contains $238-92=146$ neutrons. The difference in the number of neutrons between two isotopes of uranium is $238-235=3$
Note: Atomic masses are different for isotopes of uranium.
Different isotopes of uranium have different number of neutrons and different atomic masses. They have same atomic number, same number of electrons and same number of protons.