All isotopes of an element have the same atomic number. The atomic number defines the element.

Uranium (U) has an atomic number of 92. This means that all isotopes of uranium will have 92 protons.

Isotopes differ in the number of neutrons they have. For example, uranium-235 and uranium-238 are both isotopes of uranium, but uranium-235 has 143 neutrons, while uranium-238 has 146 neutrons.