Question

Explain, in terms of protons and neutrons, why U-235 and U-238 are different isotopes of Uranium.

Answer 1

235 and 238 are just the mass numbers of Uranium (mass number consists of the number of protons and neutrons). They are different because each isotope of an element has a different mass number. However, the number of protons of Uranium remains the same ( because the number of protons of an element never changes; it is a feature of each element).
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Answer 2

U-235 and U-238 have identical numbers of protons (otherwise the element would be different), however they have different numbers of neutrons, hence the nucleon number is different. This means they are isotopes of one another.