Question

In comparing two isotopes of the same element, the atomic number:

changes to reflect the number of protons.
changes to reflect the new mass number.
changes to reflect the number of new neutrons.
stays the same.

Answer 1

In comparing two isotopes of the same element, the atomic number, stays the same.

Allow me to explain: The protons in an isotope ALWAYS stays the same, but the neutrons change. But, what's the atomic number made up of?

Protons, which means it won't change.

However, the atomic mass is made up of neutrons + protons, so only the atomic mass would change - not the atomic number.