Question

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

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.