Answer: The correct answer is mass number.
Explanation:
An isotope is a chemical compound of the same element which only differ in the number of neutrons. The number of protons and electrons remain same for two isotopes. This means isotopes have different mass number but have same atomic number.
When we write an isotope, the number at the end of the isotope's name is the mass number.
When writing an isotope: Chlorine-35, the number 35 represents the atomic mass and not atomic number.
Hence, the correct answer is mass number.
