Question

what is the number at the end of an isotopes name?
atomic number
neutron number
mass number
electron number

Explanation:

Isotopes are atoms of the same element with the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons). The naming convention for isotopes is "element name - mass number". For example, carbon - 14 has a mass number of 14. The atomic number is the number of protons (a characteristic of the element itself, not varying for isotopes in the naming context here). The neutron number is not directly stated in the name in that simple form. The electron number (in neutral atoms, equal to the proton number) is also not what the number at the end represents in isotope naming.

Answer:

mass number