The number at the end of an isotope’s name is the number.

The number written at the end of isotope is named as mass number.

Explanation:

Isotopes are defined as the chemical species of the same element which differ from its mass number. These species have same number of protons but have different number of neutrons among them.

For Example: The isotope of carbon which is C-14

Here, C represents the symbol of the element and the number represents the mass number of the element.

Hence, the number written at the end of isotope is named as mass number.