Answer:DIFFERENT NUMBERS OF NEUTRONS
Isotopes;
Isotopic notation
Explanation: The mass number is simply the total number of protons and neutrons in an atom's nucleus. This number differentiates between the isotopes of an element. The atomic mass is the actual mass of the matter in an atom measured in relative mass units called unified atomic mass units (u). Atomic masses are always decimal numbers and are used for quantitative calculations in chemical reactions. Mass number is used in nuclear chemistry