Represented by the symbol A, the total number of nucleons (Neutrons and Protons) in the nucleus of an Atom. All atoms of a chemical Element have the same Atomic Number but may have different mass numbers (from having different numbers of
neutrons in the nucleus). Atoms of an element with the same mass number make up an Isotope of the element. Isotopes of different elements may have the same mass number
but different numbers of protons.