Isotopes are atomic species of the same chemical element (and therefore with the same atomic number (Z) or number of protons) which differ with respect to their number of neutrons. Therefore each isotope of an element has a different mass number (A), which is the simple summation of the number of protons and neutrons.

For example, the element carbon has six protons and its commonest isotope on earth has six neutrons, resulting in a mass number of 12 . Our preference on Radiopaedia is to express this in longhand as carbon-12, or in abbreviated form as C-12. For further explanation, see isotope notation.