<P> The mass number (symbol A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus . It determines the atomic mass of atoms . Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion . The mass number is different for each different isotope of a chemical element . This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element . Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N = A − Z . </P> <P> The mass number is written either after the element name or as a superscript to the left of an element's symbol . For example, the most common isotope of carbon is carbon - 12, or, which has 6 protons and 6 neutrons . The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: C . This is technically redundant, as each element is defined by its atomic number, so it is often omitted . </P>

What does the mass number of an isotope represent