Mass number Mass number The mass number (symbol "A", from the German word "Atomgewicht" (atomic weight), 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
Valley of stability atomic and neutron numbers, forming what has been called an island of stability outside of the valley of stability. All atomic nuclei are composed of protons and neutrons bound together by the nuclear force. There are 287 different nuclei that occur naturally on earth, each corresponding to a unique number of protons, called the atomic number, Z, and a unique number of neutrons, called the neutron number, N. The mass number, A, of a nuclide is the sum of atomic and neutron numbers, A=Z+N. Not all nuclides are stable, however. According to Byrne, stable nuclides are defined as those having