<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 12, 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: 12 6C . This is technically redundant, as each element is defined by its atomic number, so it is often omitted . </P> <P> Different types of radioactive decay are characterized by their changes in mass number as well as atomic number, according to the radioactive displacement law of Fajans and Soddy . For example, uranium - 238 usually decays by alpha decay, where the nucleus loses two neutrons and two protons in the form of an alpha particle . Thus the atomic number and the number of neutrons each decrease by 2 (Z: 92 → 90, N: 146 → 144), so that the mass number decreases by 4 (A = 238 → 234); the result is an atom of thorium - 234 and an alpha particle (4 2He2 +): </P> <Dl> <Dd> <Table> <Tr> <Td> 238 92 U </Td> <Td> → </Td> <Td> 234 90 Th </Td> <Td> + </Td> <Td> 2He2 + </Td> <Td> </Td> </Tr> </Table> </Dd> </Dl>

Mass number of a oxygen atom with 7 neutrons