The
mass number (
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.