In
chemistry and
physics, a
nucleon is one of the particles that makes up the
atomic nucleus. Each atomic nucleus consists of one or more nucleons, and each atom in turn consists of a cluster of nucleons surrounded by one or more electrons. There are two known kinds of nucleon: the
neutron and the
proton. The
mass number of a given atomic
isotope is identical to its number of nucleons. Thus the term nucleon number may be used in place of the more common terms mass number or atomic mass number.