The small, central, positively charged region of an atom which carries essentially all the mass. Except for the nucleus of ordinary hydrogen, all atomic nuclei contain both protons and neutrons. The nuclei of isotopes of a given element contain the same number of protons, but differ in the number of neutrons. Thus, they have the same atomic number, and so are the same element, but they have different mass number (and masses). The nuclear properties (e.g. radioactivity, fission, neutron capture, etc.) of a given element are determined by both the number of neutrons and the number of protons.

See Electron, Neutron, Proton.