- The positively-charged dense central core of an atom. All atomic nuclei, except protium, which is hydrogen-1, consist of protons and neutrons. The nucleus of hydrogen-1 consists of a single proton. In an uncharged atom, the number of electrons is the same as the number of protons. When two or more atoms have the same atomic number, but different atomic masses, they are isotopes of the same element. A nucleus is held together by the nuclear force, and comprises almost all of the mass of an atom. Also called nucleus (1).