Definition
Nucleus (Physics)
The nucleus is the dense central region of an atom that carries essentially all of its mass and its positive electric charge. It is bound together by the strong nuclear force and comprises nucleons: protons and neutrons.
Formally, writing for the mass number and for the atomic number, a nucleus is the pair
with charge . The proton number fixes the element and so the chemical identity of the atom; the neutron number fixes the isotope. The nuclear energy stored in the binding of these nucleons is far larger per unit mass than the chemical energy of the surrounding electrons.