The dense central core of an atom that contains its protons and neutrons is the nucleus. It occupies only a tiny region at the atom’s centre but contains nearly all of the atom’s mass. The surrounding electrons occupy regions described by orbitals and electron shells.
Ernest Rutherford’s 1911 gold-foil experiment provided the decisive evidence for the nucleus. Most alpha particles passed through thin gold foil, but a small fraction were strongly deflected, suggesting that positive charge and mass were concentrated in a compact central core rather than spread throughout the atom.
The nucleus is held together by the strong nuclear force, which overcomes the electrical repulsion between positively charged protons at extremely short distances. Neutrons contribute mass and nuclear stability but carry no electric charge. The number of protons determines the element’s atomic number; changing the neutron count produces different isotopes of that element.
The nucleus is not the same as an electron shell or orbital. An orbital is a quantum-mechanical region where an electron is likely to be found, while a shell groups orbitals by principal energy level. Nuclear size is typically measured in femtometres, whereas atoms are roughly 100,000 times wider.