⚛️ The nucleus — Z protons and (A−Z) neutrons packed into a femtometre
Almost all of an atom's mass sits in a nucleus ten-thousand times smaller than the atom itself. Red spheres are protons, blue are neutrons; together they are the nucleons, held together by the short-range, charge-independent nuclear force. Watch it turn.
A dense cluster of protons (red) and neutrons (blue) — the nucleus, turning slowly so you can see it is a genuine three-dimensional ball of nucleons.