Atomic Nucleus
The nucleus is the small, dense region consisting of protons and neutrons at the center of an atom.
The nucleus 1911 by Ernest Rutherford through the 1909 Geiger–Marsden gold foil experiment.
Geiger–Marsden gold foil experiment.
Rutherford discovered that every atom contains a nucleus where its positive charge and most of its mass are concentrated. They deduced this by measuring how an alpha particle beam is scattered when it strikes a thin metal foil.
Rutherford knew that Alpha particles had had mass and velocity. He hypothisized that these particles would go through the gold foil, and may scatter a bit because of the original belief of "the blueberry muffin".
After shooting alpha particles through the thin sheet of gold, Geiger, Marsden and Rutherford discovered that a small amount of molecules were scattered at larger than 90° angles.
The original thought of "the blueberry muffin" cannot explain why these particles scattered larger than 90.
This caused them to conclude that there was a small fraction of the total volume of the atom that held most of the mass of the atom.