The atomic nucleus was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford.
The diameter of the nucleus is in the range of 1.6 fm (1.6×10−15 m) for hydrogen to about 15 fm for the heaviest atoms, such as uranium. These dimensions are much smaller than the diameter of the atom itself.
The branch of physics concerned with studying and understanding the atomic nucleus, including its composition and the forces which bind it together, is called nuclear physics.
The nuclear force is highly attractive at very small distances, and this overwhelms the repulsion between protons which is due to the electromagnetic force, thus allowing nuclei to exist. However, because the residual strong force has a limited range because it decays quickly with distance, only nuclei smaller than a certain size can be completely stable. The largest known completely stable nucleus is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons).
The nuclear radius (R) is considered to be one of the basic things that any model must explain. For stable nuclei the nuclear radius is roughly proportional to the cube root of the mass number (A) of the nucleus, and particularly in nuclei containing many nucleons, as they arrange in more spherical configurations.
In other words, packing protons and neutrons in the nucleus gives approximately the same total size result as packing hard spheres of a constant size (like marbles) into a tight spherical or semi-spherical bag.
