The diameter of the nucleus is in the range of 1.6 fm (10−15 m) (for a proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as uranium). These dimensions are much smaller than the size of the atom itself by a factor of about 23,000 (uranium) to about 145,000 (hydrogen).
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 strong force is so named because it is significantly larger in magnitude than the other fundamental forces (electroweak, electromagnetic and gravitational). The strong force is highly attractive at very small distances, and this overwhelms the repulsion between protons due to the electromagnetic force, thus allowing nuclei to exist. However, because the residual strong force has a limited range, only nuclei smaller than a certain size can be completely stable. The largest known complete stable nucleus is lead-208 which contains 208 neutrons and protons. Nuclei larger than this maximal size of 208 particles generally become increasingly short-lived as the number of neutrons and protons which compose them increases beyond this number.
The residual strong force usually acts over a very short range (a few fermis, roughly one or two nucleon diameters) and causes an attraction between nucleons (protons and neutrons). However there are also halo nuclei such as lithium-11 or boron-14, in which dineutrons, or other collections of nucleons, orbit at distances of about ten fermis (similar to the size of lead-208). Such nuclei are always short-lived; for example, lithium-11 has a half-life of less than 8.6 milliseconds.
Nuclear radius is a basic experimentally measurable fact that any rational model simply has to explain. It is proportional to the cube root of the nuclear mass
This very important experimental fact implies that spherical nuclei have radii directly proportional to the cube root of their respective volumes (volume of a sphere = ). It says that nuclear volumes are generally equal to the sum of the respective volumes of constituent nucleons, like oranges packed together in a string bag, exactly as assumed in meson theory. This single experimental fact shows that protons and neutrons behave in some ways like spheres with fixed volumes that are packed together in a nucleus with a volume which approximates that which would result if the spheres were touching.
Early models of the nucleus viewed the nucleus as a rotating liquid drop. In this model, the trade-off of long-range electromagnetic forces and relatively short-range nuclear forces, together caused behavior which resembled surface tension forces in liquid drops of different sizes. This formula is successful at explaining many important phenomena of nuclei, such as their changing amounts of binding energy as their size changes, but it does not explain the special stability which occurs when nucleii have special "magic numbers" of protons or neutrons.
A number of models for the nucleus have also been proposed in which nucleons occupy orbitals, much like the atomic orbitals in atomic physics theory. These wave models imagine nucleons to be either sizeless point particles in potential wells, or else probability waves as in the "optical model", frictionlessly orbiting at high speed in potential wells.
In these models, the nucleons occupy orbitals in pairs, due to being fermions, but the exact nature and capacity of nuclear shells differs somewhat from those of elections in atomic orbitals, primarily because the potential well in which the nucleons move (especially in larger nucleii) is quite different from the central electromagnetic potential well which binds electrons in atoms. Nevertheless, the resemblance to atomic orbital models may be seen in a small atomic nucleus like that of helium-4, in which the two protons and two neutrons separately occupy 1s orbitals analagous to the 1s orbitals for the two electrons in the helium atom, and achieve unusual stability for the same reason. This stability also underlies the fact that nuclei with 5 nucleons are all extremely unstable and short-lived.
For larger nuclei, the shells occupied by nucleons begin to differ signifficantly from electron shells, but nevertheless, present nuclear theory does predict the "magic numbers" of filled nuclear shells for both protons and neutrons. The closure of the stable shells predicts unusually stable configurations, something like inert gases in chemistry. An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other element.