The density of ordinary matter is of the order ofbut the density of the nucleus is much higher. Most of the volume of an atom is taken up by the cloud of electrons around the nucleus, the nucleus itself occupying a much smaller volume at the centre of the atom.
The nuclear density in a typical nucleus can be approximately calculated. The radius of a typical nucleus iswhereis the mass number andisThe nuclear densitysatisfies
- The mass ofa uranium nucleus, isand The density is then
The components of an atom and of an atomic nucleus have varying densities. The proton is not a fundamental particle, being composed of quark-gluon matter. Its size is approximately and its densityUsing deep inelastic scattering, it has been estimated that the "size" of an electron, if it is not a point particle, must be less thanThis would correspond to a density of roughly
Probing deeper within particles, one finds quarks which appear to be very dense and very hard. There are possibilities for still higher densities for quark matter, gluon matter, or neutrino matter.
Nuclear densities are not limited to the nucleus. A neutron star contains the mass of a star in a sphere only a few km in radius. The star consists of neutrons bound together in what is basically a giant atom, consisiting only of neutrons.