However, basic liquid drop models of the nucleus imagine a fairly uniform density of nucleons, theoretically giving a more recognizable surface to a nucleus than an atom, the latter being composed of highly diffuse electron clouds with density gradually reducing away from the centre.
A single nucleon needs to be regarded as a "color confined" bag of three valence quarks, binding gluons, and a so-called "sea" of quark-antiquark pairs.
It could be difficult to decide whether to include the surrounding Yukawa meson field as part of the proton or nucleon size or to regard it as a separate entity.
Fundamentally important are realizable experimental procedures to measure some aspect of size, whatever that may mean in the quantum realm of atoms and nuclei.
This definition of charge radius is often applied to composite hadrons such as a proton, neutron, pion, or kaon, that are made up of more than one quark.
[2] For deuterons and higher nuclei, it is conventional to distinguish between the scattering charge radius, rd (obtained from scattering data), and the bound-state charge radius, Rd, which includes the Darwin–Foldy term to account for the behaviour of the anomalous magnetic moment in an electromagnetic field[3][4] and which is appropriate for treating spectroscopic data.
[9][10] There is most interest in knowing the charge radii of protons and deuterons, as these can be compared with the spectrum of atomic hydrogen and deuterium: the nonzero size of the nucleus causes a shift in the electronic energy levels which shows up as a change in the frequency of the spectral lines.