The problem of defining a radius for the atomic nucleus has some similarity to that of defining a radius for the entire atom ; neither has well defined boundaries. 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. For individual protons and neutrons or small nuclei, the concepts of size and boundary can be less clear. 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. Also, the nucleon is surrounded by its Yukawa pion field responsible for the strong nuclear force. 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.