The proton, or hydrogen atom nucleus, has been known since Rutherford’s scattering experiments of 1911. The neutron was discovered much later, in1923, by Chadwick. All nuclei in our world of matter are believed to be builtout of neutrons and protons, although there may somewhere exist a worldof antimatter in which all atoms would be constructed out of antiprotons (ofnegative charge) and antineutrons (particles distinct from neutrons). The forces which bind neutrons and protons together to form complex nuclei are not electromagnetic forces. Instead the attraction between twonucleons, the generic name for neutrons and protons, is an intense forcewhen it is acting, but it only acts when the nucleons are touching. Because ofthis resemblance of nucleons to sticky impenetrable spheres, the volume of allnuclei is approximately proportional to the total number of nucleons involved;i.e., the atomic weight A. This agrees with the experimental observation thatthe radius of a nucleus is given by R = r 0 A 1/3 . If one discounts the Coulomb repulsion between two protons, the scat- tering experiments reveal that the nuclear force between two nucleons is thesame, irregardless of whether the nucleons are protons or neutrons. In fact,it is possible to consider the neutron and the proton as two different statesof the same particle. The slightly higher mass of the neutron might be ac-counted for because of the differing electromagnetic interactions of the twoparticles.

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