Discovery of the Neutron  It is remarkable that the neutron was
not discovered until 1932 when James Chadwick used scattering
data to calculate the mass of this neutral particle. Since the
time of Rutherford it had been known that the atomic mass number
A of nuclei is a bit more than twice the atomic number Z for
most atoms and that essentially all the mass of the atom is
concentrated in the relatively tiny nucleus. As of about 1930 it
was presumed that the fundamental particles were protons and
electrons, but that required that somehow a number of electrons
were bound in the nucleus to partially cancel the charge of A
protons. But by this time it was known from the uncertainty
principle and from "particle-in-a-box" type confinement
calculations that there just wasn't enough energy available to
contain electrons in the nucleus.  A rough scale of the energy
required for the confinement of a particle to a given dimension
can be obtained by setting the DeBroglie wavelength of the
particle equal to that dimension