Evidence for Neutrons
Rutherford, based on experimental evidence,
concluded that the positive parts of all atoms were made of hydrogen nuclei,
which he gave the name protons. There were still some things that
didn't make sense, though...
Well, take helium, for instance. An average helium nucleus has is four times as heavy as a proton, which seemed to imply that it was made of four protons. But then it ought to have four times the electric charge of a proton, whereas in fact it has only two times this charge. All the heavier nuclei showed a similar mismatch between mass and charge. Even worse, different isotopes of some elements had been discovered--atoms of the same element with different masses.
And then Rutherford actually found some neutrons, right?
Using the velocity of the knocked-out protons and the laws of conservation
of energy and momentum, Chadwick calculated the mass of the unknown
particle. It was just a little heavier than the proton. He had no doubt
that these were Rutherford's neutrons.