Thus far, our model of the atom contains two subatomic particles: the proton and the electron. We know that the protons are found within the small, dense nucleus and that the electrons are somewhere in the empty space surrounding the nucleus. Much work after the discovery of the nucleus involved explaining the behavior of the electron.
Another problem still remained, however, with the nuclear model. The mass of each element could not be accounted for solely by the proton and the electron. It was shown that the electron had a mass 1,836 times smaller than the mass of the proton and can therefore be considered essentially massless.
For example, it was known that an atom of carbon was typically 12x more massive than an atom of hydrogen. However, an atom of carbon contained only 6x more protons than an atom of hydrogen. What is the cause of this extra mass? Rutherford proposed a third type of subatomic particle, contained within the nucleus, which had approximately the same mass as a proton, yet it could not carry any electrical charge. He proposed to call this unidentified subatomic particle the neutron due to its lack of charge; unfortunately, the neutron remained elusive to scientists for over a decade after Rutherford suggested its existence in atoms.
We will quickly turn our attention to the neutron before discussing the electron. Please realize, however, that its discovery actually took place last in the chronological ordering of the atomic model.
