jan 1, 1932 - James Chadwick confirmed the existence of the neutron.
This was the third and final subatomic particle that was discovered. It was called the neutron, a particle with no charge and mass equal to that of a proton and was another very important discovery. The neutron is located in the nucleus along with the proton and without the its discovery, it would be impossible to determine the atomic mass of an atom.
Added to timeline:
The Atom vs the Telephone