jan 1, 1932 - James Chadwick's Atomic Theory
In 1932, James discovered the neutron. While experimenting with beryllium atoms with alpha particles, an unknown radiation was produced. Chadwick concluded this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. The neutrons were very hard to find because it did not repel the protons when it was in the atom.
Added to timeline:
Atomic Theory Timeline - Adrianne Moran 4th Period Chemistry