The British physicist Sir Joseph John Thomson (1856–1940) in 1897 researched electrical conduction in gases, which led to the important discovery that cathode rays consist of negatively charged particles called electrons. The discovery of the electron inaugurated the electrical theory of the atom, and this, along with other work, entitled Thomson to be regarded as the founder of modern atomic physics.

Ernest Rutherford discovered the proton in 1919. He also predicted the existence of the neutron, later discovered by his colleague, James Chadwick (1891–1974). Chadwick was awarded the 1935 Nobel Prize in Physics for this discovery.