This was the first atomic particle discovered by J.J. Thomson in 1897. He characterized the properties of cathode rays, as a stream of negatively charged particles or electrons. Thomson found the particle to be negatively charged. He was also able to measure the charge-to-mass ratio of the cathode rays. The value he obtained was independent of the gas used in the cathode ray tube.
It was not until 12 years later that Robert Millikan was able to determine the charge of an electron. He experimentally measured a value of -1.6022 x 10-19 Coulombs. Using Thomson's charge-to-mass ratio the mass of an electron had a value of 9.109 x 10-31 kg.
Thomson experimentally determined the existence of positively charged particles in the cathode ray tube, but he was unable to characterize these particles further. In 1919 Ernest Rutherford characterized the proton as a particle with a charge equal in magnitude to that of the electron but with the opposite sign. The mass was measured as 1.673 x 10-27 kg.
The neutron was characterized by James Chadwick in 1932. The neutron has almost the same mass as the proton and no charge.
Particle | Charge | Mass |
electron | -1.6022 x 10-19 Coulombs | 9.109 x 10-31 kilograms |
proton | -1.6022 x 10-19 Coulombs | 1.673 x 10-27 kilograms |
neutron | 1.675 x 10-27 kilograms |
Our current view of the structure of the atom was described as a result of experiments performed under the direction of Ernest Rutherford. In his experiment alpha particles (which he had characterized by 1908) were 'shot' at a thin piece of gold foil. The behavior of the scattered particles lead Rutherford to postulate a new model of the atom. His model, which we currently hold, locates almost all of the mass of the atom in the nucleus with the electron located outside the nucleus.