The next major breakthrough was was to begin long before Mendeléev periodic table, as an extension of one of Michael Faraday's failed experiments. Faraday had tried to drive an electric current through a vacuum, but was unable to obtain a sufficiently high vacuum to do so. In 1855, a German Physicist Julius Plücker (1801-68) employing a glass tube made by glassblower, Heinrich Geissler (1814-79) succeeded in driving an electric current between two plates sealed into a tube under high vacuum. He noticed that the current produced a glowing effect that depended on the purity of the vacuum. In 1875, the English physicist William Crookes (1832-1919) produced and even better tube (a Crookes tube) in which the electric current could be more easily studied. It became clear that the electric current started at the cathode and traveled to the anode where it struck the glass creating a glow. Crookes demonstrated that he could cast a shadow by placing a piece of metal in the path of the current, showing that the electric current travels in a straight line. The current was called "Cathode Rays" by German physicist Eugene Goldstein (1850-1930). Very rapidly scientists began to debate the nature of these "cathode rays", were they light and so made up of waves or speeding charged particles. It wasn't long before experiments were being devised to prove one view or another. Charged particles were deflected by magnets in Pücker's experiments but it was the English Physicist Joseph John Thompson (1856 - 1940), who in 1897 finally showed that they could also be deflected by and electric field. From then on it was generally accepted that cathode rays were indeed particles that carried a negative charge. This known, Thompson used the idea that magnetic deflection is controlled by the particles mass and by the size of its electric charge to measure the charge to mass ratio of the cathode ray. He could not however, measure them independently.
The separation of charge and mass was achieved by American physicist Robert Millikan (1868 - 1953) in 1911. Millikan succeeded in measuring the minimum electric charge that could be carried by a particle that if it was a cathode ray must be 1/1837 the mass of a hydrogen atom. It was the first of the subatomic particles to be discovered. This was the "atom of electricity". It had already been given its name by the Irish Physicist George Johnstone Stoney (1826 - 1911) in 1891, the "electron." Thompson is credited with its discovery.
The next question was "How were the electrons and atoms related?' It was the German physicist Heinrich Rudolf Hertz (1857 - 1897) who, in 1888 discovered radio waves. This paved the way for another German physicist Philipp Eduard Anton Lenard (1862 - 1947) in 1902, to show that electrons could be emitted from a metal in a process known as the "Photoelectric Effect." In this experiment, light of different colors is shone on a metal until electrons are ejected from the atom. After this demonstration it seemed reasonable that all atoms contained electrons and must be balanced by some positive charge since atoms didn't carry a charge. It was Thompson who suggested that the atom was a solid mass of positive charge with the charged electrons stuck into it like a raisins in pound cake to neutralize the positive charge. He also predicted that adding more electrons would increase the negative charge and removing them would give a positive charge.
It wasn't long before the idea of a solid atom was being challenged. The New Zealand-born physicist Ernest Rutherford (1871-1937) accepted the idea of a unit of positive charge existed and that it was a particle different to the electron. In 1914, he suggested that the positive particle was the mass of a hydrogen atom, an idea confirmed by later experiments, in 1920 he named it the proton. The idea sat around for about 12 years until in 1932, in an experiment suggested by Rutherford, the English physicist James Chadwick discovered a particle the size of a proton but without a charge, it was named the neutron.