Atomic theory has made progress since the discovery of radioactivity in 1898. And much of modern technology is based on these advances. They could not have happened without the discovery of electrons (e-), protons (p+) and neutrons (n0) through experiments conducted by four key scientists. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Atomic theory began to develop in 1897 when JJ Thomson discovered e- through his cathode ray experiment. This experiment (figure one) had a partially evacuated tube (a) with a cathode (b) and an anode (c) attached to each end. The cathode and anode were connected to a power source with the cathode negative and the anode positive. An electric shock was sent through the cathode and emitted negative cathode rays that traveled in a straight line (d). But when a positive magnet (e) was placed next to the rays, the rays were deflected towards the positive magnet (f), proving that they are negative because opposites attract. Another important discovery in atomic theory are protons. In 1910 at the University of Chicago a scientist named Rutherford was puzzled by the structure of the nuclear atom. J. J. Thomson had suggested a "plum pudding model" (Figure 2) of the nuclear atom in which the atom is a ball of positive charge with e- stuck inside it, but Rutherford wanted to test this theory, so he conducted the gold foil experiment (Figure 3). At the entrance there was a round fluorescent screen with a radioactive source. From the radioactive source Rutherford fired partial α- onto a thin gold foil. He expected everyone to move forward with small detours, but that wasn't the case. (Figure 4) He observed some particles moving off course from the straight line he assumed they were traveling, and some particles were bounced back entirely. From this observation we came to the conclusion that when a particle gets very close to the point where all the positive charge is located, it will move off course and when it hits this nucleus it will be deflected back. Rutherford called the nucleus he found the nucleus which also makes up most of the mass of an atom and is made up of protons. He discovered (figure 5) that the nucleus is surrounded by positive charge and contains particles and stuck inside it. In 1911 Millikan returned to electron work. Thomson had demonstrated what the mass/charge of an e- was, he wanted to determine what the actual charge on each particle was. To do this he created the oil drop experiment (Figure 6). Drops of oil were sprayed into a chamber with a very small hole in the bottom. When a drop of oil passed through the hole it was observed under a microscope and hit with an x-ray which removed all air particles from it. The positive charge transmitted through the plate above the particles was manipulated and monitored so that the scientist could see how much positive charge was needed to balance the negative charge below and the positive charge above and suspend the oil in mid-air. From this they could determine the negative charge of an electron. Another thing that puzzled Rutherford was why atomic mass was larger than the combined mass of protons and electrons in a substance. He proposed that this was due to a neutral particle, but never did any experiments to prove it. In 1932 Chadwick accepted the challenge and designed an experiment (Figure 7) in which he fired α-partials at a beryllium target. This then emitted particles that were dropped onto the paraffin wax, then releasing other particles, protons. From the energy calculations he saw that the particles released.