Atomic theory has progressed 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 by four key scientists. Say no to plagiarism. Get a tailor-made essay on 'Why violent video games should not be banned'?Get the original essayAtomic theory began to develop in 1897 when JJ Thomson discovered e- through his experiment with cathode rays. In this experiment (Figure 1), he 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 being negative and the anode being positive. An electric shock was sent through the cathode and it emitted negative cathode rays which 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), which proved that they were negative because opposites attract. Another important discovery in atomic theory concerns protons. In 1910, at the University of Chicago, a scientist named Rutherford was perplexed about the structure of the nuclear atom. JJ Thomson had suggested a "plum pudding model" (Figure 2) of the nuclear atom in which the atom is a positively charged ball with e- stuck inside, but Rutherford wanted to put this theory to the test, so he conducted the study on gold. leaf experiment (Figure 3). There was a round fluorescent screen installed with a radioactive source at the entrance. From the radioactive source, Rutherford shot α partials onto a thin sheet of gold. He expected them all to pass with minor deviations, but that wasn't the case. (Figure 4) He observed that some particles moved away from the straight line he assumed they were traveling, and that some particles bounced back completely. From this observation came the conclusion that when a particle approaches extremely close to where all the positive charges are, it veers off course and when it hits this nucleus, it moves backwards. Rutherford called the nucleus he found the nucleus which also makes up most of an atom's mass and is made up of protons. He discovered (Figure 5) that the nucleus is surrounded by positive charges and contains electronic particles trapped inside. By 1911, Millikan was back to working with electrons. Thomson had proven what the mass/charge of an e- was, he wanted to determine what the actual charge of each particle was. To do this, he created the oil droplet experiment (Figure 6). Drops of oil were sprayed into a chamber with a tiny hole in the bottom. When a drop of oil passed through the hole, it was observed under a microscope and zapped with an x-ray which removed all air particles. 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 cause the oil to suspend in the air. From there, they were able to determine the negative charge of an electron. Another thing that confused Rutherford was why the atomic mass was greater than the combined mass of the protons and electrons in a substance. He proposed that this was due to a neutral particle, but never did experiments to prove it. In 1932, Chadwick took up the challenge and designed an experiment (Figure 7) in which he shot.