An Evolution of The Atomic Theory

The atomic theory has advanced since the discovery of radioactivity in 1898. And so much of modern technology is based on these advancements. They couldn’t have happened without the discovery of electrons (e-), protons (p+), and neutrons (n0) through experiments done by four key scientists.

Atomic theory starts to develop in 1897 when J. J. Thomson discovered the e- through his cathode ray experiment. In this experiment (figure one) he had a partially evacuated tube (a) with a cathode (b) and an anode (c) attached at each end. The cathode and anode were hooked up to a power source with the cathode being negative and the anode being positive. An electrical shock was sent through the cathode and it 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 deflected toward the positive magnet (f), proving they are negative because opposites attract.

Another important discovery in atomic theory is protons. In 1910 at the University of Chicago a scientist named Rutherford was puzzled over the structure of the nuclear atom. J.J. Thomson had suggested a “plum pudding model” (Figure 2) of the nuclear atom where the atom is a ball of positive charge with e- stuck in it, but Rutherford wanted to put this theory to the test, so he conducted the gold foil experiment (Figure 3). There was a round florescent screen set up with a radioactive source at the entrance. From the radioactive source Rutherford fired α- partials at thin gold foil. He expected them to all go through with minor deflections but this wasn’t the case. (Figure 4) He observed some particles moving off course from the straight line he assumed them travel, and some particles being bounced back altogether. From this observation came the conclusion that when a particle comes extremely close to where all the positive charge is located, it will move off course and when it hits this core it will deflect back. Rutherford called the core he found the nucleus which also makes up most of the mass of an atom and consists of protons. He found (figure 5) that the nucleus is surrounded by positive charge and has e- particles stuck in it.

In 1911 Millikan was back on the job of electrons. Thomson had proven what the mass/charge of a e- was, he wanted to determine what the actual charge on each particle was. To do this he created the oil droplet experiment (Figure 6). Oil drops were sprayed into a chamber will a very tiny hole at the bottom. When an oil drop passed through the hole it was observed through a microscope and zapped with an x-ray that removed all air particles from it. The positive charge given through the plate above the particles was manipulated and monitored so the scientist could see how much positive charge it took to balance the negative charge underneath and the positive charge above and make the oil suspend in mid-air. From this they could determine the negative charge on an electron.

Another thing that stumped Rutherford was why the atomic mass was larger than the combined mass of protons and electrons in a substance. He proposed that it was because of a neutral particle, but never did any experiments to prove this. In 1932 Chadwick took on the challenge and designed an experiment (Figure 7) where he fired α- partials at a beryllium target. This then emitted particles that were allowed to fall onto paraffin wax, then releasing more particles, protons. From energy calculations he saw that the particles released from beryllium, as a result of the arrival of α- partials on it, are uncharged and have essentially the same mass as protons, he called them neutrons.

Without these important advances in atomic theory, it would be still assumed that the atom is the smallest particle of matter. Technology would be stuck in 1803 with John Dalton’s theory, and until a scientist came along with an experiment as brilliant as these, there would not be modern technology or a understanding of science as current as the one today.