DISCOVERY OF THE ATOMIC NUCLEUS

In his early experiments with radioactivity, Ernest Rutherford found that radioactive atoms emitted three kinds of radiation which, as we have mentioned, he called α rays, β rays and γ rays. The α rays turned out to be heavy positively charged particles, later identified as helium nuclei. The β rays were beams of negatively charged particles later determined to be electrons, and the neutral γ rays turned out to be high energy particles of light (photons).
In the early 1900s, before 1912, it was not clear how these particles were emitted or what the structure of the atom was. Since J. J. Thomson’s experiments with electron beams in 1895, it was known that atoms contained electrons, and it was also known that complete atoms were electrically neutral and much more massive than an electron. Thus the atom had to have mass and positive charge in some form or other, but no one knew what form. By 1912 the plum pudding model was quite popular. This was a picture in which mass and positive charge was spread throughout the atom like the pudding, and the electrons were located at various points, like the plums. A rather vague picture at best.
In 1912 Rutherford and Hans Geiger began a series of experiments using beams of radioactive particles to probe the structure of matter. These experiments could begin after Geiger had developed a tube to detect radioactive particles. This device later became known as a Geiger counter, and is still used through the world to monitor radiation.
In the first set of experiments, a beam of α particles were aimed at a gold foil. It was expected that some of the α particles would be slightly deflected as they passed through the positive matter in the gold atoms, or came near electrons. To the utter amazement of both Rutherford and Geiger, some of the α particles bounced straight back out of the gold foil, with essentially the same kinetic energy they had going in.

We have seen from our analysis of the elastic collision of two equal mass particles, that the incoming particle stops and the struck particle continues on. Only if the mass of the struck particle is greater than the mass of the incoming particle, will the incoming particle bounce back. And only if the mass of the struck particle is much greater than the mass of the incoming particle will the incoming particle rebound with nearly the same energy that it had coming in. Thus Rutherford and Geiger’s observation that some of the α particles bounced right back out of the gold foil, indicated that they struck a solid object much more massive than an α particle.
Most α particles passed through the gold foil without much deflection, indicating that most of the volume of the gold foil was devoid of mass. The few collisions that did result in a recoil indicated that the mass in a gold foil was concentrated in incredibly small regions of space. A more detailed analysis showed that the scattering was caused by an electric force, thus they knew that both the mass and positive charge were located in a tiny region of the atom. In this way the atomic nucleus was discovered.