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Section 3: The Nucleus
In 1911, two years after the measurement of the mass and charge of the electron, Ernest Rutherford (1871–1937), a New Zealand native and protégé of Thomson, began experiments to explore the atom and the reigning plum pudding model by bombarding a thin gold foil with fast-moving particles. The results of the famous gold foil experiment provided new evidence that would lead to new discoveries and the need for a new model for the atom.
Rutherford's research, conducted at the Cavendish Laboratory at the University of Cambridge, focused on radioactive elements, such as radium, thorium, and uranium, and the particles emitted by them. Radioactive elements are unstable, and when they undergo radioactive decay, they change into other elements. (For more on radioactive decay, see Unit 12.) In the process, they emit one or more types of particles. For example, the decay of some heavy elements, such as radium, emits alpha particles. In alpha decay, the particles are ejected at high speed and can travel a few centimeters in air but can easily be stopped by a sheet of paper.
Figure 3-5. Rutherford's Gold Foil Experiment
Rutherford expected the alpha particles to pass through the Plum Pudding Model of the atom undisturbed, but he observed that a small portion of the particles were deflected, indicating a small, concentrated positive charge.
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Rutherford and his students, Hans Geiger and Ernest Marsden, used a source of alpha particles to bombard a target of an extremely thin piece of gold, which was surrounded by a detector that would indicate when it was struck by the alpha particles. Even though the foil had a thickness of 20,000 atoms, according to the plum pudding model, the alpha particles should pass right through the foil without any deflection. Rutherford explained this by hypothesizing that the diffuse positive charge of the "goo" would not have been strong enough to repel the alpha particles. When they ran the experiment, most of the particles did travel directly from their source in a line straight through the gold foil. However, to their astonishment, for about 1 out of 8,000 particles, Rutherford and his team discovered that the alpha particles were deflected from their straight path by over 90 degrees. From this observation, they proposed a new theory that the atom must be mostly open space with a heavy and dense region of positive charge in the middle. It had to be positive because it repelled the positively charged alpha particles. Forever putting to rest the plum pudding model, Rutherford is reported to have said about their discovery, "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
In effect, by sending this beam of fast-moving, relatively heavy positive particles through the foil, and finding that a certain number were strongly deflected, Rutherford had probed inside the atom and found a deep, dense core that had previously been undetected. In 1912, Rutherford named this small, dense area of positive charge the "nucleus." His new model of the atom placed most of the atom’s mass and all its positive charge in this tiny, dense nucleus, which is surrounded by much smaller, negatively charged particles (the electrons) occupying mostly empty space. Rutherford did not know what was in the nucleus exactly, just that it had to be small, densely heavy, and positive. Later experiments would define the nucleus more precisely.