Measuring the Size of a Nucleus

In this activity you used indirect measurement to find the area of a penny. A key scientific discovery-the discovery of the atomic nucleus-was made using a similar method. Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden made the discovery. In the lab, the team bombarded a piece of thin gold foil with a beam of alpha particles. They thought that an atom had positive charges spread evenly through it. If this idea was true, the alpha particles would go through the foil in nearly straight lines. The alpha particle beam was their "dropping pen." The foil was their "card." To their surprise, Marsden saw that some of the alpha particles bounced back towards the source of the beam. Rutherford thought deeply about these observations. He also thought about all the ideas scientists had put forth about what was in atoms. He concluded that the alpha particles bounced back when they hit an area of concentrated positive charge. He also thought that the charge was in the center of the atom, the nucleus. Rutherford used the experimental results to calculate the size of the nucleus of these atoms. He could have compared the number of "hits" particles that bounced back-with the total number of particles sent toward the foil. Then he could have used that ratio to determine the area in the foil where atomic nuclei could be found compared to the total area of the foil. Rutherford's mathematics were a bit more complicated. He calculated that the diameter of the atomic nucleus was10^-15 m, while the diameter of the atom is 10^-10 m.

Indirect measurement has been very useful in science. It continues to be useful. Rutherford's discovery of the nucleus of an atom led to other ideas that could be tested further. All good scientific theories lead to new ideas and new tests. Finding the size of a penny without directly measuring it could be considered a good magic trick. It is actually good science. It is a method that can be repeated. The same method, done correctly, will give the same results no matter who uses it or where it is done. Probability was important in this indirect method of measuring the size of the penny. If you had aimed the pen, the experiment would not have given good results. In your chapter challenge, you will be required to discuss how predictions can be made using probability. You will also have to explain how the size of the nucleus can be determined without a "ruler." This activity should help you in completing this aspect of the challenge.