Sherry M. Burgess
Introduction
There are times when scientists are unable to actually touch or see whatever they are examining. The required technology that would allow them to do direct examination may not have been developed. This was especially true for the scientists during the nineteenth and early twentieth centuries who were investigating what was matter actually made of. A few believed that, all matter was made of atoms or tiny indivisible particles (that could not be seen). However although they could not see them, scientists of that day were able to make observations of indirect evidence. The work which Ernest Rutherford did in 1909 is one example. It was for this experiment that he was credited with the discovery of the nucleus.
At the time of this event Rutherford was very involved with the study of radiation. It was during one of his earlier investigations (1890’s) that Rutherford named two types of rays emitted from radioactive material; alpha rays and beta rays. Rutherford was investigating the penetrating ability of these rays. He used uranium for the radioactive material. By methodically increasing the layers of sheets of aluminum foil of different thickness and measuring the amount of radiation that passed through the foil, he concluded that two types of rays existed.3 This was based on how far the rays penetrated the foil. The name beta rays was given to those rays which penetrated thicker sheets.4 It was later proven that these rays were actually particles.
A short time later, a German physicist by the name of Hans Geiger began working with scattering alpha particles. The purpose of these experiments was to test J. J. Thomson’s theory of the structure of the atom. (For anyone not familiar with this model, information can be obtained from any high school chemistry text book). In his experiments Geiger fired alpha and beta particles at a piece of metal, foil. He then analyzed how they were scattered. Geiger devised a way to count the reflected particles. The angles of some deflections were found to be very large; 90 degrees or greater, sometimes 180 degrees. According to the account given in The Atomic Scientists, it was the large deflections reported in Geiger’s work that prompted Rutherford’s questioning the validity of Thomson’s model of the atom.5 The fact that radioactive particles could pass through thin sheets of metallic foil served as an additional stimulus for a different idea about the structure of the atom; that is the atom consisted mostly of empty space.
So it was in this setting that Ernest Rutherford developed his famous experiment in 1909. To test his idea of the structure of the atom, Rutherford believed he could use the scattering of alpha particles. He shot a beam of alpha particles at a very thin sheet of gold foil. In order to produce the beam he placed some radioactive material into a lead box consisting of one tiny hole in one of its walls. He measured the angle of deflection by surrounding the foil with a screen coated with zinc sulfide. Wherever the alpha particles hit the screen, a flash of light was given off. The results of the experiment were that a large majority of the particles went straight through the foil. A very small amount, about one in 8,000, 6 were deflected at angles greater than or equal to 90 degrees. Also, just as Geiger had observed some were even deflected at 180 degrees. Rutherford concluded that since most of the alpha particles passed straight through, the atoms that made up the gold foil had to consist of mostly empty space. (There were no masses for them to ricochet off). He stated that most of the mass was concentrated in a tiny center. He further stated that the center consisted of positive material which accounted for the deflection of the positive alpha particles (like charges repelling each other).
As previously alluded to, this was an example of an experiment in which the actual object (in this case the atom) was never directly observed. However based on the data obtained from Rutherford’s experiment new information about atomic structure was obtained. The first activity of this section involves investigating some of the physical properties of air using indirect evidence. Students are expected to have a working knowledge of the definitions of mass, volume, density, as well as the difference in particle arrangement found in solids, liquids, and gases before engaging in this investigation.
Activities
This first activity involves inflating one balloon with air and comparing it to the non inflated balloon in terms of mass and appearance. It from the Prentice Hall Activity Book: Matter, Building Block of the Universe; entitled “Using Indirect Evidence”. Although it is not directed in the instructions, I recommend that both balloons be weighed before inflating one. This would insure that both balloons are identical. (The directions do not instruct the student to determine the mass of the balloon that is to be inflated before hand). Once one of the balloons is inflated, it is examined in terms of density, response to pressure, ability to make noise, volume, and response to temperature change. Following these investigations a few questions are asked pertaining to critical thinking and applications. Please refer to the actual book for directions and questions; pages 77-79.7
The second exercise involves students examining the contents of small containers without opening them. It is an adaptation of the Laboratory Investigation, Shoe Box Atoms, found in the same activity book mentioned above.7 In this exercise eight small numbered containers (small empty boxes) are filled with different items. The items include the following: packing peanuts, small wooden block or flat piece of wood (small enough to fit inside the container), ping-pong ball, golf ball, small magnet, screws, sheet of paper, plastic beads. Students can work in groups or if the class is small enough individually. The boxes can be divided up between the groups or students. More than one set of boxes can also be used. Before the boxes are distributed, the class is given the set of instructions. They are informed that they are to gather as much information as they can about the object (s) inside the boxes without opening them. They will determine the following properties for the item (s): magnetic, mass, number of items and shape, ability to bounce or flip. Magnetic properties will be checked by observing the results of placing a magnet near the box. This will tell if the object (s) contain some type of metal and how much of an attraction to a magnet it has. It can also show if it contains magnetic material by repelling the magnet. A iron nail will also be placed near the box. It will also demonstrate if the material is magnetic material by attracting the nail. If it is, it should attract the nail. If the content (s) of the box is non-magnetic, nothing should happen when the nail is placed near the box. Mass will be determined by first weighing the container and subtracting its mass from that of the other container. Tilting the boxes will give information that will help determining the quantity and shape of the object (s). If one hears the sound of something rolling, it can be assumed that the object (s) have rounded sides. On the contrary, if a sliding noise is heard, one can assume that the shape is something with flat sides. Bouncing or flipping can be determined by shaking the box (es). Data can be recorded on a chart made by the students. See appendix _ for the actual chart. Next, students are to draw a picture of what they think is in each box they investigated based on their observations. Only after they have made their drawing should they be told the contents of each box. (In order to simplify reuse of the boxes, it is advisable to tell them and show an example of each box content, rather than have them open each box. The boxes can be referred to by number). In the lab write up, students should describe the content for the box (es) they had. This should include the number of objects, as well as an explanation of why they described it the way they did. For example: The object in box #1 is a round object rolled. It has a mass of 10g. It does have metallic properties. When a metal screw was placed near the box it stuck, etc. An example of this kind should be explained to the class. At the end of their write up, students should say whether or not their description fit the object (s). If it didn’t, they should give an explanation based on their data. During the post lab discussion the class should address the issue of how information can be obtained on something even though it can’t be seen. They should refer to their write ups.
