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Textbook Analysis - Rachel Horsting
Part 1: Material Description
Date of Publication: 2002
Authors: A. C. Wilbraham, D. D. Staley, M. S. Matta, E. L. Waterman
Ability Levels: all
Victor and I have the same book; it is not surprising since we are teaching in the same district.
Part 2: Unpacking of a Learning Goal
. I focused on C4.8B Describe the atom as mostly empty space with an extremely small, dense nucleus consisting of the protons and neutrons and an electron cloud surrounding the nucleus.
Part 3: Analysis
Criterion III.A: Providing a Variety of Phenomena
in making the key ideas real.
to the relevant key idea
The textbook provides a few phenomena dealing with this aspect of atomic structure.
First, it has a description of Rutherford's famous gold foil experiment, with an accompanying schematic of the experiment (p.111). This description is excellent for indicator 1 (I1). It is the experiment that originally gave scientists a lot of information about atomic structure, and by showing the set-up and the surprise of the researchers it demystifies the scientific process. This text scores excellent for I2, because it describes how the key idea was orginially discovered.
Accompanying the text are two pictures with captions. Picture A is a diagram of the experiment layout. Picture B is a diagram of the alpha particles going through the empty space in the gold foil, or being deflected off of the nuclei. The captions only summarize the main text; they add nothing new. Diagram A is useful in showing how the experiment actually looked while it was being done, and what all the components were. Diagram B is useful in showing how empty space can exist in a solid sheet of gold. They are both linked to the main ideas in the captions. Both diagrams score excellent for both indicators.
The textbook has a CD-Rom that has a video of the gold foil experiment. I do not know where the CDs are, but no one seems to have one. Thus, I did not watch the video. However, it is likely that it clarifies the design of the experiment. I am not going to analyze it because I have not watched it.
There is also a picture of a football stadium, with a caption that reads "If and atom were the size of this stadium, then its nucleus would be about the size of a marble!" (p.112) For such a small representation, it does a lot. I think it scores execellent for I1, because practically everyone (especially in Ann Arbor) has been in a very large sports stadium, or has seen one on TV, and everyone knows how big a marble is. Therefore, it is very useful in conveying just how much of an atom is empty space. It is also explicitly linked to the key idea because the caption explains the relative size of the atom and the nucleus. There fore it also gets an excellent for I2.
The last representation is part of the description of the Rutherford experiment, but it is a quote from Rutherford himself, "[the alpha particles bouncing back toward the source] was about as credible as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you." This quote has the virtures of the stadium/marble comparison. Tissue paper is a common experience, and although most people have not handled a fifteen inch shell, we know how big fifteen inches are and that it is some kind of bullet. Therefore this representation scores and excellent for I1 and I2.
Overall, I would only give this textbook an excellent for this criterion, because each element is useful and explicitly linked, and because there seem to be a sufficient number. The only reservation I have is that most of the phenomena are linked to the one experiment, so they do not seem varied. However, it seemed logical to analyze them all seperately, so I think they are seperate enough to count.
Criterion III.B: Providing Vivid Experiences
Each firsthand experience is
(when compared to other firsthand experiences) and, if several firsthand experiences target the same idea, the set of firsthand experiences is efficient. (The efficiency of an experience equals the cost of the experience [in time and money] in relation to its value.)
The experiences that are
firsthand (e.g., text, pictures, video) provide students with a
sense of the phenomena. (Please note that if the material provides only firsthand experiences, this indicator is not applicable.)
The set of firsthand and vicarious experiences is
The textbook provides no first hand experience for this concept. I1 is not applicable.
The second hand experiences, the text of the experiment, the two pictures, the picture of the football stadium with the previously described caption, and Rutherford's quote, get a satisfactory, satisfactory, excellent, and excellent respectively. The experiment text only gets a satisfactory because it is not clearly enough explained for all students to be able to visualize it (see IV.B for more). This pictures are clear, but are not vicarious or immediate. There is no link to anything the students have done or seen. The experiment is contrived in that it is not an intensification of something easily observed in real life. The stadium example gets an excellent score for I2, because it is easy to imagine oneself sitting in the stadium, holding a marble. The same is true for Rutherford's quote; it immediately brings to mind a big gun firing at a piece of tissue paper.
I think that if the experiment and the more vicarious representations are put together, they are satisfactorily sufficient. The experiment give the real history of the discovery, and the stadium provides an intuitive picture of what the results mean. Each can fill some of the holes potentially left by the other.
Overall, I give the textbook a satisfactory on this criterion. Although some parts did some things very well, there were also no firsthand experiences. Even though the other experiences are probably sufficient, I don't think it should score an excellent when there are no firsthand experiences.
Criterion IV.B: Representing Ideas Effectively
(or, if not accurate, then students are asked to critique the representation).
Representation is likely to be
to the real thing.
The description of the gold foil experiment scores excellent for I1. It is an accurate account of the experiment. It scores poor for I2. The introduction and discussion of alpha particles is potentially very confusing. It is confusing to be describing various kinds of particles when the nature of an atom and an element have not been completely established. It is completely new, and the students likely have no idea at this point what the relative size of an alpha particle is to a gold atom. It is possible that they will be so confused by the alpha particle that they won't understand the rest of what they read, because they don't understand all the actors involved in the experiment. The other part that is potentially confusing is why the scientists predicted the outcome they did. Why did they think the particles would go through the gold foil at all? This section of text only scores a satisfactory for linking to the real thing. Although it explains how this aspect of atomic structure was discovered, it does not explain clearly the thought process that deduced the interpretation from the results. It only says that Rutherford suggested a new theory of the atom (p. 112). It does not explain why his theory was plausible in light of the results, or why everyone believed him, or if any other scientists repeated his experiment or came to the same theory through other experiments. In this way the linking is not fully explicit.
Picture A (a diagram of the experiment) is accurate (excellent for I1), but it is not likely to be comprehensible for the same reasons the text is potentially confusing (poor for I2). It is linked to the experiment through a caption that summarizes the text above.
Picture B (a scheme of alpha particles shooting between the gold nuclei) is not particularly accurate, because the ratio of empty space to the nuclei is wrong; the nuclei are far to big. However, I would only let that make the score a satisfactory, because it is impossible to accurately represent the ratio in a drawing. I do think they could have made a little more empty space; in the picture there are as many alpha particles deflected as there are particles that get through. This drawing is more comprehensible, but the caption only summarizes the text, and has the same faults regarding Rutherford's process and conclusion. Therefore it gets satisfactories for both I2 and I3.
Both the stadium and the quote do an excellent job representing different aspects of the main idea, that atoms are mostly empty space, and that the nucleus is very small and strongly held together. They are both accurate, comprehensible, and explicitly linked.
Overall, I think the text gets a satisfactory for this criterion. Some of the phenomena and representations did a poor job, some did an excellent job, but together I think that a satisfactory representation of the atom can emerge.
Part 4: Modifications
I'm not sure it is worthwhile to have the students read this. Students are often not good at reading captions or studying diagrams. In that case, about half the value of the section is lost. They could potentially be very confused by the description of the experiment. I think that it would be valuable to use the stadium example, and maybe even the pictures, during class. I think that going over Rutherford's experiment is a good way to talk about this concept, because it brings in the history of ideas and the process of science. I think topics of electric attraction and the nuclear forces would need to be included so that the model of the atom is consistent, and the students know why the nucleus is small and compact, and why the electrons take up so much space. I think it would be important to make sure that the students understood what the scientists thought they would get. I also think that I would try to make them come up with conclusions about atomic structure based on experimental results like these; instead of giving them the answer I would get them to understand the questions and the set up, and see what different results would mean, before telling them the results that actually happened. That would give them firsthand experience about how scientists draw conclusions from experiments that are indirect.
In general, I would use the representations differently, rather than change them. They are useful, and rich with history and the scientific method as well as classic.
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