For the past two years I’ve tried (and mostly failed) Modeling in Physics. I didn’t take the class/workshop over the summer. I was cheap and haven’t even become a member of the AMTA. (I know, I know: I should do that.) Finally, I came upon the awesome Kelly O’Shea and her awesome blog, in which she basically explains what she does, for some lessons word-for-word, on top of providing all of her packets of work, which is awesome. Finally I feel like my students are learning Physics in this totally new, intuitive, cool way. They’re solving problems by drawing graphs rather than crunching numbers. But I’ve lectured in Chemistry for the past 2 years cause I think I know what I’m talking about in that subject. I’ve heard of modeling in Chemistry, but I haven’t had the time (to take the workshops) or money (to join the AMTA and get materials). Now that I’m finally feeling a little good about it in Physics, I think I can picture how it should happen in Chemistry, and so one day I decided to try out what was in my head. These are Junior in Chemistry, and all we’ve done so far is background work: significant figures, scientific notation, SI units of measurement and conversions.
Part 1: Dalton’s Atomic Theory
Students broke into groups of 4, acting as scientists from previous centuries, and received a large whiteboard. On the whiteboard, they created three columns: “What we learned”, “Our group’s 5 rules about atoms”, and “The class’s 5 rules about atoms”. I explained the following points (putting most of these on the board):
- At this time, nobody had heard of an “Atom”. Your job is to come up with 5 rules that you think might be true about atoms.
- People knew about certain elements and how they seemed to always react in certain ways.
- For example: ~16 g of Oxygen and ~2 g of Hydrogen would react to form ~18 g of water, but if you had more Oxygen or more Hydrogen (not both) then you would have leftover of that.
- About 8 g of Oxygen and ~1 g of Hydrogen would also react to form ~9 g of water, but again, too much of either would leave you with extra of that substance.
- Other elements reacted in similar ways with certain amounts, where it was possible to have “too much” of one or the other.
I left students with that information and they were required to come to their own conclusions. Students started off terribly confused by what they should do, so I shuffled around the room and asked provocative questions: “How large are these so-called atoms? Be specific!”, “What are those numbers on the board and why might they be important?”, and “What do you think about the chemical and physical properties of atoms?” These questions led different groups to come up with different rules, while led to a good discussion, in which the class had to decide on 5 rules. I was impressed that both classes ended on 5 points that were comparable to Dalton’s.
Part 2: Thompson’s Cathode-Ray Tube Experiment
Here I explained Thompson’s Cathode-Ray Tube experiment, highlighting the following points:
- Clearly something was moving through the vacuum, but it wasn’t entire atoms.
- The thing that moved through the vacuum was attracted to a positive magnet and repelled by a negative magnet.
I asked them to make 3 columns again in groups of 4 with the following headers: “What we already knew (Dalton’s/the Class’s 5 rules)”, “What can we learn from the Cathode-Ray Tube Experiment” and “What our picture of the atom looks like now”. After some very leading questions, groups moved in the right direction, and I made sure that at least one group in each class had an atom somewhat resembling the Plum-Pudding model before moving into the group discussion. The group discussions were a little better this time around, partly because they had done it once now and party because they had drawings to fall back on. It was still kinda like pulling teeth, but both classes at least saw something like the Plum-Pudding model before either accepting or rejecting it. I followed up on this discussion by giving Thompson’s own model (the students really wanted to see it at this point!), which maybe I should not have done? I want them to think that their own “Discoveries” are valuable, and those are undermined when I show them “the answer” (or what they perceive in their minds to be “the answer”).
Part 3: Rutherford’s Gold Foil Experiment
This discussion occurred on a different day, and so I was a little more prepared, but it was still foreign and difficult for them. I introduced the experiment and asked them to make a prediction, based on our previous model of the atom, sketching where they thought the alpha particles would land. After this prediction, I highlighted the following points of the experiment:
- The alpha particles we were bombarding them with were essentially positively charged Helium atoms.
- Sometimes they bounced back at an odd angle, but rarely: they deflected backwards only about 1 out of every 8000 times.
Again, with some pushed upon individual groups, students came to the conclusion that there must be a small, dense, positively charged center of the atom. We decided to randomly call it the “nucleus” and I even managed to convince some of them that it was reasonable to believe that the electrons are orbiting this nucleus. 
Pros and Cons
Things I liked about this activity and format:
- Very different from lecture format: students struggling with information and coming to conclusions on their own. I know students need when I hear them say “no, we just want you to tell us how things work.”
- More students, through the struggle, understand why we know certain things about the atom.
- I just found a cool article about how it’s better for students to argue in science labs than for them to mundanely accept what the teacher says, or worse: finagle the data to avoid an incorrect hypothesis. More arguing = better formed foundation and understanding.
Things I didn’t like, or need to improve on:
- Many students sitting out, both when working in groups of 4 and especially in the class discussions.
- Very time-consuming. We took about 3-4 days in what I could have lecture on in less than an hour. But would they have gotten as much out of the 1 hour lecture? I’m not sure, but there has to be some balance.
- As we went along, I showed them what the conclusions of the scientists of the day was, and we discussed how it compared to our conclusions. Should I have done this, possibly undermining their own confidence in coming to scientific conclusions?
- Students didn’t seem to jump into the activity on their own. I was planning on circulating the room anyway, but it seemed like it was necessary for me to be around a group to “keep them going”, so to speak, which isn’t the case in our hand-on labs or even when I give them time to do their (ungraded) homework!
- This may be because I didn’t have good entry material, and I could have better thought out the background information to lower the entry-level. As it was, it took a good bit of creativity and thinking to even start thinking about the experiments, and so I’m worried I loss a large percentage of the class before the activity started.
What are your thoughts on it all? Help me out if you’ve ever done something like this or have other helpful ideas!
Here are some pictures of the students’ boards after part 2: Thompson’s Cathode-Ray Tube experiment, so you can see where the different groups took the information (with some heavy questioning and prompting on my part).
This group liked the smiley-face model of the atom.
This group jumped to the conclusion that we knew what protons are a bit early. Because of that, I continually asked them “and how do you/we know THAT?”.
This group had fascinating theory that all mass is positively charged and electrons are mass-less particles. Definitely one of the more creative approaches to solve the problem.
 Well, okay, so I have enough money but I’m too cheap, again.
 Soon I’ll explain to them that this is a lie. 🙂