The picture above shows the table of contents from the chemistry textbook that is currently in my high school classroom. This book had been purchased by the school in '03 so it had already been in my current classroom for nearly 10 years before I arrived.
So what's wrong with this picture with regard to the atomic structure?
The problem that I, and probably most modelers, have with the positioning of where learning about atomic structure is expected is far too early. If you take a moment to essentially exclude the typical chapters on "Into to Chem" and "Scientific Measurement", the atomic structure is basically the 2nd unit in the curriculum! What frustrates me so much about this approach is that the entire inner-workings of the atom are all of a sudden thrown right at the students and it encourages them to accept all we know about such inner-workings before any evidence has been brought to their attention that atoms exist in the first place! I mean, let's be real, the idea of "atomic structure" didn't become a big topic of discussion within the scientific community until the very late 1800s. Is there nothing that students need to learn prior to J.J. Thomson's famous cathode ray experiment besides classifying the different between a physical and chemical change?
Since this post is related to the electron, surely the current textbook will compensate for having such a chapter so misplaced by provided a mountain of evidence for the electron's existence and its properties right? You tell me....
The discovery, charge, and the mass of the electron are given an entire 3/4 OF A PAGE!! Are you kidding me? Now, it's entirely possible that many teachers who use books such as this have access to cathode ray tubes, or do experiments that are analogous to what Thomson and Millikan did but how many of them simply rely on the following notion:
"we know of the electron's existence, here are some of it's properties that we know of, now let's move on to protons"
Not only do Thomson and Millikan deserve more respect but, most importantly, our students deserve a greater depth into the scientific reasoning and evidence that led to such a fundamental discovery.
Though this entire topic of proper placement of units could easily fuel a writing binge for several hours, I digress. After all, I'm here to talk about how I approach the electron.
It is now 1/2 way through our THIRD quarter and, as of one week ago, we just started to begin our unit on Atomic Structure: The Inner Workings of the Atom. After finishing up our Counting Particles unit where the concept of the mole model was introduced, I felt that moving on to atomic structure was appropriate. Because it has not been completely decided in my mind how I will be approaching the proton, neutron, electronic configuration, radiation, and isotopes, I will only focus this post on how I approach the electron in my classroom. I will bring up those other ideas in another post I'm sure.
Sticky Tape Lab/Activity:
This is an awesome paradigm lab that was first shown to me using the Modeling Chemistry curriculum. Students place two pieces of tape on top of one another with the bottom piece sticky-side down on a base piece of tape and the top piece sticky-side down on the bottom (middle) piece of tape. Then they peel the top two pieces of tape off slowly from the base tape, ground the bottom tape with their fingers so that both pieces of tape are essentially neutral (though the students don't yet know why they're doing that part) and hold them up. Next they quickly rip both pieces of tape from each other and immediately they notice that new properties have arisen in both pieces of tape--they're charged. To investigate this new property of charge, students place their top tape, bottom tape, strip of Al foil, and strip of paper somewhere in the room so that they are all hanging over a ledge. Making a new top tape and bottom tape, in the same fashion they made the original ones, and cutting out two new strips of Al foil and paper, they are able to investigate how each strip reacts when any one of the new four strips are placed near them. Students organize their observation in a table like the one below:
After the data has been collected, a nice discussion follows between myself and the class that is structured in the following following presentation which was mostly taken from a presentation within the modeling chemistry folder for this unit.
The most important part of the discussion is what we are able to eventually conclude. Whatever is responsible for changing the properties of our tapes is:
1.) Smaller than an atom
2.) It's mobile
3.) It has a charge
Just based on those conclusions alone, we are able to deduce that our current model of the atom, which is essentially a billiard-ball-like structure needs to be changed--something is inside it!
Thomson's Cathode Ray Experiment:
In the past, I've focused heavily on this experiment mainly via lecture due to our current inability to replicate the experiment (no cathode ray rube). The pure beauty of this experiment had always driven my passion to talk about it. It wasn't until around my 3rd year that I realized I was making the learning involved about this experiment more about myself than my students.....I LOVED talking about it! Because of this, I've toned it back quite a bit and have instead used a variety of videos on youtube where others are replicating the experiment with an actual cathode ray tube. Students use their iPads to watch a variety of videos that correlate to specific questions. In addition, I give them the opportunity to read through small excerpts of Thomson's Nobel speech from 1906. The main goal of this part in the development of the electron in our model of the atom is to understand how we know the electron has a NEGATIVE charge. Thomson's experiment confirms what we had already known from the sticky tape lab. That is, we knew it was mobile and smaller than an atom. Now we know its charge is negative. It is at this point in time we develop the "plum-pudding" model since we now know the electrons have a negative charge. Also, we go back to discussing why the Al foil and Paper were attracted to both the top and bottom tape even though the foil and paper were neutral. The PHET simulation below that involves a balloon, a wool sweat, and a wall has served useful for visual purposes.
How Much Charge Does the Electron Have? Millikan's Oil-Drop Experiment:
Though I would LOVE to replicate the actual experiment, it's simply not possible with current resources. Instead, what I do is split this idea of the magnitude of the electron's charge into 2 steps:
1.) Watch an 11-min video made by me that has questions sprinkled throughout, using Educanon, on how Millikan performed his oil-drop experiment. I briefly discuss the apparatus, the purpose, the logic, and an extremely over-simplified version of his data. The video can be seen below. You will not be able to see the questions because the students view it using Educanon. The quality was below what was originally on my iPad and I just didn't know the quality goes down from iPad to youtube but I suppose that's part of my learning experience in relation to making videos.
2.) After watching the video, many students still have questions about how Millikan was able to determine the charge. Because of this, we do an activity that serves as a great analogy to the oil-drop experiment. This activity was taken directly from Flinn Scientific and it produces great results and discussion.
The purpose of the analogy activity is to determine the mass of a single BB without weighing any KNOWN quantity of BBs. Just like Millikan determined the charge of a single electron without knowing how many electrons he was dealing with, students determine the mass of a single BB indirectly and eventually use this calculated individual mass to successfully predict how many BBs were collected using a certain length of magnetic stip in a particular weigh boat.
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The magnetic strips are analogous to the oil droplets and the BBs are analogous to the electrons. Predicting the number of BBs that are attracted to a certain magnetic strip is analogous to determining the charge of an oil drop. Determining the smallest difference in mass between two trials of BB collection is analogous to determining the charge of a single electron. All of this is done without ever knowing, until the very end, how many BBs you are actually dealing with at any given point in time. My own description of this may be a bit hard to follow so I recommend reading over Flinn's description or my own modified procedure and analysis of this activity below.
At this point in the unit we have determined the following things ALL BASED ON EVIDENCE.
- electrons are smaller than atoms (how much smaller will come later)
- electrons have a negative charge
- the charge of an electron is approximately -1.6 x 10^-19 C (the actual magnitude of the charge was derived based on over-simplified data from me....but it was still derived from data)
- electrons are mobile
As mentioned above in the first bullet point, I plan to do some sort of investigation tomorrow or the day after that is centered on determining the mass of the electron--or at least how Thomson determined the charge to mass ratio and what that indicated about the mass of the electron. I will edit this post at a later point once I've come up with an activity to do so.
Anyways, if you made it this far into my post, thank you! By no means do I think this is the best approach. I am never complacent with how I teach anything so any advice is strongly recommended. However, I do know that this is a better approach to learning about atomic structure with regard to evidence-based claims, scientific reasoning, and concept-building rather than simply telling students to read 3/4 of a page in a textbook dedicated to one of the most important subatomic particles in nature.
