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viernes, 17 de enero de 2014

An atom is the thousandth part of...Or what my students believe is the size of an atom (Part 1)

This week my syllabus got to a part I was dreading... Periodic Properties. Why? First of all because according to Mexican curriculum I shouldn't be teaching them and still somehow manage to make them get why there are covalent and ionic bonds, but at the same time like just don't tell them to much because they might learn something (seriously I'm going to dedicate a post to Mexican science curriculum and why they think 15 year olds are stupid) 

Point is.. I really felt electronegativity is basic to understand ionic and covalent bonding so teaching it for me is a must, but then well maybe atomic radii wouldn't hurt either. So here I am embarking on something  where I might be wrong (maybe it's not pedagogical incorrect to teach this to my students). 

So I began preparing my class about atomic radii and i realized something. I haven't discussed with my students anything about the size of an atom. We have covered, yes, everything is made of atoms and well yes they must be very tiny because nobody has ever seen them... but really how tiny is the atom? Like if I said the atomic radii of Mg is 150 pm... How do I expect them to imagine 150 pm ? Or to understand that's really really really really tiny. 

So I decided to make a series of questions and asses the ideas they were arriving with (this questions were hidden in a surprise test to make sure they remembered the key concepts we studied last semester). 

First question was "All atoms are the same size" True or False
And besides answering  true or false they had to answer: a) I'm sure of my answer b) I'm not sure c) I have no idea. Why? Because I wanted to asses the confidence they had for answering this kind of statements.

The results were the following:

  • Of 47 students 13 (27.6%) said it was true and 34 (72.3%) false. So that means most of them get a sense that more electrons mean more size. I believe that's a reasonable. Then how to explain the minority? Well I believe we spend so much time telling them most of the size is in the nucleus that they start to think... "well if the electron has almost no mass it shouldn't affect size" though I don't know why they think protons don't affect that much either. 
  • I teach two groups, first group only 4 girls said it was true so in the other group it was the double that thought all atoms were the same size. 
  • First group: of the four persons that thought all atoms were the same size 3 said they were sure of their answer the other one not so sure. 
  • Second group: 4 said they were sure all atoms were the same size 5 said they weren't sure.
  • From the 34 that answered "false" we can forget 7 that said they didn't know the answer so they just guessed, that leaves 27 (16 from group one 11 from group 2) roughly half of them in both groups said they were sure of their answer
Second question was "How would you explain the size of an atom?" They were a little baffled by it but when they got tired of me not giving them any clues about how to address the question they started writing.

The most interesting answers from the first group were:

Student 1: I think an atom is the size of a marble divided in five
Student 2: Like when you are looking at the window and there's light coming from it and you can see the dust in the air. I believe that's kind of the size of an atom
Student 3: We could say an atom is a million time smaller than the tip of a pencil
Student 4: I think the size of an atom can be compared to the fourth part of a lemon
Student 5: The thousandth part of a rheum
Student 6: Anything divided in a million parts
Student 7: The hundredth part of a grain of sand
Student 8: The millionth part of a centimeter
Student 9: The thousandth part of dust
Student 10: The size of an atom is 1x10-18 mol of the size of a cat
Student 11: Fifteen times smaller than a cell
Student 12: Thousandth part of an ant
Student 13: It depends on what it is made of
Student 14: Too small to see it at plain sight

Second group most of the answers were:

- Depends on how many protons, electrons and neutrons it has
- Depends on the kind of element or material
- They are just so small you can't see them
- The smallest thing that can exist
Not a single one of them made an analogy

I'm just so surprised at how different both groups reacted to the question. First group was so imaginative even if their answers were wrong, on the other hand the other group is like they understood the question in a completely different way (I have a feeling that there's a research topic in here)

And even though second group seemed to get better at least what it takes to have atoms of different sizes truth is first group had more girls with the correct answer. Still all this analysis gives us enough information to know what ideas they are bringing and now we can work from them in order to get to a better idea of how small is the atom.

Next post will deal with teaching how small is the atom, and how to help them grasp big numbers and small numbers and a version of "Avogadro goes to court" for middle school students.

Greatest misconception of the week: Mol is a measure of the size of an atom (more on that later)

Comment if you have any thoughts about this post and thanks for reading :)

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