Lewis, Rules and the Heart of the Matter....

WHAT AM I DOING TO HELP KIDS ACHIEVE?

HOW DO I KNOW WHEN THEY ARE THERE?

WHAT IS THE EVIDENCE?

     Lewis dot structures are an interesting part of chemistry.  Why teach them?  It goes like this...if we can start with a symbol and draw a 2-D structure with a little help we can get the 2-D structure into a 3-D structure.  If we can draw a 3-D structure, then with a little electronegativity, we can begin to understand if the molecule is polar or non polar.  If we can do that, we can begin to predict behaviors on a large scale.   This is the same thing scientists do when they are trying to design new drugs.

     Because of that reason, I believe these little structures have a somewhat important place in the curriculum.  There is a problem.  Research has shown that there are all these "rules" for making lewis dot structures.  Where did these "rules" come from?  Primarily science text books.  Not exactly a good science explanation.  You can imagine...each book has it's own "rules" and the rules work...for some items but then if you draw a larger structure, exceptions (and confusion) occurs.  Interestingly enough, if you go back to the work of Linus Pauling and electronegativity, you start to see a different pictures.  One major idea Pauling examined that influenced electronegativity was bond energy.  It turns out that molecules generally form in a way to have the lowest potential energy in bonds.  The lower the bond energy, the greater the difference in electronegativity....a nice little pattern.  Now, instead of "rules", we are trying to learn a science concept.

     My first approach at teaching this is to first introduce the "rules".  This causes minor frustration.  Students figure out what all other students figure out...don't make much sense and have exceptions.  I have tried to create a "need to know".  Now we are jumping into a POGIL on bond energy and then the TIMU activity on Lewis dot structures.  I did have one student who already started to make the connection of bond energy and electronegativity.  Greater bond energy....lower potential energy (usually favored by molecules) ....also greater change in electrongegativity between atoms in bonds.  One student said, "So if I want to make a molecule, the change in electronegativity might tell me what bonds I might have an easier time forming in a reaction...."  Wow....Much better discussion than saying, "Hey, memorize these rules that only work for a few cases....".  The problem is that as we dip our big toes into the world of energy, there are a million places we can go.  However, it is a nice problem to have.

     POGIL's are great resources.  Sometimes it is a challenge to find the best way to use and assess these.  Here is one thing I am going to try.  We are doing one on bond energy.  I assigned it for homework.  Students will have time to work in groups, check there answers and I will go around to do a "checkpoint".  Then I have a few in which I cut out the key questions and copied them.  I will pass them out at random.  Students will have to find other students with the same key question and then they will have to present to the class.  In academic, we are doing nomenclature.  I am going to do it this year through the POGIL's and then end with "Chem Poker" as the assessment.  We shall see.....  By the way...just did the "moleculer shape" POGIL with the PHET program.  Good program, but hands on models I think would work better...just a hunch and a feeling form the feedback from the kids...