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Copyright © 2010 jsd

1  Types of Chemical Bonding

1.1  The Small Picture

Here are some examples of chemical substances. Each one is made of smaller units. We categorize them in terms of the structure within each unit, and in terms of the bonding between units.

Structure
within unit
     Bonding Between Units
     Induced Dipole    Ionic     
     ——    ——     
Monatomic  |  solid neon    rock salt: {Na+ Cl}     
Covalent bonds  |  dry ice: CO2    {NH4+ NO3}     

1.2  The Big Picture

Same as above, with more examples and more categories.

Structure
within unit
     Bonding Between Units
     none    Induced Dipole    Permanent Dipole    Ionic    Covalent
     ——    ——    ——    ——    ——
Monatomic  |  neon gas    solid neon    x    rock salt: {Na+ Cl}    quartz: {O Si O}
Ionic bonds  |  NaCl vapor    x    rock salt: NaCl    rock salt: {Na2Cl+ NaCl2}    x
Covalent bonds  |  CO2 gas    dry ice: CO2    ice: H2O    {NH4+ NO3}    quartz: SiO2

Notes:

1.3  Pedagogical Remarks

For several reasons, it is important for the teacher to have the big picture, even if the students are only looking at the small picture. Perhaps the most important reason is that you want to draw the small picture in a way that is consistent with the big picture. You want to be able to extend and refine the picture without forcing the students to do any more “unlearning” than necessary.   When students are just starting out, we do not expect them to have the big picture, and we do not expect them to learn the big picture all at once. They can start with the small picture, as shown in section 1.1.

If you draw the small picture properly, it serves as a stepping stone for further learning. We want it to be a stepping stone, not a stumbling block.   If the teacher does not have the big picture, it is all-too-easy to draw the small picture in a way that seems reasonable at the time but is, alas, not consistent with the big picture. See below for an example of what I mean.

I emphasize that doing it properly makes life easier – not harder – for the students. As a rule, it is easier to learn things that make sense.

Here’s why I mention this: It is appallingly common to find the expression “ionic compound” used as it were the opposite of “molecular compound”. I get nearly 500,000 hits from http://www.google.com/search?q=ionic-compound+molecular-compound ... which makes no sense.

There are good categorization schemes and not-so-good categorization schemes. If somebody chooses to dichotomize everything as “milk” versus “poodles” I tend to think that is not a very wise choice. Similarly I think categorizing things as “ionic” (such as rock salt) versus “molecular” (such as dry ice) is not a very wise choice.

To the extent that those terms make any sense at all, “ionic” tells us about bonding between one unit and another in the rock salt crystal, whereas “molecular” tells us something about the bonding within a single unit in the dry ice crystal. Those are two dramatically different ideas. It’s milk versus poodles.

Also: rock salt and dry ice are not the only possibilities. This should be obvious from a brief glance at the small picture (not to mention the big picture). You could say “neon will not be on the test, so don’t worry about it” and similarly “ammonium nitrate will not be on the test, so don’t worry about it” ... but that is not what I would call a good attitude or good pedagogy. Even if such things are not on the test, they exist in the real world, and any student who thinks about the topic will realize they exist and will wonder where they fit into the alleged “ionic” versus “molecular” categorization scheme.

1.4  Rationale

The primary, fundamental, and overarching goal of any class – and of the education system as a whole – should be to teach students to think more clearly. This is what I care about.   In contrast, I care only indirectly about individual puddles of nonsense such as:

  • "ionic compound" versus "molecular compound"
  • "chemical change" versus "physical change" (as distinguished by macroscopic qualitative observations)
  • "significant digits"
  • filled "Lewis octets" in molecules
  • entropy = "disorder" and/or entropy = "spreading"
  • the cut-and-dried five-step "scientific method"
  • et cetera

Every time you teach something that doesn’t make sense, it puts the students on notice that rote regurgitation is rewarded, whereas thinking just gets them into trouble. This is Not Good. This needs to change.

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Copyright © 2010 jsd