Thursday, May 27, 2010

Molecules: Shapes of Molecules

Determining the shape of molecules can be a tricky business.

For example,
Carbon dioxide (O=C=O) has a central atom (C) with two atoms bonded to it and it has a linear shape.

Water (H-O-H) has a central atom (O) with two atoms bonded to it, but it has a bent shape.


Here’s a demonstration to help you students visualize molecular shapes. And catch your students attention!

Use balloons to represent the number of “things” around a central atom.

As mentioned before, carbon dioxide has two atoms around the central atom, so we tie two inflated balloons together, and voila! The linear molecule is formed:


How about carbon tetrachloride (CCl4) or another molecule that has four atoms surrounding the central atom? Tie four balloons together (I make two pairs of balloons and twist them together) and you’ve got the tetrahedral shape!



Now, back to that tricky water example….

If you draw the electron dot diagram for water, you’ll notice that there are not just two atoms around the central atom, there are also two electron pairs hanging out there. Those need to be accounted for as “things” around the central atom.

So… The oxygen atom in water has four “things” around it that need to be represented with our balloons. Just like with the carbon tetrachloride, I make two pairs of balloons, only this time, I make each pair out of a different color (one color represents the atoms, one color represents the electron pairs).

Twist the two balloon pairs together, and…


Let’s say the green balloons represent the hydrogen atoms…take a look, the green balloons are bent down into a bent shape. The yellow balloons are pushing them down, just as the electron pairs push the hydrogen atoms down!


Cool stuff!

Try it yourself and see what happens if you have 3, 5 or 6 “things” surrounding a central atom.

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This activity was presented by C. Lee in a workshop entitled “The End of Boring Biology and Confusing Chemistry” at the 2006 New Jersey Science Teachers Association Convention

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