Molecular Geometry

Today the boys all played with my molecules set. These have wooden balls of various colors with small holes drilled in them that fit little springs also included in the set. This is much like when we made molecules with marshmallows in that they both follow the same rules.

The black balls have four holes and represent carbon atoms. The yellow balls have one hole and represent hydrogen atoms. So, this configuration is CH4. The difference these sets make is the fact that they are set at the correct angles, making the angle between the hydrogen molecules at 109 degrees.

Why does this matter, you ask? Well, if you tell a student that H2O has two hydrogen atoms and one oxygen atom then a student might think that they are aligned so that they are a linear configuration with hydrogen atoms straight across from each other with a oxygen atom sandwiched in between, when in reality those hydrogen atoms are at a 105 degree angle. They are not even at the 90 degree, right angle configuration that you often see when one draws the molecule on paper.

Speaking of linear molecules, Carbon Dioxide (one carbon and two oxygen molecules) is a linear molecule. This configuration has two double bonds.

The springs allow the bonds to bend to make double bonds.

This pyramidal shape is NH3 (one nitrogen and three hydrogen atoms). They form these three-dimensional shapes because of their covalent bonds. Covalent bonds are made up of electron pairs that are shared between two atoms. Even though they are bonded, they tend to repel one another because they each have the same type of electrical charge. For this reason, they tend to move as far away from each other as possible. In moving as far as possible from each other, they tend to form three dimensional shapes. For example, if drew this molecule flat on paper, the hydrogen molecules would be at 90 degree angles from each other, but if you raise this molecule off the paper, as a three dimensional shape, the hydrogen can now be 107 degrees apart.

My younger students do not yet understand the differences, but their playing with these atoms helps them begin to see the shapes the molecules form as the atoms bond together.
Kits like this are easy to get at any science supply store or through Amazon.



sources:

• Exploring Creation with Chemistry, Jay Wile
• Real Science 4 Kids, Chemistry
• Friendly Chemistry