10.14.13 - VSEPR, Lab Data, Formal Charges, and Resonance Structures.

VSEPR Theory Lab

This week, we started off by finishing the VSEPR theory lab. In the worksheet, VSEPR Theory Lab (1), we went through 13 examples of molecules and found it's Electron Domain Geometry and Molecular Domain Geometry.

In order to determine the Electron Domain Geometry, you must...
1. Draw a Lewis Structure for the molecule.
2. Count the number of bonded atoms around the central atom.
3. Count the number of lone pair electrons around the central atom.
4. Figure out its molecular class.
5. Find out its Geometric shape.

• Total of two bonded atoms and lone pair electrons: linear.
• Total of three bonded atoms and lone pair electrons: Trigonal Planar.
• Total of four bonded atoms and lone pair electrons: Tetrahedral.
• Total of five bonded atoms and lone pair electrons: Trigonal Bipyramidal.
• Total of six bonded atoms and lone pair electrons: Octahedral.

In order to determine the Molecular Domain Geometry, you must...

1. Find the Electron Domain Geometry.

2. Think of what the shape that the Bonded atoms would take (the red balloons, in context of our lab)

• They would be either linear, bent, trigonal planar, tetrahedral, trigonal pyramidal, trigonal bipyramidal, seesaw, T-shape, octahedral, or square pyramidal.

To follow this main idea, we did a lab using balloons and gum drops. Balloons helped us figure out the Electron Domain Geometry by tying the red balloons (bonded electrons) and the white balloons (lone pair electrons) together. The gum drops represent the Molecular Domain geometry (Orange was the central atom, black was the bonded atoms). The gum drops were tied with toothpicks that depicted the shape of the molecules relatively accurately.

Lab Data

We also finished figuring out the answer to our lab questions from last week (What is the mass percent of copper in a brass sample?). This is our class data:

By using the information given, we figured out the mass percent of copper.

After doing some stoichiometry, we found that the mass percent of copper was 58.7%

Later next week, Dr.J will (try to) give us our lab reports from the last lab back in order to finish our lab this week of brass.

Formal Charges

We had a lecture quiz on formal charges, as well as a Pogil (Lewis Structures (III)) to further become comfortable with the material. To calculate formal charge, you must...

1. Make a Lewis structure for the molecule.

2. Count the number of electrons corresponding to the atom you are trying to figure out.

3. Subtract the number of original electrons (from the periodic table) by the number of assigned electrons.

Here's an example.

Resonance Structures

We also did a Pogil for Resonance Structures (Lewis Structures (III)) to understand which formal charge is the best for each molecule. By figuring out the resonance structures of the molecules, we were able to decide which structure was the best for that molecule (the one with the least number of formal charges).

Main Ideas

I understood most of the material that we learned this week, such as formal charges, resonance structures, bond order, and hypervalancy, however I still have trouble finding the Electron Domain Geometry and Molecular Domain Geometry for molecules. Why can't H2O be linear, instead of bent? Does the structure look like this?

All of the material we learned this week helps us understand the deeper facts of Lewis structures, beyond their models. VSEPR theory is a really interesting idea to me, however I wish I could become more comfortable with it. Next week, I will probably have more questions about the lab as well.