Molecular Geometry

By: Katie Coy

Reminders:

We started off class today with a few reminders:

1. Make sure if you have not taken the quiz that we took in class on Friday that you do so ASAP
2. If you were absent on Friday and today, make sure to show Ms. Friedmann your notes that you took on the two videos (Thursday nights homework.)

Molecular Geometry:

Today we began a new concept called Molecular Geometry. Below are the notes that I took, but you can always go get a copy of Ms. Friedmann's notes off the Unit 7 Notes box on Moodle. Without further ado, here are the notes that we took in class today:

Molecular Geometry (essentially creating a 3D model of a Lewis Structure for a molecule)

 

     A. Molecular geometry is a 3D picture that is representing a more accurate model of how the electrons are spaced around a central atom (the atom in the middle of a Lewis structure.) Lewis Structures show only a 2D model of electrons, whereas Molecular geometry attempts to show a more accurate model of how electrons in the electron cloud (explained in part C) are actually formatted. 

    B. VSEPR- stands for Valence Shell Electron Pair Repulsion

          1. 'clouds' of electrons surrounding a central atom are attracted to the central atom, but since electrons all have a negative charge, they repel each other. Therefore, the most stable state for an electron is to position itself as far away from other atoms as possible. 

     C. There are two kind of electron clouds or things that the electrons live in around the central atom

          1. a pair of electrons in a cloud (bond)

          2. a pair of unbonded electrons (aka a lone pair)

     D. In deciding molecular geometry ask yourself two questions:

          1. how many clouds (either bonded or unbonded) are around that central atom?

          2. how many clouds are bonding and how many clouds are unbonded (or just have a pair of electrons in it?)

The Different Geometries- aka how to actually draw the 3D shapes of molecules

    A. Code: this code is used by chemists to represent all the stuff (atoms, bonded electrons, and unbonded                       electrons) in a molecule. The code is as follows:

          1. A= central atom

          2. X= terminal atoms (atoms that are attached to the central atoms by bonds)

          3. E= electrons that are on the central atom itself (aka lone pair electrons)

   B. Vesper Code                           # of Clouds                            Geometry name

       AX2                                             2 clouds                          Linear (180 degrees)

       AX3                                             3 clouds             Trigonal Planar (120 degrees)

       AX2E                                           3 clouds                     Bent (about 120 degrees)

       AX4                                             4 clouds                 Tetrahedron (109.5 degrees)

       AX3E                                           4 clouds    Trigonal pyramid (about 105 degrees)

       AX2E2                                         4 clouds                      Bent (about 105 degrees)

    C. The VESPER code as seen above is not complete because the # of clouds can go up to 6. In order to fill out the two worksheets homework for tonight, you need to know up to the 6th cloud. This information can be found on this website: http://www.sfponline.org/Uploads/15/VSEPR%20handout.pdf. Also, do not worry if you do not understand completely what this means at all. For the people who were absent today, we just began to familiarize ourselves with this EXTREMELY new concept and confusion is expected. My best explanation for how to use the VESPER code (as seen above) is through an example:

     D. For example,  lets say we have CO2 and the Lewis structure looks like this:

Lewis Structure of CO2

        As one can see there are two electron clouds, because there is the left double bond with carbon and the right double bond with carbon. Anyways, to figure out the correct molecular shape of CO2, consult the table seen above. Which VESPER code shows the formula for having one central atom (carbon) and two terminal atoms (the left and right oxygen?) That's right... AX2 seems to be the right VESPER code so that means that the geometry name for CO2 is linear with a bond angle (the angle between bonds) of 180 degrees. In turn this results into the shape of CO2 on a 3D model being as follows... 

Focus on the right diagram for this is the molecular geometry of CO2

I hope that my example was helpful. If you need more explanation or help, Ms. Friedmann posted a link to a site in the Unit 7 Moodle box that has a Java program that will help you visualize molecular geometry much better! (Just a side note, this program will need Java to run so it will not run on the Chromebooks!)

Homework:

1. Two Webassigns are due by 11:59 pm tomorrow night
2. Try the two handouts 'Molecular Shapes form Lewis structures" due tomorrow
1. Just a note that this can be found in the Unit 7 handouts folder and again this concept is new and confusion is expected.
3. Have a wonderful night :)

I almost forgot.. the next blogger is Lauren Benson