Experiment 11

VSEPR and Lewis Structures (Day 1)

View information on VSEPR Theory of Molecular Geometry Pages online.  These suppplemental pages describe what VSEPR is, and how to use VSEPR to make molecular models, how to predict molecular shapes (molecule geometry), and how to predict molecule polaity, based on the number of electron domains and their arrangement around the center atom.  Please note, that an electron domain is a single electron region, a pair of electrons (either lone pairs or bonded pairs), multiple pairs of electrons (such as double and triple bonds).

In order to get correct VSEPR results, you must draw Lewis Dot structures.  Follow the procedures below to draw correct Lewis structures:

  1. Count the total number of valence electrons for each atom, or group of atoms, in a molecule (e.g., CHCl3 has 26 electrons).  For ions, add electrons corresponding to the total negative charge (-), or deduct electrons for each positive (+) charge (e.g., SO42- has 32 electrons) to the number of valence electrons.
  2. Identify the center atom (usually the atom present in smallest quantity, or usually the least electronegative), and attach the other atoms to this center atom.  Remember, that "H" can never be a center atom, and must be attached to one of the outlying "O" atoms for acids, although the "H" is attached to the center atom if the molecule or ion is not an acid.
  3. Add back all the electrons (count the electrons in bonds first, and then add the remainder electrons), adding eight (8) electrons to all outlying atoms ("H" can have only a 2-electron bond) before adding any electrons to the center atom.  If you do not have enough electrons to provide the center atom with eight (8) electrons, share a pair of electrons from an outlying atom with the center atom to form a double-bond (outlying halogens never form a double bond).  Usually, all atoms will have eight (8) electrons (octet), except for "H" which wants only two (duet).
  4. Any extra electrons, beyond what is necessary to achieve the octet rule for outlying and center atoms must be placed on the center atom only.  The center atom can have more than eight (8) electrons.
  5. Based on correct Lewis Dot structures, use the table described below to determine its number of VSEPR electron domains, predicted shape, and polarity.

You should print a figure using VSEPR to predict molecule geometry (shape), polarity, and electron domain arrangments for molecules having Three to Six Electron Domains.  This table is more complete than the table found in your experimental protocols.

The following worksheets must be printed prior to coming to the first lab period for this experiment:

Computer Modeling (Day 2)

Today, we will complete VSEPR Worksheet 2. Before you make models, your instructor will check your Lewis Dot Structures for correctness. Your instructor will also check to make certain you have listed the correct molecular geometry and whether the molecule (or ion) is polar (an ion is not necessarily polar!). Get your instructor's signature before making your models. You will need to make models for three structures on the first page and three structures on the second page (6 models total; you can make more if you desire). When the models are made, show them to your instructor.

Using Chem3D™ you will create computer models for the following compounds:
  • NH3 (ammonia)
  • CH4 (methane)
  • HCN (hydrogen cyanide)
  • H2O (water)
  • CO2 (carbon dioxide)
  • CH2O (formaldehyde)

To start the Chem3D™ program, go to the "Apple" Icon in the upper left hard corner of the computer screen. Hold the mouse button down and scroll down to the Chem3D™ program. After you start the program, in the upper left hand corner of the program window, there is a text box where you can enter the formula of the compound you wish to model (you cannot model ions). You must use capital letters, in addition to the appropriate subscript numbers. Type in the formula for the compound (e.g., NH3 for ammonia) and hit the [RETURN] key ([ENTER] key on a PC) to display the compound in the display window. Select the rotation tool (the "globe-like" symbol), and, holding the mouse button down, you can rotate your molecular image. Rotate it until you have an image that you find visually pleasing.

Preference Settings: Under the "View" dropdown menu, select "Preferences." For Model Type, choose "Ball and Stick" and for Atom Fill, select "Pattern by Element."

You will analyze your molecule by choosing the "Analyze" drop down menu. Choose "Show bond lengths" and the calculated bond lengths will appear in a new window. Choose "Show bond angles" and the bond angles will appear in the same window as the bond lengths. Copy these values to the Microsoft Word document, setting up the page similar to the example shown by your instructor.

You will also need to copy your molecule image to the Word document. You may need to select the arrow tool before choosing the SELECT ALL option under Edit. Copy the molecular image to the Word document and paste it into the document. To resize and to move your image in the Word document, double-click on the image to get the Picture Toolbar (or go to View and select the Picture Toolbar link). Click on the Layout option (icon with a diamond in it) and choose "Square" which will now allow you to resize and to move the image. Make the image smaller by selecting one of the corners, and, while holding the mouse down, move the corner to make the image smaller. To move the image, click on the image and move it to the right side of the page, but horizontal to the bond length and angle data.

Be sure to add the name of the compound for each set of data and compound included in the Word document. You should arrange your data similar to the example shown below. On the left, it shows the bond lengths in Angstroms (Å) along with the actual and optimal bond angles. The example below is for formaldehyde.

 
C(1)-O(2) 1.208 1.208
C(1)-H(3) 1.113 1.113
C(1)-H(4) 1.113 1.113
O(2)-C(1)-H(3) 121.266 120.300
O(2)-C(1)-H(4) 121.266 120.300
H(3)-C(1)-H(4) 117.464 116.500

Repeat the above process for each of the six compounds listed above. Make certain all of your data is on no more than two pages. Print only one copy of your data. Make copies of your data to be included in your lab notebook. These data will be used during the lab exam, so they must be part of your lab notebook.

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