VSEPR and Molecular Models

In this experiment, you will refresh your understanding of VSEPR (Valence Shell Electron Pair Repulsion) theory.  You will draw Lewis Dot Structures of several compounds, make models of these structures and predict some physical properties, such as polarity.

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 molecular polarity, based on the number of electron domains and their arrangement around the center atom.  Please remember, that even though VSEPR suggests pairs of electrons, VSEPR really refers to electron domains or regions.  For our sake, we consider VSEPR electron domains to be any electron region.  Any of the following electron domain can be classified as a "VSEPR electron pair."

• A single, unbonded electron is an electron domain
• A pair of electrons (either lone pairs or bonded pairs)
• Multiple bonded pairs of electrons (such as a double bond or a triple bond).

Remember, an electron region can exist as a single electron (one electron), two electrons (lone pair or bonded pair), four electrons (a double bond), or six electrons (triple bond).

In order to get correct VSEPR results, you must first draw correct Lewis Dot Structures.  Follow the procedures shown below to draw correct Lewis structures.  Remember that you must account for all valence electrons and you must show all of these electrons in the Lewis Dot Structure.

1. Determine the total number of valence electrons for the atoms present in a molecule or ion (e.g., CHCl₃ has 26 valence electrons; C=4, H=1, Cl=21 [3x7]).  For ions, after you determine the number of valence electrons for the atoms, add the number of electrons corresponding to the total negative charge (-), or deduct electrons for each positive (+) charge (e.g., SO₄^2- has 32 electrons and NH₄⁺ has 8 electrons) to the number of valence electrons.
2. Identify the center atom (usually the atom present in the smallest quantity, and usually the atom which has the smaller electronegativity value), and attach the other atoms to this center atom.  Remember, that "H" can never be a center atom, and, if "O" is present, will usually 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 formed in connecting the atoms 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, and "H" can 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 all outlying atoms (as well as the center atoms) must be placed on the center atom only.  The center atom can have more than eight (8) electrons if that atom is in the third energy level or higher (e.g., S, Cl, and P can have more than eight electrons, but only if they are the center atom).
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.

As a reminder, remember that as a general rule, each of the following atoms will usually have the number of bonds shown.

• "H" will always have one bond, and will never have more than two electrons surrounding it.
• "O" will usually have two bonds (either two single bonds or one double bond), but occassionally will have three bonds (e.g., in H₃O⁺ and CO).
• "C" will always have four bonds
• "N" will usually have three bonds and a lone pair (exception is NH₄⁺, which has four bonds)
• Halogens (F, Cl, Br, and I) will always have one bond as an outlying atom (not center atom).  "F" will never be a center atom.  When Cl, Br, or I is a center atom, these atoms will always have at least eight electrons, but may have more.  As center atoms, Cl, Br, and I can have more than one bond, as well as more than eight electrons.
• Period 2 elements (e.g., C, N, O, and F) will never have more than eight electrons, whether they are a center atom or an outlying atom.

You must print the worksheets, which are available online to record your results.

The compounds or ions that you will use for this experiment are listed below:

For each compound, you will need to do the following:

• Show the correct name of the compound or ion listed
• List the total number of valence electrons present
• Draw a correct Lewis Dot Structure, accounting for all valence electrons, and give correct molecular shape
• Make a model of the compound or ion, and show it to your instructor
• Determine whether the compound or ion is polar (Y/N)
• Indicate the orbital hybridization of the center atom in each model (e.g., sp, sp², sp³, dsp³, d²sp³, etc.)

At the end of this period, after you have finished and handed in the work sheets for the compounds or ions shown above, you will take a quiz on VSEPR and molecular structure.  You will not be able to use any materials for the quiz except the model set.  

Go To Experiment: VSEPR 1a 1b 1c 2 3 4 5 6 7 8 9 10
Return to Chem210 Experiments Index