Molecular Shapes Computer Modeling Lab
Question:
What molecular shapes and bond angles occur for different covalent molecules?
Data:
Draw the Lewis Dot Diagram for the following molecules. You must show all work including
how you calculated the total number of valence electrons. (The molecules in bold were not
on the homework list.)
BeCl2
H2O
NH3
Cl2
NH4+
XeF4
CH3Cl
SF6
CN1PO33PBr5
BH3
IF3
SiO32Br31ICl5
CH2O
NO21- and SF4
Procedure:
For each of the Lewis Dot Diagrams above, use the PhET computer modeling program to “build” the
molecule and then record (in your lab notebook) both the shape of…and bond angle(s) present
in…each of the molecules.
1. Using the PhET program…click on the boxes labeled “Molecular Geometry” and “Show Bond Angles”
(do not include the electronic geometry at this time).
2. Using your LDD…”build” the molecule by clicking on the needed items in the upper right box…the lone
pair item designates a non-bonding pair of electrons.
3. Once you have the molecule “built”…record the shape name and bond angle generated by the
program. This should appear beneath each of your LDD for the list of covalent molecules above! Be
clear and legible with your work…missing or confusing dots yield an incorrect LDD.
4. The modeling program allows you to place either bonded electrons or nonbonding electrons around a
central atom. Build a molecule with four single bonds and examine the bond angle. Now remove one
of the bonded atoms and replace it with a nonbonding pair. Record the given bond angle. Also,
record your observations of the apparent sizes of the bonding pair of electrons vs. the nonbonding pairs
of electrons (the apparent volumes occupied by the electron pairs).
5. Now (using the same example from #4 above) click on the “Electron Geometry” button and record the
molecular and electronic names listed for both of these molecules. Do this for a number of molecules
which have the same number of attachments but different bonding pairs and nonbonding pairs of
electrons and record.
6. Using the computer model…build the CH2O molecule. Compare the molecular shape and bond
angles to that of BH3. What do you notice? Analyze the following statement: “Any number of shared
electrons b/t two atoms (including double and triple bonds)…oriented in a single direction…acts as a
single effective pair in terms of molecular geometry.”
7. Click on the “Real Molecules” tab. Activate all buttons and then designate H2O on the drop-down
menu. Toggle b/t real and model and record the bond angles. Do this for the following molecules
(which are paired with some of your earlier molecules and compare.
XeF4; BF3/BH3; ClF3/IF3; BrF5/ICl5
Using the Table of Electronegativity Values you tape/glue into your lab book, calculate the
difference in electronegativity between all bonded atoms and indicate whether the bond is
nonpolar covalent, polar covalent, or ionic. Be sure to show all work.
Data Analysis:
Using the data above, generate a sequential chart from the simplest molecule shape to the most
complex that indicates a relationship between bonded atoms to the central atom and lone pairs on
the central atom. Do not include redundant molecular shapes with the same combination of
numbers!
Use the following table as a guide:
Table I – Molecular Analysis
Number of
Number of
Lone Pairs
Atoms Bonded
(Nonbonding
to Central
pairs) of
Atom
electrons on
Central Atom
2
1
Total Number
of Attachments
(electron
domains) on
the Central
Atom
3
Shape Name
Bond Angle(s)
Example
Molecule
Bent
120°
SO2
Conclusions:
1. State in your own words the basic idea of VSEPR theory?
2. How does VSEPR relate to the different molecular shapes?
3. Examine the NH4+ molecule. If you analyze this molecule in two dimensions (as you have it drawn on your
paper) what bond angle and shape seems to be present? How does the actual molecule shape and bond
angle appear (based on the computer model…which is 3-D)? Explain.
4. The modeling program allows you to place either bonded electrons or nonbonding electrons around a
central atom. You “built” a molecule with four single bonds and examined the bond angle in the procedure
#4 above and then removed one of the bonded atoms and replace it with a nonbonding pair and recorded
the angle.
(a) You examined the size of the electron orbital for a bonded pair of electrons with the size of the orbital of a
nonbonding pair…what did you notice? Explain this answer.
(b) The computer modeling program is actually incorrect on the bond angle for the molecule with three
bonded pairs and one nonbonding pair (the “Real Molecules” tab is correct). Would you expect the bond
angle between bonding pairs to increase or decrease when the nonbonding pair is placed for a bonding pair?
Explain. Examine your data from procedure #6 for the “corrected bond angles” for some of the molecules.
5. CH3Cl is a 5 atom molecule and is a perfect tetrahedron, but AsCl4- , also a 5 atom molecule, is given a
different shape name. Explain.
6. BeCl2 and TeCl2 are both covalent molecules, yet BeCl2 is linear while TeCl2 is nonlinear (bent). Explain.
7. “In molecules or polyatomic ions that contain nonbonding pairs of valence electrons on the central atom,
the electronic geometry and the molecular geometry cannot be the same. Explain this statement using your
observations from procedure step #5.
8. Distinguish between polar and nonpolar covalent bonds. Define polar vs nonpolar molecules. Examine the
Lewis Dot diagrams for Cl2 and HCl…both are linear, two atom molecules.
(a) Designate the more electronegative atom in HCl. Indicate the direction the shared pair of electrons will be
more tightly held and appropriately place a (+) and (-) charge on the end of this molecule. What about doing
the same for Cl2? Explain.
(b) Now, compare BH3 and NH3 for polar molecule status. Explain.