Molecular Modeling – Why Do Molecules Have Different Shapes?
Go to: http://www.stolaf.edu/depts/chemistry/mo/struc/explore.htm
The initial screen should look like the insert below.
Select: Shape Entry level
Use the drop-down box to select different molecular geometries. A list of molecules that exhibit
the shape will appear below the chart. To see a structure, click on View.
Notice features displayed: formula, physical state of the substance, bond length (‘distance’),
bond angle (if any), shape…
Note: the background color here
is white. When you view the
molecules, the background will be
black. Right-click in the molecule
display area for options including
color. Change the background
color if you wish.
Click on the molecule to rotate it with your mouse…
Right-click in the molecule display area for a drop-box of options. Some of the options will
allow you to enhance the structure, change colors, label the atoms, spin the molecule, etc.
(If you will be making screen shots of these molecules, think about what options you will want to
display in your finished product.)
Assignment: PowerPoint with 8 slides…
1) Prepare a title slide. The topic is 3-D molecular shapes according to VSEPR Theory: Valence
Shell Electron Pair Repulsion Theory.
2) Dedicate one slide to each of the following molecular shapes:
 Linear (having >2 atoms)
 Bent
 Trigonal planar (planar)
 Trigonal pyramidal (pyramidal)
 Tetrahedral
 Diatomic (H2, N2, O2, F2, Cl2, Br2, I2 are diatomic elements, but there are other
diatomic structures that contain 2 different elements such as CO)
3) Find 2 molecules in the database for each one of the shapes. I recommend that you select
fairly simple examples – those with 2-5 atoms altogether are good.
4) Prepare a screen shot of each one to put on the slide. Before taking the screen shot, you
should enlarge the molecule (Zoom to 20%), change background color (white is better than
black…or you select), and rotate the molecule to get the best viewpoint of its 3-D shape.
5) On loose leaf paper, sketch a Lewis Dot diagram of each molecule. Use the rules we discussed
in class previously. Make sure the sketch shows atoms with octets. Use double or triple
bonds as needed. Beware: any molecule with an “odd” number of valence electrons does
not obey the Octet Rule – I advise you not to use that molecule in your presentation.
Also, any molecule with boron (B) as the central atom will not satisfy the Octet Rule.
Likewise, do not use it. Once you are certain your Lewis diagram “fits” the molecule you
copied from the database, use the Paint program for your Lewis diagrams. Draw, crop,
copy and paste the sketch into your PowerPoint.
You must draw a Lewis Dot diagram for each molecule. This is NOT on the
molecular structure website – you must use the skills recently shown in class.
However, there is a good site that might help you check some of your diagrams.
http://www.stolaf.edu/depts/chemistry/mo/struc/lewis/
6) Add 3 facts about each shape onto the slide. The facts to identify are:
a) How many atoms surround the central atom?
b) How many sets of bonding electrons surround the central atom? (A single bond
is 1 set of bonding electrons. A double bond is also 1 set of bonding
electrons. A triple bond is 1 set of bonding electrons.)
c) How many sets of non-bonding electrons surround the central atom? (Nonbonding electrons are also known as “lone pairs” of electrons – they are not
shared between two atoms.)
8) Name the presentation as follows: chem2_VSEPR_your last name; submit to the drop box