Larry L. Brandon Thornwood High School
South Holland, IL 60473
This was a presentation of a relatively inexpensive model system for use in
chemistry or some biology classes. It could also be used in junior high science
I make use of some solid geometry shapes and the Valence Electron Shell
Repulsion theory. The shapes are tetrahedra for 1-bond, 2-bond, 3-bond and 4-bond
atoms. There are two sizes -- the smaller one is for carbon, nitrogen, oxygen and
fluorine. The larger size is for silicon, phosphorus, sulfur and chlorine. There
must also be an accounting for the resonance structures of nitrogen, phosphorus
Nitrogen and phosphorus occasionally seem to show an oxidation number of 5
(e.g., the nitrate ion). The trigonal bipyramid can be used to show this. There
are two sizes of this shape available. The smaller one is for nitrogen, the
larger for phosphorus. Sulfur will show an oxidation number of 6. The octahedron
accounts nicely for this.
We use the standard model colors for the elements -- hydrogen is white, carbon
is black, nitrogen is blue, oxygen is red, fluorine and chlorine are green,
phosphorus is purple, and sulfur is yellow. Students are told to use magic
markers on the back of the sheet. Then when the shapes are cut out and folded on
the lines, the lines are on the inside. This makes a better looking model. I
also insist on using Elmer's School Glue. I know that there is nothing in the
stuff that will cause harm to any student.
A razor blade (single edge, available at hobby stores and from supply houses)
is used to slice the vertex which is used to create a bond. A portion of a flat
toothpick or wood splint is used to brace connections between figures. VSEPR is
applied when putting models together. Each vertex represents either a non-
bonding pair of electrons or a single bonding electron.
/ \ / \
The above figure is a representation of what two tetrahedra look like when
This is a culminating activity with molecular structure and bonding. It is
most definitely NOT an introductory activity. Again, try other applications of
this modeling. Let me know what you did and how you liked it.
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