Ch 7: Using Molecular Model Kits to Understand Molecular Shapes
Name __________________________________ Date ________________ Period _______
Although we represent molecules two-dimensionally on paper, most actually are three-dimensional. The valence shell electron pair
repulsion theory (VSEPR, often pronounced ‘vesper’) allows us to predict the shapes of organic molecules. In most cases the shape of
a molecule determines its properties. By building models and predicting shapes, chemists come to understand the properties of even
the most complex organic molecules, such as vitamins, enzymes, hormones and nucleic acids.
Key Terms: VSEPR, isomer, tetrahedron/tetrahedral, trigonal planar, trigonal pyramidal, bent, linear, single bond, double bond, triple
bond, unshared electron pair, electron cloud, lewis dot structure, molecule shape, electron cloud geometry, bond angle.
Complete the following on the back of this paper:
1. Use a ruler, pencil and scissors to cut a piece of paper that is exactly 20.0 cm long and 8.9 cm wide.
Roll this paper into a cylinder and tape it, without overlapping, to provide a circumference of 20.0 cm. Pinch one end of the
cylinder closed and tape it, then pinch the other end perpendicular to the first and mark the fold points. Crease from each of these
points to each end of the taped end, then tape this end closed to form a tetrahedron. A tetrahedron has four points, each equal
distance from the center point, and as far as possible from each other. This shape is very important in understanding the three
dimensional shape of many organic molecules, since the clouds of the four electron pairs in a stable octet will arrange themselves
as far apart as possible (109.5o), resulting in a tetrahedral geometry of electron clouds.
Check here when finished
2.
Assemble a methane molecule, CH4, using white atoms for hydrogen and black for carbon. Draw the lewis structure. Use lines to
represent shared electron pairs, and dots to represent unshared electron pairs, throughout this activity. Sketch the threedimensional shape of this molecule. Describe how this shape compares to your tetrahedron. (3 items)
3.
Assemble a chloro-ethane molecule, C2H5Cl. Use green to represent the chlorine. Note the tetrahedral arrangement of the bonds
on each of the carbon atoms. Single bonds spin freely, so there is only one possible arrangement of atoms in this molecule. Draw
the lewis structure, then sketch the actual shape of the molecule. (2 items)
4.
Assemble a dichloro-ethene molecule, C2H2Cl2, which has a double bond between the two carbons. You must use the long
flexible bonds to form double bonds with this model set. In the case of double and triple bonds, as with a single bond the
electrons of these bonds are all forced to stay between the two bonded atoms, so they form a single electron cloud. Furthermore,
there is no spin about a double or triple bond. There are three electron clouds surrounding each carbon, resulting in a shape called
trigonal planar. What is the approximate angle between the bonds around each carbon atom? Describe the shape of this
molecule? There are three different isomers (substances that share the same formula, but have different structures) for this
molecular formula. Draw the lewis structure of each using the bond angles you observe in your models. (5 items)
5.
Acetylene, also called ethyne, C2H2, has a triple bond between the carbon atoms. Acetylene’s shape is linear because there are
just two electron clouds surrounding each carbon, one triple bond, and one simgle bond. What is the angle between these clouds?
Draw the lewis structure of acetylene. (2 items)
6.
There are two different isomers of C2H6O, an alcohol and an ether. Assemble models, using red atoms to represent oxygen, then
draw lewis structures of each. Oxygen will have two unshared electron pairs and two bonds, for a total of four electron clouds,
giving it a tetrahedral cloud geometry with 109.5o bond angles. Note that since there are only two bonds on the oxygen, the
molecule shape is bent at that point in the molecule. We see this same bent shape in a water molecule (check it out!). (2 items)
7.
Assemble an ammonia molecule, NH3, using the black atom for nitrogen. Draw the lewis structure and sketch the shape of this
molecule. Nitrogen has an unshared electron pair and three bonds, resulting in tetrahedral cloud geometry. However, since
there are only three bonds, the molecule shape is not tetrahedral, but trigonal pyramidal, a low triangular pyramid. (2 items)
8.
Draw the lewis structures of the seven isomers of C4H10O. Take care that the structures you draw are different and not just the
same structure flipped over. Four are alcohols and three are ethers. (7 items)
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