Lab Activity 16.2 - VSEPR Theory

Purpose: To use Lewis dot structures to predict the geometry of a molecule from the Valence Shell Electron Pair Repulsion (VSEPR) model.

Materials:

Play-Doh (1 large ball, 6 small balls)	6 toothpicks
protractor	black, green, purple, yellow and red markers
scissors	Scotch tape or glue stick

Procedure:

VSEPR Tutorial: http://wblrd.sk.ca/~chem20bb/covmolec/vseprmovie.html

1. Go to the website above and work through the tutorial.

2. You will be asked to build molecules using styrofoam balls and the toothpicks. Replace the styrofoam balls with balls of Play-Doh. Each molecule that you build will contain the large Play-Doh ball as the central atom in the molecule. Please actually build each model so that you can see the model in three-dimensions. Verify the angles between atoms with a protractor.

3. As you build your molecules, record the entries in Table 1. Remember that the electron pairs around the central atom in a molecule will repel each other and move as far apart as is possible.

4. Use your VSEPR chart to work through the examples in Table 2 and predict the geometry of the given molecules. Check your answers using the tutorial.

Molecular Origami

5. On the origami models, use a marker to draw color coded circles over each of the chemical symbols of the atoms. You are doing this because some of them will be cut off when you cut out the patterns. Use this color scheme: C – black, Cl – green, F – purple, S – yellow, P – red, H - blue

6. Cut out the origami models. Hold the paper with the print side up – we’ll call this the front and the non-printed side the back. Fold along the lines. On a solid line, fold towards the back. On a dotted line, fold towards the front. Make sharp creases and corners. Tape or glue your models together.

7. Describe the shape of each of your molecules. Use the shape of the molecule to help draw the Lewis dot structures. Some of these dot structures may be exceptions to the octet rule. Fill in Table 3 as you go.

Table 1 - VSEPR Theory Chart

Number of Atoms	Number of electron pair around the central atom	Number of lone pair and bonding pair		Sketch of the molecule	Bond Angle (Angle between two bonds)	Molecular Shape
		LP	BP

Table 2 – Examples: Predicting Molecular Geometry

Chemical Formula	Lewis Dot Structure	Number of lone pair and bonding pair (count multiple bonds as one pair)		Molecular Shape	Bond Angle (Angle between two bonds)
		LP	BP
H20
NH3
CO2
HBr
PF5
SO2
CH4
BeCl2

Table 3 – Molecular Origami

Chemical Formula	Molecular Shape	Lewis Dot Structure	Number of lone pairs and bonding pairs
			LP	BP
PCl2F3
CCl2F2
SF6
PHF2

Conclusions and questions:

1) In a trigonal pyramidal molecule, why are the angles less than 109.5^o?

2) When would a bent molecule have bonds of less than

a) 120^o?

b) 109.5^o?

3) Which of your “origami” molecules were exceptions to the octet rule?

4) Look at your origami model of CCl₂F₂. Give some reasons why the shape of this molecule is not a regular tetrahedron (all sides equal, all angles equal to 109.5^o).

5) Consider the ammonium ion, NH₄⁺.

a) Draw the Lewis dot structure.

b) What is its molecular shape?

c) What are the bond angles?

d) How is this like and unlike the CCl₂F₂ molecule?

6) Draw the Lewis Dot structure and give the molecular shape and the bond angle(s) for each of the following:

a) CO molecular shape: bond angle(s):	b) H2S molecular shape: bond angle(s):
c) CCl4 molecular shape: bond angle(s):	d) CH2Cl2 molecular shape: bond angle(s):
e) SiF6^2- molecular shape: bond angle(s):	f) AsF3 molecular shape: bond angle(s):

Molecular Origami

phosphorus dichloride trifluoride

PCl₂F₃

dichlorodifluoromethane

CCl₂F₂

sulfur hexafluoride 

SF₆

difluorophosphine

PHF₂

Please note that the origami models in this packet are not all to the same scale.