Three Dimensional Structures of Molecules

Description

Balloons are used to illustrate the valence shell electron pair repulsion model for predicting molecular shapes.

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1. Have students blow up nine balloons to the same size.
2. Tie the balloons together so that you have three sets of two and one set of three.
3. Display the shape of one pair of balloons. Point out that they are on a line. Explain to students that this arrangement is analogous to a molecule such as BeH₂.
4. Display the shape of the three balloons tied together. Note that they form a planar triangle, with "bond angles" of 120°. This might represent a molecule such as BH₃.
5. Take two sets of two, and push them together in a square planar arrangement.
6. Mention to the students that that is one possible way that they might illustrate a molecule such as CH₄. Then push or "pop" the one set against the other, and the balloons will form a three-dimensional, tetrahedral shape. Note that the electrons are further away from one another in the three-dimensional structure.
7. Place a 3x5 card on one of the balloons in the tetrahedral arrangement to represent a nonbonding electron pair. Mention that the electron pair occupies the same space as a bond, perhaps even more. Note that the resulting pyramidal molecular shape (pretending that the balloon with the card is not seen) represents molecules like NH₃ (three electron pair bonds, one unshared pair).
8. Place another 3x5 card on a second balloon to represent another non-bonding pair of electrons. Note the bent planar molecular shape, as in H₂O, which has two bonded atoms and two unshared pairs (the balloons with the cards). You may wish to color code the lone pair balloons; for example, all lone pair balloons could be green. This option requires more balloons than the card scheme.
9. Display the tetrahedral arrangement one more time with another index card in place on a third balloon. Note that there is only one bonding site available, and that the resulting molecule is again linear. This is analogous to the bonding in HF.
10. Extend the demonstration by placing five balloons together (1 set of two and the set of three). These five form a structure called trigonal bipyramid (e.g., PCl₅).
11. Place six balloons together by adding another pair of balloons to the tetrahedral arrangement. An octahedral structure will form. Show the eight sides or faces that are the basis of the name. An example of an octahedral molecule is SF₆.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 040 Three Dimensional Structures of Molecules

Watch the movie.

Sketch the molecular structure of each type of structure predicted by the valence shell electron-pair repulsion theory.

BeH₂

BH₃

CH₄

NH₃

H₂O

PCl₅

SF₆

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Purpose

To model the valence shell electron-pair repulsion using balloons.

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• 9 10" balloons
• 6 3" x 5" file cards (used for identification of bonded atoms and lone electron pairs)
• double-stick tape (used to stick the cards to the balloons)

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• Have students blow up the balloons so as to all have nearly the same size. Not only is this convenient, but it also helps students participate in the demonstration from the beginning.
• Practice this demonstration at least once before trying it in front of the class.

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Teacher preparation: 10 minutes

Presentation: 20-30 minutes

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There are no unusual hazards in this experiment.

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No special precautions are required in this experiment. Follow routine laboratory precautions.

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Discard used balloons with routine solid waste.

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• valence electrons
• bonding
• linear
• planar
• tetrahedral
• pyramidal
• bonding pair
• non-bonding pair
• lone pair
• trigonal bipyramidal
• octahedral
• bond angles
• electron probability shapes

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