How to Teach Molecular Geometry

Written by john brennan
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Teaching molecular geometry doesn't have to involve molecular orbitals and quantum mechanics. In fact, you can teach it to nearly any class or audience in terms of valence-shell electron pair repulsion (VSEPR) theory. As long as your students or listeners already know how to draw and interpret Lewis dot structures, you can teach them how to use these structures with VSEPR to understand the shape of a molecule in 3-D.

Skill level:
Moderately Challenging

Things you need

• Whiteboard or chalkboard

Instructions

1. 1

Draw pictures of Lewis dot structures for the following molecules: carbon dioxide, sulphur dioxide, ammonia, methane, phosphorus pentachloride, sulphur hexafluoride.

2. 2

Explain that for the purposes of the VSEPR model, each lone pair and single/double/triple bond in a Lewis dot structure represents a region of electron density. Electrons repel each other, so these regions want to be as far apart from each other as possible. The regions of electron density around each atom will arrange themselves in such a fashion that they maximise their distance from each other.

3. 3

Point out that in VSEPR, when it comes to figuring out how bonds and electron pairs are arranged around an atom, single, double and triple bonds are not treated any differently from each other. In other words, a single bond is treated no differently than a triple bond, even though the triple bond contains more electrons. A lone pair, however, is assumed to take up slightly more space than bonding pairs, so where a lone pair is present, the bond angles between the other bonds are slightly less than they would normally be.

4. 4

Start with your drawing of carbon monoxide. Point out that the carbon atom in the centre of CO2 has two double bonds hence two regions of electron density. These two regions will want to be as far apart from each other as possible, so the angle between the two double bonds will be 180 degrees, which means they will be pointing in opposite directions. The molecule will be linear.

5. 5

Proceed to the structure of sulphur dioxide. There are three regions of high electron density around the central sulphur atom -- a lone pair and two double bonds. The configuration that will keep all three as far apart as possible will be an equilateral triangle in one plane with bond angles of 120 degrees. This type of configuration is called trigonal planar. Because the lone pair takes up slightly more space than the bonding pairs, the angle between the two sulphur-oxygen double bonds will be slightly less than 120 degrees in this case.

6. 6

Note that ammonia has four regions of electron density, so the resulting configuration will be tetrahedral, where the three hydrogen atoms and lone pair point toward the corners of a pyramid. The resulting bond angles would ordinarily be about 109.5 degrees, but the lone pair takes up slightly more space, so it will be slightly less than 109.5 in this case.

7. 7

Explain that the bond angles in methane will be about 109.5, because there are four carbon-hydrogen bonds arranged around the central carbon atom, so the tetrahedral configuration is most favourable.

8. 8

Move on to phosphorus pentachloride, which has five regions of electron density around the central phosphorus atom, so it will adopt a type of configuration called trigonal bipyramidal. Three of the chlorines lie in the same horizontal plane with bond angles of 120 degrees between the phosphorus-chlorine bonds. The other two phosphorus-chlorine bonds point straight up and down at 90 degrees to the plane containing the first three.

9. 9

Point out that sulphur hexafluoride has six regions of electron density, so its configuration will be octahedral, where each sulphur-fluorine bond is at an angle of 90 degrees with respect to its four neighbours. This configuration is completely symmetrical.

10. 10

Build models of each molecule (or have them ready ahead of time). Use a molecular modelling kit or a set of foam balls and toothpicks. If you have a large audience, use a PowerPoint slide. Show your audience what each of the molecules looks like from different angles and explain that, once again, they can predict which geometry the molecule will adopt by choosing the one that maximises the distance between regions of electron density.

Tips and warnings

• The VSEPR model is inaccurate in many ways. The predictions it makes, however, are qualitatively accurate in most cases, so you and your audience can use it to understand and predict molecular geometry. Nonetheless, it might be wise to explain that VSEPR isn't really an accurate picture of how electrons are actually arranged around atoms in molecules -- just a tool you can use to find the shape of a molecule.

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