Demonstration 2: Displaying Structures in Two Dimensions

To reinforce the usual pencil-paper (or chalkboard) presentation of Lewis-dot formulas

• homemade or commercial flannel board (about 2 x 3 feet)
• 30-40 circles, 1-in. diameter, of tag board or light cardboard with a felt backing
• 10-20 circles, 3-in. diameter, of tag board or light cardboard with a felt backing
• Overhead projector and screen
• 30 circles, about 1-in. diameter, cut from colored transparency film
• 30 circles, about 1/4-in. diameter, cut from colored transparency film (These are easily produced from colored transparency film and an ordinary paper hole punch.)
• Transparency grid with 1 cm x 1 cm rulings

Demonstrate the Lewis-dot formulas for several atoms, molecules and ions-for example, C, O, N, Ne, H, Al3+, K+, CH4, H2O, NH3, Cl2O, H2S, etc. Limit examples to binary (two-element) molecules if possible. Write the same structures on the chalkboard while demonstrating to reinforce the concept and to show students how to use paper and pencil to do this.

After your demonstration, consider asking a student to come to the flannel board or overhead projector to try to illustrate the bonding and nonbonding pairs for a particular molecule. If time permits, have other students do the same.

Hand out the duplicated construction paper to students and encourage them to practice at home. Provide a list of elements, molecules, and ions for them to use.

This demonstration may be used to introduce the concept of Lewis-dot structures along with a chalkboard presentation. It requires about 90 min. to prepare the flannel board and cutouts. Presentation time may vary from 5 to 50 min. depending upon how much is presented and how much discussion is generated.

Use small circles to represent electrons and large circles to represent atoms or ions. If colored transparency stock is available along with an overhead projector, then it is probably easier and simpler to use this method. This will also cut down on storage requirements.

Students can be furnished with circles of two diameters (1-in. and 1/4-in. are good) duplicated on colored construction paper to take home and cut out for practice with Lewis-dot formulas.

While showing students the Lewis-dot structures, point out steps involved in writing Lewis-dot formulas. State and apply the octet rule as the demonstration progresses. If students are called on, have them show that the octet rule is followed.

This is a good time to introduce the use of double and triple bonds to show the octet rule and common patterns when C, N, O, and S are involved. For example, CO2 and CO illustrate this nicely.

Point out that the use of physical models such as these to represent atoms, and, especially bonds, is only an approximation of what chemists think is "real". [Based on Demonstration B23, Displaying structures in two dimensions. (1988).Doing Chemistry. Washington, DC: American Chemical Society.]

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