Displaying Structures in Two Dimensions

Description

This is a teacher-oriented demonstration of the use of two audio visual techniques: flannel boards; and cut-outs at the overhead projector. No student components.

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Purpose

• To illustrate the use of flannel boards for teaching electron dot structures.
• To illustrate the use of transparent cut-outs for teaching bonding in molecules and in solids.

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•  1 homemade, or commercially prepared, flannel board (about 2 feet by 3 feet is a good size)
• 30-40 1" circles of tag board or light cardboard (with a felt backing)
• 10-20 3" (or larger) circles of various colors of tag board or light cardboard (also with a felt backing)
• overhead projector, screen
• 30 cut out circles of different colors
• transparency grid with markings at 1-cm intervals

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• Use the smaller circles to represent electrons, and the larger ones to represent the atoms or ions that are being illustrated.
• Try to show the relative sizes of atoms; that is, metals within a row being smaller than nonmetals and so on.
• Though the "particles" do not have to be shown to scale, students will have already learned that cations are smaller than anions in general. So if you mix up the sizes and use larger ones where smaller is appropriate, you can "test" the students' recollection of the size relationship.

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Construct board, cut-outs: 1.5 hours

Teacher preparation: 5 minutes

Presentation: 5-30 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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Save the materials for future use.

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• When bonds or molecules form between atoms that are coming in contact, the electrons that are most important in understanding those bonds are the outer-shell electrons. The number and arrangements of these electrons (sometimes called valence electrons) determines the number of bonds that a particular atom can form, as well as the size of the atom and the size of the particle formed after reaction.
• In addition, atoms become particularly stable when they achieve an octet of outer-shell electrons (i.e. when they become isoelectronic with a noble gas atom). Lewis structures, or electron dot diagrams, employ a special notation to identify any particular atom's valence electron situation. The valence electrons are shown as dots around the elemental symbol.
• The size of an atom (and the size of its corresponding cation or anion as it loses or gains electrons) depends both upon the number of protons and the number of electrons present. As electrons are added to a neutral atom, the size of the resulting anion increases (since the same nucleus is dealing with more electrons) and vice versa.

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1. Demonstrate one or two atoms or ions, such as C, O, N, Ne, H, Al³⁺, K⁺, and then one or two molecules, such as BH₃, CH₄, and H₂O. Note octet formation and bonding capacity where appropriate.
2. After the teacher's demonstration, a student can be called up to the front of the class to try to illustrate the bonding and nonbonding pairs for a particular structure. Have other students do the same as time allows.
3. Individual students may use the board at their convenience for extra help or reinforcement.
4. Students may use the idea of cutting out paper electrons to practice the Lewis structures at home.
5. Transparent plastic cut outs used at an overhead projector play a similar role.
6. These are effective when describing bonding in ionic solids, for example.

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Students should be aware that the use of any physical classroom model such as a flannel board to represent real atoms and molecules and, especially, bonds is only an approximation of what chemists think is real.

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• overhead
• transparency
• flannel board
• valence electrons
• Lewis diagram
• anions
• cations

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