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ISM_chapter9 - Chapter 9 Molecular Structures 401 Chapter 9...

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Chapter 9: Molecular Structures 401 Chapter 9: Molecular Structures Teaching for Conceptual Understanding Introduce this chapter by explaining the importance of understanding molecular shapes. Good examples include the structure of DNA, designer drugs, and physical properties of molecules. If you (or a colleague) use molecular modeling in your research, tell your students about the work and show them examples. Explain that understanding covalent bonding and an ability to draw correct Lewis structures are prerequisites for understanding and predicting correct molecular geometries. If necessary, the students should review Chapter 8. Electron-pair geometry and molecular geometry can be confusing for students. Use balloon models (Figure 9.3) to illustrate the differences and similarities between the two. For example ammonia has a tetrahedral electron-pair geometry and a triangular pyramidal molecular geometry. Construct a four-balloon model from three balloons of the same color and one of a different color. Explain the four balloons represent the regions in which electron pairs are located. The same colored balloons are bonding regions and the different colored balloon the lone pair region. Although a lone pair of electrons contributes to the arrangement of the atoms in the molecule, it is not part of the chemical formula or structure and can be removed from view. Pop the different colored balloon and have the students focus on the three remaining balloons (it is important you continue holding the three balloons in a triangular pyramidal arrangement). A similar example explains the structure of water. Figures 9.4-9.6 are good study aids. Hydrogen bonding is a concept for which there are many student misconceptions, the most obvious being it is a bonding between two hydrogen atoms or two hydrogen molecules. Another is that hydrogen bonding occurs only between two like molecules. Questions for Review and Thought 96-99 assess student understanding of hydrogen bonding. If you are teaching students whose majors involve the study of living systems, it is most important that they have a solid understanding of the concept. Take the time to teach and test it well. Suggestions for Effective Learning Models are an absolute necessity for teaching molecular geometry. Ball-and-stick models will work best to clearly illustrate the different structures and they will correspond to the drawings in the textbook. Balloon models, as described above, work well when explaining regions of bonding and lone-pair electrons. If you draw three-dimensional structures in a two-dimensional plane, use the solid ( ), dashed line (- - - -), and wedge ( ) notation explained in Section 9.1. This notation is used throughout the textbook and in other (organic, biochemistry, biology) textbooks.
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