Studio_10_Lewis_Structures_FINAL_FALL_2006 - Chem. 25:...

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NAME: _________________________ STUDIO: __________________________ Lewis Structures and the VSEPR Model The structure of molecules in large part determines their physical and chemical properties. Lewis structures are most useful for providing an initial picture of a most fundamental part of structure – a view of the connections among atoms. Because covalent bonds are pairs of valence electrons shared between two atoms and Lewis structures concentrate on electron pairs, a Lewis structure gives us a starting point to consider where the electrons are in a molecule, because it shows us how the valence electrons are distributed among the atoms. Lewis structures are sometimes an end in themselves; frequently, they are just the starting point for more complex analyses of structure. Lewis structures are flat, 2-dimensional representations of molecules. To take the next step and describe the 3-dimensional structure of molecules, we use the valence shell electron-pair repulsion (VSEPR) model. The basic principle that one applies when suggesting the 3-D structure of a molecule using VSEPR is to first recognize that electron pairs repel each other. Remember that electrons are negatively charged, and ‘like repels like.’ The VSEPR model suggests that the organization of electrons about a given atom in a molecule is principally determined by repulsive forces; electron pairs localize about a central atom in a fashion that minimizes electron-pair repulsions. Lewis structures provide us with the number of bonds and lone pairs of electrons about a central atom; this is called the steric number. VSEPR tells us where these bonds and lone pairs are located. These statements are the underlying principles of VSEPR; we’ll modify them a bit, too, as we look at the 3-dimensional structures of increasing variability and complexity. In this studio, we will first go over the steps to generate acceptable Lewis structures for simple inorganic and organic molecules and then we’ll explore some of the finer points of drawing Lewis structures. At the same time, we’ll consider the VSEPR model and generate pictures of the 3-D architecture of molecules. Note that there is no pre-lab for this; the entire exercise will be done in the studio and turned in at the end of the period. PART I: LEWIS STRUCTURES: THE BASIC IDEA Lewis structures describe where the valence electrons of constituent atoms are located in a molecule. Essentially all you have to do is consider the number of valence electrons available and distribute them according to the following guidelines. Notice the liberal use of two words: ‘tend’ and ‘guidelines.’ The statements are suggestions, not rules, and we will begin by dealing with generalities that usually work. We will modify and extend these guidelines as we deal with larger molecules with more diverse options. The convention:
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Studio_10_Lewis_Structures_FINAL_FALL_2006 - Chem. 25:...

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