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Models - Experiment 12 Revision 1.1 Molecular Modeling of...

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Experiment 12 Revision 1.1 Molecular Modeling of Covalent Compounds To learn about the geometry of covalently bound molecules. To learn about VSEPR theory. To learn about Isomerism. To learn about Molecular Polarity. In this laboratory exercise we will build models of some simple molecules that are in accordance with the geometries suggested by the Valence Shell Electron Pair Repulsion (VSEPR) Theory. We will note the influence Lone Pairs of electrons and Multiple Bonds have upon the geometry of these molecules. We will also note the overall polarity of the molecules modeled. VSEPR theory provides a simple extension of Lewis bonding theory; predicting molecular geometries for covalently bound molecules, polyatomic ions and networks. This theory assumes the shape of a molecule is influenced by the number of electron pairs about each central atom in the molecule. Although largely accurate in predicting molecular geometries, it is a bit superficial and must be supplemented with Valence Bond Theory in order to provide a more accurate picture of the orbital structure of said electron pairs. In any case, both VSEPR and Valence Bond Theory are being supplanted by the more robust, although more computationally demanding, Molecular Orbital Theory to describe both the electronic structure and geometry of simple molecules. The basic idea underlying VESPR is that each valence shell electron pair around an atom will mutually repel all the other valence shell electron pairs about that atom. Therefore, the electron pairs and covalent bonds will find a geometric arrangement which minimizes these repulsions. These arrangements, for the common cases, are: Num. Electron Domains Geometry Angle Between Domains 2 Linear 180 o 3 Trigonal Planar 120 o 4 Tetrahedral 109.5 o 5 Trigonal Bipyramidal 120 o (equatorial) & 90 o (axial) 6 Octahedral 90 o This Electronic Geometry defines the type of Molecular Geometry possible. For each type of Electronic Geometry, there is a subset of Molecular Geometries which depend on the number of atoms covalently bound to the central atom, as well as the number of Lone Pairs about that atom. As an example, each of the following molecules, CH 4 , NH 3 , H 2 O and HF, is tetrahedral in its Electronic Geometry; each has four electron domains about the central atom. However, they each exhibit different Molecular Geometries. CH 4 is tetrahedral but NH 3 is trigonal pyramidal; CH 4 has 4 bonding atoms about the C atom, whereas NH 3 has only 3, with the fourth electron
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pair being a Lone Pair. When determining Molecular Geometries, Lone Pairs of electrons are not considered. The Lone Pairs influence the Molecular Geometry, but do not participate in it. A list of possible Molecular Geometries for 2-6 electron pairs about a central atom E is provided below:
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Several points need to be considered when identifying Molecular Geometries: i) Double and triple bonds count as a single VSEPR domain . This is because a second covalent bond between two atoms shortens and strengthens the overall bonding, but it does not change the geometry of the bonding.
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