lecture 4 - important resonance structures. I. Count the...

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318M – Krische, Lecture 4: F - 09/5/03, Ch. 1, Covalent Bonding and Shapes of Molecules Today, we continue and conclude our discussion of VSEPR theory by considering compounds with multiple bonds (i.e. double and triple bonds). Like single bonds and lone pairs, double and triple bonds are treated as single regions of electron density . Using VSEPR theory, we saw that the carbon atoms of ethylene have distorted trigonal geometry (approx. 120 o ). Also, we saw that methyl cation, BF3, and acetylene have ideal trigonal geometry. We also saw that acetylene, which contains a C-C triple bond, has a digonal geometry (180 o ). In class, we summarized the rules for using VSEPR theory. These rules can also be found on the on- line Chapter 1 Study Guide, Part I. Rules for Applying VSEPR : For molecules that are best represented as combinations of resonance forms (resonance structures are discussed in the forthcoming lecture), it is important to apply VSEPR each
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Unformatted text preview: important resonance structures. I. Count the number of groups around the atom of interest. Single bonds, double bonds, triple bonds and lone pairs each count as a single group. II. If the atom of interest is bound to four groups , it should have a tetrahedral geometry (109 o bond angle). If the atom of interest is bound to three groups , it should have a trigonal geometry (120 o bond angle). If the atom of interest is bound to two groups , it should have a digonal geometry (180 o bond angle). We next examined acetate anion. Here, VSEPR did not accurately describe the geometry of the anionic oxygen. Why? Because a single Lewis structure does not adequately describe acetate ion. We found that acetate anion is best described as a composite of two equivalent Lewis structures. These composite structures are referred to as resonance structures and the molecule is said to be a resonance hybrid....
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