13-The Shapes of Molecules(VSEPR)

13-The Shapes of Molecules(VSEPR) - 5.111 Lecture Summary...

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5.111 Lecture Summary #13 KAN October 9, 2009 13.1 Reading for today: Sections 3-1,3.2 Reading for Lecture 14: Sections 3-8-3.11 VSEPR The Shapes of Molecules: Valence Shell Electron Pair Repulsion (VSEPR) Theory Shape (geometry) has an important influence on the physical and chemical properties of molecules, such as melting points, boiling points, and reactivity. Shape is particularly important in biological systems where molecules must fit precisely into active sites of enzymes. Shapes (geometries) of molecules can be predicted with considerable accuracy using a simple method based on Lewis structures. Valence Shell Electron Pair Repulsion (VSEPR) Theory is based on the principle that valence electron pairs repel each other, and thus, the geometry around the central atom will be such as to minimize the repulsion. Nomenclature: A = central atom X = bound atom E = lone pair VSEPR rules: The Steric Number (SN) is used to predict geometries. SN depends on the number of atoms bonded to the central atom and the number of lone pairs on the central atom: SN = (# of atoms bonded to central atom) + (# of lone pairs on central atom) Note: As far as electron-pair repulsion is concerned, double bonds and triple bonds can be treated like single bonds. This approximation is good for qualitative purposes. So, the
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13-The Shapes of Molecules(VSEPR) - 5.111 Lecture Summary...

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