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Unformatted text preview: Chapter 9 9.2 Valence-shell electron-pair repulsion (VSEPR) Based on t h e idea t h a t repulsions occur a mong regions of electron density, such as repulsions a mong pairs of bonding and lone pair electrons Electron pairs are oriented as far away from each other as possible Electron-pair geo m e try De t er mined by t h e nu mb er and arrange m en t of t h e electron pairs around t h e cen tral a to m M olecular geo m e try The arrange m en t of t h e a to ms in space *For molecules w hose cen tr al a to ms do not have a lone pair, t h e electron- pair geom e try and molecular geom e t ry ar e t h e same * Bond angles The angles be t w e en t h e bonds of t wo a to ms t ha t ar e bonded to t h e sa me t hird a to m Linear and triangular planar have three bonding pairs and have less than an octet The tetrahedral molecule has an octet The triangular bipyramidal and octahedral exceed the octet rule and so usually they are elements in Period 3 or higher For de t er mining molecular geo m e t ry, t he electron pairs in a m ul tiple bond constitu t e a single region of electron density as in a single bond Steps to pr edict t h e electron-pair geom e try, molecular geo m e t ry, and bond angles by applying t h e VSEPR m odel to t h e to t al nu mber of valence electron pairs: 1. Draw the Lewis structure 2. Determine the number of bonds and the number of lone pairs around each atom 3. Pick the appropriate electron-pair geometry around each central atoms, and then choose the molecular shape that matches the total number of bonds and lone pairs 4. Predict the bond angles, remembering that lone pairs occupy more volume than do bonding pairs Lone pair-lone pair > lone pair bonding pair > bonding pair-bonding pair *Experimentally determined bond angles of NH are 107.5 As the number of lone pairs on the central atom increases, the bond angle decreases Every carbon atom in an alkane has a tetrahedral environment. The tetrahedron is arguably the most important shape in chemistry because of its predominance in the chemistry of carbon and the carbon-based chains that form from the backbone of many biomolecules. Equa torial positions The positions in t h e t riangular plane lie in t h e equa tor of an imaginary spher e around th e centr al a to m Axial positions The nor th and south poles Any lone pairs, because t hey ar e m ore ext ensive t han bonding pairs, will occupy equa torial positions ra th er th an axial positions Unlike the triangular bipyramidal, the octahedron has no distinct axial and equatorial positions; all six positions around the central atom are equivalent Six atoms bonded to the central atom in an octahedron can occupy three axes at right angles to one another (the x-axis, y-axis, and z-axis) If a molecule with octrahedral electron-pair geometry has one lone pair on the central atom, it makes no difference which apex the lone pair occupies (the...
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