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Unformatted text preview: Chapter 10. Chemical Bonding II. Molecular Geometry and Hybridization of Atomic Orbitals (Only Sections 10.1–10.5) Recommended problems: 2, 8-10, 12, 18-22, 32-34, 36, 39-42. In chapter-9, we looked at number of electrons that each atom in a molecule have after distributing their valence electrons. We were able to do this by drawing Lewis dot structure. But… we never worried about the molecular geometry. Well, molecules are like any other objects should have some shapes. In fact, the molecular shape influences its physical and chemical properties! How do you arrive at the molecular shapes (or geometry)? Consider water and ammonia molecule: Is H 2 O linear (H–O–H) or bent O H H ? Is NH 3 planar N H H H or non-planar N H H H ? In order to answer these questions, we use another theory called Valence Shell Electron Pair Repulsion Theory (VSEPR) – Pronounced as VESPER theory. Valence Shell Electron Pair Repulsion Theory (VSEPR): Proposed by Gillespie and Nyholm in 1957. The theory assumes that the shape of a molecule depends on repulsions between valence shell electron 1 Reading assignments!! Read ahead all the sections and come prepared to answer some questions in the class. pairs, which stay as far apart from each other as possible. Remember! When we talk about valence electron pairs, it includes both bonded electrons and lone pairs of electrons. Again look at the Lewis structure of water, H 2 O lone pairs (LP's) bonding pairs (BP's) total of 4 valence shell e-pairs H H O Look at another example, Formaldehyde, H 2 CO around C, three bonding pairs around O, one bonding and two non-bonding pairs H H double and triple bonds are treated like single bonds C O According to the VSEPR theory, the electron pairs will tend to arrange in such way that it reduces the repulsion between them. Thus, water molecule has 4 electron pairs around central atom O and the geometry that can allow these 4 pair to have minimum repulsion would be a tetrahedral arrangement with angle of 109 o at the carbon center. In general, the repulsion between a lone pair (LP) of electrons and another LP is much greater than LP and a bonded pair (BP) of electrons which is again greater than a BP – BP repulsion. Since, water molecule has two LP’s, it pushes the BP’s and hence, the angle between H-O-H is less than 109 o . Similarly, in formaldehyde, we have 3 electron pair (double bonds are considered as single bond in this case), the geometry that can allow these electrons to have minimum repulsion would be a triangular planar with angle of 120 o around the central carbon. In fact, these are the molecular geometries determined experimentally....
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