chapter10 - Onceyoucanseethe3dshape, youcanpredictpolarity

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1 Chapter 10 Once you can see the 3d shape, you can predict polarity What’s polar? Charge separation: The electron density is not equally shared. Example HF Is a molecule polar? Two things are required: Is the difference in electronegativity greater than .4? .4 or less = not polar. Is the molecule fully symmetric? If it is, it’s not polar Examples CF 4 CH 2 F 2 SI 2 COCl 2 Hybrid Orbitals & Valence Bond Theory Chapter 9 Predicting Molecular Shapes Molecules and polyatomic ions take on particular shapes based on how many atoms and electron pairs are attached to the center atom We know that atoms take up space electrons take We know that atoms take up space, electrons take space as well How do we know we’re working in the correct direction? We look at experimental results. Our theories should match experimental results. VSEPR (Valence Shell Electron Pair Repulsion) Theory VSEPR (Valence Shell Electron Pair Repulsion) Theory will allow us to analyze the shapes of molecules based on the idea that the electron regions in bonds and lone pairs repel regions in bonds and lone pairs repel. In VSEPR theory, each bonding region (single, double, or triple) and lone pair region have approximately equal repulsion for one another and count as one electron region each.
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2 Practically: What does this look like? Balloon time: How can the balloons keep max distance from each other? 2 balloons, 180 deg apart, 3 balloons 120 deg, 4 balloons; 109 deg, 5 balloons 120 and 90 deg 6 balloons 90 deg We can make a table to show shapes: E pairs and atoms connecte d to center Bond an gle Electron/at om shape Shape, no electron pairs Shape, one electron pair Shape, 2 electro n pairs 2 180 Linear Linear Linear 3 120 Trigonal planar Trignoal planar Bent 4 109.5 Tetrahedral Tetrahedral Trigonal pyramidal Bent 5 120,0 90 Trigonal bipyra mid Trigonal bipyrami d See saw T shape 6 90 Octahedral Octahedral Square pyramid Square planar How do we get from this result to a theory to explain it? This theory should build on the atomic theory, already defined, and enhance it to create a molecular theory. It should fit with experimental results Think back to atomic theory: Carbon: 1s 2 2s 2 2p 2 We know that the 2s 2 2p 2 electrons are valence but they are different, 2 s orbitals, 2 p orbitals Experimentally; we know that CH forms with all Experimentally; we know that CH 4 forms, with all 4 bonds being the same. What happened? How do we get a theory that allows the 2s orbitals and the 2p orbitals to be the same? Definition time: Hybrid From biology, you learn that a hybrid offspring results from selecting parents with specific traits. Female horse + male donkey Æ mule Hybrid corn: selective breading to make corn with the desired traits.
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chapter10 - Onceyoucanseethe3dshape, youcanpredictpolarity

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