Chapter 9a Molecular Geometry and Bonding Theories

Chapter 9a Molecular Geometry and Bonding Theories -...

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Unformatted text preview: Chapter 9: Molecular Geometry and Bonding Theories Shapes and Sizes of Molecules q Chemical basis of odor Shapes and Sizes of Molecules q Chemical basis of taste q Chemical basis of vision: 11-cis-retinal 11-trans-retinal q Double-helical structure of DNA q Explains solubility of substances q Explains melting and boiling points q Responsible for molecular interactions 9.1 Molecular Shapes q CO2 is linear, SO2 is bent. Why are they different? q The shape of a molecule is determined by its bond angles, the angles made by lines connecting the nuclei of the atoms in a molecule. q The bond angle in CO2 is 180o; the bond angle in SO2 is 120o. Molecular Shapes q How can we predict the shape of a molecule? q Pairs of electrons, being negatively charged, repel one another; we can use this to predict their arrangement around a central atom 9.2 The VSEPR Model q Valence-Shell Electron-Pair Repulsion q Electron pairs (or groups of pairs) try to avoid one another because of repulsions between like-charged particles. q Regions where electrons are likely to be found will be called electron domains. q A domain can be qa nonbonding electron pair qan atom (single, double, triple bond or anything in between) q Electron domains occur as far apart as possible Electron-Domain Geometry Predict the Electron-Domain Geometry q CO2 q SO3 q BH3 q NH3 q IF4- How many electron domains are there around the central atom in XeF4? 33% 33% 33% 1. 4 2. 5 3. 6 1 2 3 Molecular Geometry q Molecular geometry describes the shape of only those electron domains that contain bonding electrons. q Molecular geometry is predicted by writing a Lewis structure, then determining the electron-domain geometry, and finally determining the molecular geometry. 1 Molecular Geometry 1 Molecular Geometry 1 Molecular Geometry 1 Molecular Geometry 1 What are the structures of the following molecules and ions? Draw structure and list molecular shape name and bond angles. BeF2(g) XeF2 CO2 SO2 HCN NO H2O SF2 2 BH3 SO3 NH - ClF3 1 What is the EDG and molecular geometry for this molecule? 25% 25% 25% 25% 1. 2. 3. 4. Trigonal bypyramidal; tetrahedral Tetrahedral; tetrahedral Octahedral; see-saw Trigonal bypyramidal; see-saw 1 2 3 4 1 9.3 Polarity of Polyatomic Molecules q Bonds can be polar if electrons are shared unequally q If molecules are polar, they have a separation of positive and negative charge within the molecule, and they exhibit a dipole moment, which is the product of the charge and the distance between centers of positive and negative charge 1 Polar Bonds q For diatomic molecules (with a polar bond), the molecule is polar because the bond is polar. q + H :F Polarity is in the direction of the bond 1 Are molecules polar if they have polar bonds? q Yes, if they are diatomic. q Maybe, if they are polyatomic. Bond polarity of two or more bonds can cancel one another. q - + + Cl : Be : Cl 1 Nonpolar Molecules with Polar Bonds q The molecules SO3 and CCl4 have polar bonds, but their symmetric geometries with no nonbonding electrons on the central atom cause them to be nonpolar molecules. Trigonal Planar Tetrahedral 20 2 Symmetry and Polarity q If equal forces are pulling in different directions, they can cancel out (molecule is not polar) 2 Symmetry and Polarity q If the forces are unequal, they don't cancel (molecule is polar) 2 Molecular Polarity 2 Polarity of Molecules q Can a molecule be polar if its bonds are all nonpolar? q F2 q N2 q O3 q P4 2 Polarity of Molecules q What structures will give rise to nonpolarity even when bonds are polar? q Any completely symmetrical molecule (a symmetrical structure with identical outside atoms) is always nonpolar. q CF4 PCl5 q BCl3 SF6 q CO2 2 Polarity of Molecules 2 Draw the 3D shape and identify each molecule as polar or nonpolar qPCl3 BCl3 Electronegativities F 4.0 O 3.5 Cl, N 3.0 C, I, S 2.5 H, P 2.1 B 2.0 2 ...
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This note was uploaded on 04/06/2011 for the course CHM 113 taught by Professor Anbar during the Spring '06 term at ASU.

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