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Molecular Geometry and Polarity

Molecular Geometry and Polarity. http://www.scl.ameslab.gov/MacMolPlt/Surface.JPG. Bond Angles in Carbon Compounds. electron configuration = 1s 2 2s 2 2p 2. 2p orbitals with one electron in each. Can p orbitals with one electron in each find the place where the 3 rd p orbital should be?.

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Molecular Geometry and Polarity

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  1. Molecular Geometry and Polarity http://www.scl.ameslab.gov/MacMolPlt/Surface.JPG

  2. Bond Angles in Carbon Compounds electron configuration = 1s22s22p2 2p orbitals with one electron in each. Can p orbitals with one electron in each find the place where the 3rd p orbital should be? Orbitals with one electron in each will overlap to form single bonds. If they can, the bond angles should be 90o. But…the bond angles are 109.5o!

  3. It’s All in the Shape… • So what’s going on? • Think back to the lab… • What is the primary reason molecules form the geometry we find? • Electron Pair Repulsion

  4. A - central atom X -surrounding atom E -nonbonding valence electron-group integers VSEPR - Valence Shell Electron Pair Repulsion Theory Each group of valence electrons around a central atom is located as far away as possible from the others in order to minimize repulsions. These repulsions maximize the space that each object attached to the central atom occupies. The result is five electron-group arrangements of minimum energy seen in a large majority of molecules and polyatomic ions. The electron-groups are defining the object arrangement, but the molecular shape is defined by the relative positions of the atomic nuclei. Because valence electrons can be bonding or nonbonding, the same electron-group arrangement can give rise to different molecular shapes. AXmEn Silberberg, Principles of Chemistry

  5. linear tetrahedral trigonal planar trigonal bipyramidal octahedral Figure 10.3 Electron-group repulsions and the five basic molecular shapes. Silberberg, Principles of Chemistry

  6. Figure 10.4 The single molecular shape of the linear electron-group arrangement. Examples: CS2, HCN, BeF2 Silberberg, Principles of Chemistry

  7. Class Shape Figure 10.5 The two molecular shapes of the trigonal planar electron-group arrangement. Examples: SO2, O3, PbCl2, SnBr2 Examples: SO3, BF3, NO3-, CO32- Silberberg, Principles of Chemistry

  8. The three molecular shapes of the tetrahedral electron-group arrangement. Figure 10.6 Examples: CH4, SiCl4, SO42-, ClO4- NH3 PF3 ClO3 H3O+ H2O OF2 SCl2 Silberberg, Principles of Chemistry

  9. Figure 10.8 The four molecular shapes of the trigonal bipyramidal electron-group arrangement. PF5 AsF5 SOF4 SF4 XeO2F2 IF4+ IO2F2- XeF2 I3- IF2- ClF3 BrF3 Silberberg, Principles of Chemistry

  10. Figure 10.9 The three molecular shapes of the octahedral electron-group arrangement. SF6 IOF5 BrF5 TeF5- XeOF4 XeF4 ICl4- Silberberg, Principles of Chemistry

  11. 1220 Effect of Double Bonds 1160 real Effect of Nonbonding(Lone) Pairs Factors Affecting Actual Bond Angles Bond angles are consistent with theoretical angles when the atoms attached to the central atom are the same and when all electrons are bonding electrons of the same order. 1200 larger EN 1200 ideal greater electron density Lone pairs repel bonding pairs more strongly than bonding pairs repel each other. 950 Silberberg, Principles of Chemistry

  12. The steps in determining a molecular shape. Figure 10.10 Molecular formula Step 1 Lewis structure Count all e- groups around central atom (A) Step 2 Electron-group arrangement Note lone pairs and double bonds Step 3 Count bonding and nonbonding e- groups separately. Bond angles Step 4 Molecular shape (AXmEn) Silberberg, Principles of Chemistry

  13. PROBLEM: Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF3 and (b)COCl2. SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three, or Four Electron Groups The shape is based upon the tetrahedral arrangement. The F-P-F bond angles should be <109.50 due to the repulsion of the nonbonding electron pair. The final shape is trigonal pyramidal. <109.50 The type of shape is AX3E Silberberg, Principles of Chemistry

  14. 124.50 1110 SAMPLE PROBLEM 10.6 continued Predicting Molecular Shapes with Two, Three, or Four Electron Groups (b) For COCl2, C has the lowest EN and will be the center atom. There are 24 valence e-, 3 atoms attached to the center atom. C does not have an octet; a pair of nonbonding electrons will move in from the O to make a double bond. Type AX3 The shape for an atom with three atom attachments and no nonbonding pairs on the central atom is trigonal planar. The Cl-C-Cl bond angle will be less than 1200 due to the electron density of the C=O. Silberberg, Principles of Chemistry

  15. PROBLEM: Determine the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) SbF5 and (b) BrF5. SOLUTION: (a) SbF5 - 40 valence e-; all electrons around central atom will be in bonding pairs; shape is AX5 - trigonal bipyramidal. SAMPLE PROBLEM 10.7 Predicting Molecular Shapes with Five or Six Electron Groups (b) BrF5 - 42 valence e-; 5 bonding pairs and 1 nonbonding pair on central atom. Shape is AX5E, square pyramidal. Silberberg, Principles of Chemistry

  16. PROBLEM: Determine the shape around each of the central atoms in acetone, (CH3)2C=O. PLAN: Find the shape of one atom at a time after writing the Lewis structure. tetrahedral tetrahedral trigonal planar >1200 <1200 SAMPLE PROBLEM 10.8 Predicting Molecular Shapes with More Than One Central Atom SOLUTION: Silberberg, Principles of Chemistry

  17. Molecular Polarity • Just like bonds can be polar because of even electron distribution, molecules can be polar because of net electrical imbalances. • These imbalances are not the same as ion formation. • How do we know when a molecule is polar?

  18. Electric field OFF Electric field ON The orientation of polar molecules in an electric field. Figure 10.12

  19. PROBLEM: From electronegativity (EN) values (button) and their periodic trends, predict whether each of the following molecules is polar and show the direction of bond dipoles and the overall molecular dipole when applicable: PLAN: Draw the shape, find the EN values and combine the concepts to determine the polarity. SOLUTION: (a) NH3 SAMPLE PROBLEM 10.9 Predicting the Polarity of Molecules (a) Ammonia, NH3 (b) Boron trifluoride, BF3 (c) Carbonyl sulfide, COS (atom sequence SCO) The dipoles reinforce each other, so the overall molecule is definitely polar. ENN = 3.0 ENH = 2.1 molecular dipole bond dipoles

  20. SAMPLE PROBLEM 10.9 Predicting the Polarity of Molecules continued (b) BF3 has 24 valence e- and all electrons around the B will be involved in bonds. The shape is AX3, trigonal planar. F (EN 4.0) is more electronegative than B (EN 2.0) and all of the dipoles will be directed from B to F. Because all are at the same angle and of the same magnitude, the molecule is nonpolar. 1200 (c) COS is linear. C and S have the same EN (2.0) but the C=O bond is quite polar(DEN) so the molecule is polar overall. Silberberg, Principles of Chemistry

  21. More Molecular Polarity… • http://academic.pgcc.edu/~ssinex/polarity/polarity.htm • Work through the site listed above.

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