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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. VSEPR Theory • Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory • because electrons are negatively charged, they should be most stable when they are separated as much as possible • The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

  5. linear tetrahedral trigonal planar trigonal bipyramidal octahedral Electron-group repulsions and the five basic molecular shapes.

  6. Two electron pairs on central atom Examples: CS2, HCN, BeF2

  7. Electron vs Molecular Geometry • The geometry of electron pairs around a central atom is called the electron geometry. • The arrangement of bonded nuclei around a central atom forms the molecular geometry. • Lone pair electrons on a central atom will repel other pairs but will not be visible in the molecular geometry (no nuclei) • If there are lone pairs on the central atom the electron geometry and the molecular geometry will differ.

  8. Three electron pairs on central atom Examples: SO3, BF3, NO3-, CO32- Examples: SO2, O3, PbCl2, SnBr2

  9. Four electron pairs on central atom Examples: CH4, SiCl4, SO42-, ClO4-

  10. Examples: NH3, PF3, ClO3. H3O+

  11. Examples: H2O, OF2, SCl2

  12. Five electron pairs on central atom

  13. Six electron pairs on central atom

  14. Representing 3-Dimensional Shapes on a 2-Dimensional Surface • One of the problems with drawing molecules is trying to show their dimensionality • By convention, the central atom is put in the plane of the paper • Put as many other atoms as possible in the same plane and indicate with a straight line • For atoms in front of the plane, use a solid wedge • For atoms behind the plane, use a hashed wedge

  15. The steps in determining a molecular shape Molecular formula Step 1 Lewis structure Count all e- pairs around central atom Step 2 Electron-group arrangement (electron geometry) Note lone pairs and double bonds Step 3 Bond angles Consider bonding e- pairs only Step 4 Molecular geometry

  16. 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

  17. 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 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

  18. 124.50 1110 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. 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.

  19. 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 trigonal bipyramidal. 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 square pyramidal.

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

  21. 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?

  22. Electric field OFF Electric field ON The orientation of polar molecules in an electric field.

  23. Polarity of Molecules • For a molecule to be polar it must • have polar bonds • electronegativity difference - theory • bond dipole moments - measured • have an unsymmetrical shape • vector addition • Nonbonding pairs affect molecular polarity, strong pull in their direction

  24. Molecule Polarity The H─Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule.

  25. Molecule Polarity The O─C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.

  26. Molecule Polarity The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.

  27. 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: SOLUTION: (a) NH3 Predicting the Polarity of Molecules (a) Ammonia, NH3 (b) Boron trifluoride, BF3 (c) Carbonyl sulfide, COS (atom sequence SCO) Draw the shape, find the EN values and combine the concepts to determine the polarity. The dipoles reinforce each other, so the overall molecule is definitely polar. ENN = 3.0 ENH = 2.1 molecular dipole bond dipoles

  28. Predicting the Polarity of Molecules (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.

  29. More Molecular Polarity… • http://academic.pgcc.edu/~ssinex/polarity/polarity.htm

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