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1.10 The Shapes of Some Simple Molecules

1.10 The Shapes of Some Simple Molecules. Methane. tetrahedral geometry H—C—H angle = 109.5°. Methane. tetrahedral geometry each H—C—H angle = 109.5°. Valence Shell Electron Pair Repulsions.

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1.10 The Shapes of Some Simple Molecules

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  1. 1.10The Shapes of Some Simple Molecules

  2. Methane • tetrahedral geometry • H—C—H angle = 109.5°

  3. Methane • tetrahedral geometry • each H—C—H angle = 109.5°

  4. Valence Shell Electron Pair Repulsions • The most stable arrangement of groups attached to a central atom is the one that has the maximum separation of electron pairs(bonded or nonbonded).

  5. Figure 1.9 (a): Water • bent geometry • H—O—H angle = 105° H H : O .. but notice the tetrahedral arrangement of electron pairs

  6. Figure 1.9 (b): Ammonia • trigonal pyramidal geometry • H—N—H angle = 107° H H : N H but notice the tetrahedral arrangement of electron pairs

  7. Figure 1.9 (c): Boron Trifluoride • F—B—F angle = 120° • trigonal planar geometry allows for maximum separationof three electron pairs

  8. Multiple Bonds • Four-electron double bonds and six-electron triple bonds are considered to be similar to a two-electron single bond in terms of their spatialrequirements.

  9. H O C H Figure 1.11: Formaldehyde • H—C—H and H—C—Oangles are close to 120° • trigonal planar geometry

  10. O O C Figure 1.12: Carbon Dioxide • O—C—O angle = 180° • linear geometry

  11. 1.11Molecular Dipole Moments

  12. +— Dipole Moment • A substance possesses a dipole moment if its centers of positive and negative charge do not coincide. • m = e x d • (expressed in Debye units) not polar

  13. — + Dipole Moment • A substance possesses a dipole moment if its centers of positive and negative charge do not coincide. • m = e x d • (expressed in Debye units) polar

  14. O O C Molecular Dipole Moments d+ d- d- • molecule must have polar bonds • necessary, but not sufficient • need to know molecular shape • because individual bond dipoles can cancel

  15. O O C Molecular Dipole Moments Carbon dioxide has no dipole moment; m = 0 D

  16. Figure 1.13 Dichloromethane Carbon tetrachloride m = 0 D m = 1.62 D

  17. Figure 1.13 Resultant of thesetwo bond dipoles is Resultant of thesetwo bond dipoles is m = 0 D Carbon tetrachloride has no dipolemoment because all of the individualbond dipoles cancel.

  18. Figure 1.13 Resultant of thesetwo bond dipoles is Resultant of thesetwo bond dipoles is m = 1.62 D The individual bond dipoles do notcancel in dichloromethane; it hasa dipole moment.

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