1. PowerPoint to accompany Chapter 8 Molecular Geometry and Bonding Theories

2. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Shapes  The shape of a molecule plays an important role in its reactivity.  By noting the number of bonding and nonbonding electron pairs, we can easily predict the shape of the molecule. Figure 8.2

3. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

4. CH 4, C, H 2 O (SO 4 ) -2, (SiO 4 ) -4 (PO 4 ) -3, CCl 2 F 2

5. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

6. What Determines the Shape of a Molecule?  Simply put, electron pairs, whether they are bonding or nonbonding, repel each other.  By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.

7. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

8. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

9. Electron Domains  We can refer to the electron pairs as electron domains.  In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom. Therefore, they count as one electron domain. This molecule has four electron domains.

10. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom. Table 8.1

11. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron-Domain Geometries  All one must do is count the number of electron domains in the Lewis structure.  The geometry will be that which corresponds to that number of electron domains. Figure 8.6

12. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Geometries  The electron-domain geometry is often not the shape of the molecule, however.  The molecular geometry is defined by the positions of only the atoms in the molecules, not the nonbonding pairs. Figure 8.6

13. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Linear  In this domain, there is only one molecular geometry: linear.  NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain geometry is. Table 8.2

14. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Geometries Within each electron domain structure, there might be more than one molecular geometry. Given these examples, try and draw the electron domain geometry for OH-, The hydroxide ion. Once you try this, look at the notes for more chemical consequences of these simple geometries. Table 8.2

15. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Trigonal Planar  There are two molecular geometries:  Trigonal planar, if all the electron domains are bonding  Bent, if one of the domains is a nonbonding pair. Table 8.2

16. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Tetrahedral  There are three molecular geometries:  Tetrahedral, if all are bonding pairs  Trigonal pyramidal, if one is a nonbonding pair  Bent, if there are two nonbonding pairs Table 8.2

17. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Trigonal Bipyramidal  There are two distinct positions in this geometry:  Axial (Apical if degenerate)  Equatorial  Name and draw the 4 possible degenerate geometries. Figure 8.8

18. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Trigonal Bipyramidal  Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

19. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Trigonal Bipyramidal  There are four distinct molecular geometries in this domain:  Trigonal bipyramidal  Seesaw  T-shaped  Linear Table 8.3

20. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electron Domain: Octahedral  All positions are equivalent in the octahedral domain. There are three molecular geometries:  Octahedral  Square pyramidal  Square planar Table 8.3

21. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles  Nonbonding pairs are physically larger than bonding pairs (all charge and virtually no mass to constrain them).  Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule. Figure 8.7

22. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles  Double and triple bonds place greater electron density on one side of the central atom than do single bonds.  Therefore, they also affect bond angles.  For hazards and gas warefare read notes.

23. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Shapes of Larger Molecules In larger molecules, it makes more sense to talk about the geometry of a particular atom rather than the geometry of the molecule as a whole.

24. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

25. Molecular Shape and Molecular Polarity If a molecule possesses polar bonds, it does not mean the molecule as a whole will be polar. Figure 8.11

26. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Shape and Molecular Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. Figure 8.12

27. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecules Containing Polar Bonds Figure 8.13

28. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Ozone-depleting gas trends F12 F22

29. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Chapter 8 End of Part 1  Molecular Shapes  Electron Domain Shapes