Molecular Geometries and Bonding Theories

1. Chapter 9Molecular Geometriesand Bonding Theories

2. 9.1 Molecular Shapes • Lewis structures do not indicate shape of molecule • The shape of a molecule plays an important role in its reactivity. • The overall shape is determined by bond angles. • By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule. ABn geometries: (page 347)

3. 9.2 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.

4. What Determines the Shape of a Molecule? • Simply put, electron pairs, whether they be 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. • Each nonbonding pair, single bond, or multiple bond produces an electron domain around the central atom. • In the NH3 molecule below, for example, there are four electron domains around the central atom, three bonding and one nonbonding.

5. Electron-Domain Geometry (ABn molecules) • Electron domains are regions of space around the central atom in which electrons are concentrated. • The arrangement of electron domains around an ABn molecule or ion is called its electron-domain geometry. • All the electrons (to include any multiple bonds) on one side of a central atom constitute an electron domain. This molecule has four electron domains.

6. These are the electron-domain geometries for two through six electron domains around a central atom.

7. To determine the electron geometry just count the number of electron domains in the Lewis structure. • The geometry will be that which corresponds to that number of electron domains.

8. Molecular Geometries • The electron-domain geometry is often not the shape of the molecule, however. • The molecular geometry (i.e., shape of the molecule) is the arrangement of only the atoms in a molecule or ion. • The molecular geometry, though influenced by the presence of nonbonding electrons, does not include the nonbonding electron pairs in the shape of the molecule.

9. Within each electron domain, then, there might be more than one molecular geometry. • For example, in the geometries to the right, all three are the tetrahedral electron-domain geometry. • There are three molecular geometries based only on the number of atoms present.

10. Linear Electron Domain • 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 is.

11. Trigonal Planar Electron Domain • There are two molecular geometries: • Trigonal planar, if all the electron domains are bonding • Bent, if one of the domains is a nonbonding pair.

12. Nonbonding Pairs and Bond Angle • Nonbonding pairs are physically larger than bonding pairs. • Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

13. Multiple Bonds and 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.

14. Tetrahedral Electron Domain • 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

15. Trigonal Bipyramidal Electron Domain • There are two distinct positions in this geometry: • Axial • Equatorial

16. Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

17. There are four distinct molecular geometries in this domain: trigonal bipyramidal, seesaw, T-shaped, and linear.

18. Octahedral Electron Domain • All bond angles are 90o. • There are three molecular geometries: octahedral, square pyramidal, square planar.

19. Shapes of Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

20. This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.

21. 9.3 Molecular Shape and Molecular Polarity • In Chapter 8 we discussed bond dipoles. • But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

22. By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

23. Polar vs. non-polar molecules. • Molecules in which the central atom is symmetrically surrounded by identical atoms are nonpolar.