The document discusses molecular structures and resonance, molecular geometries and bonding theories, and electronegativity and polarity. It explains how to write Lewis structures, including showing resonance structures. It describes how molecular geometry can be predicted from the number of electron pairs around an atom. Valence shell electron pair repulsion theory is used to determine molecular shapes. Hybrid orbital theory is introduced to explain molecular geometries involving s, p, and d orbitals.

1. Molecular Structures and Resonance CHAPTER 9.3

2. Structural Formulas Most useful of all; predict relative location of atoms by using letter symbols and bonds.

3. Writing Lewis Structures for Ions Follow the same rules except for Cations (+) = SUBTRACT the # on the charge Anions (-) = ADD the # on the charge Place structure inside brackets and show the oxidation number outside. Ex: Phosphate ion, PO 4 3-

4. Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

5. Delocalized Electrons: Resonance In reality, each of the four atoms in the nitrate ion has a p orbital. The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.

6. Delocalized Electrons: Resonance This means the  electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.

7. Resonance The organic molecule benzene has six  bonds and a p orbital on each carbon atom.

8. Resonance In reality the  electrons in benzene are not localized, but delocalized. The even distribution of the  electrons in benzene makes the molecule unusually stable.

9. Draw Lewis structures for the following; include resonance structures (if any) and formal charges HCN C 2 H 2 HOF BF 3 HCFO COCl 2 N 2 F 2 N 2 O 3 ICl 3 COS PO 4 3- ClO 4 - NO 3 - NH 4 + SO 4 2-

10. Molecular Geometries and Bonding Theories CHAPTER 9.4

11. 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.

12. What Determines the Shape of a Molecule? Electron pairs (both bonding & lone) repel each other. The shape of the molecule can be predicted by placing the electron pairs as far away from each other.

13. Electron Domains We can refer to the electron pairs as electron domains . A double or triple bond, count as one electron domain. This molecule has four electron domains.

14. 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.”

15. Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom.

16. 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.

17. Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs .

18. Molecular Geometries One electron domain may = 2 or more molecular geometries (shape) , depending on lone pairs.

19. Linear Electron Domain (2) 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.

20. Trigonal Planar Electron Domain (3) There are two molecular geometries: Trigonal planar , if all the electron domains are bonding Bent , if one of the domains is a nonbonding pair.

21. 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.

22. 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.

23. Tetrahedral Electron Domain (4) 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

24. Trigonal Bipyramidal Electron Domain (5) There are two distinct positions in this geometry: Axial Equatorial

25. Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. Place lone pairs in equatorial positions ONLY!

26. Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: Trigonal bipyramidal Seesaw T-shaped Linear

27. Octahedral Electron Domain (6) All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar

28. 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.

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

30. Electronegativity and Polarity CHAPTER 9.5

31. Polarity Occurs when electrons are unequally shared. Dipoles - regions where atoms have large electronegativity differences creating two poles (+, -) But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

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

33. Polarity

34. Hybrid Orbitals But it’s hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize.

35. Hybrid Orbitals Consider beryllium: In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.

36. Hybrid Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2 s to the 2 p orbital, it can form two bonds.

37. Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals called sp hybrid . These sp hybrid orbitals have two lobes like a p orbital. One of the lobes is larger and more rounded as is the s orbital.

38. Hybrid Orbitals These two degenerate orbitals would align themselves 180  from each other. This is consistent with the observed geometry of beryllium compounds: linear.

39. Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1 s orbital but lower than the 2 p .

40. Hybrid Orbitals Using a similar model for boron leads to… sp 2 hybrid orbital b/c it uses the one 2s orbital and two orbitals from 2p.

41. Hybrid Orbitals The three degenerate sp 2 orbitals.

42. Hybrid Orbitals With carbon we get… 4 degenerate orbitals are sp 3 hybrid Uses the one 2s orbital and all three 2p orbitals.

43. Hybrid Orbitals The four degenerate sp 3 orbitals.

44. Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.

45. Hybrid Orbitals This leads to five degenerate sp 3 d orbitals… … or six degenerate sp 3 d 2 orbitals.

46. Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom.