Chapter 9 Molecular Geometries and Bonding Theories

1. Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Chapter 9Molecular Geometriesand Bonding Theories John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice-Hall, Inc.

2. Bell work • Write the electron configuration for the following ions: O O2- Ca Ca2+ • Draw the Lewis structure for NH3. What is the hybridization of the central atom? What is the shape? What are the bond angles?

3. Agenda • Bell work • Pass back graded stuff • Quick questions on chapter 8 • Chapter 8 quiz • HW • Chapter 9 overview (student-made!) – due Friday • Bring Ultimate Chemical Equations handbook on Thursday! • We are meeting early tomorrow. We’ll cover material from chapters 6 and 7.

4. Thursday/Friday • A lot of you are going to be absent on Thursday, so we’re switching Thursday and Friday of this week. Those of you who are absent on Thursday will be responsible for reviewing chapters 1-7 in the Ultimate handbook on your own.

6. Bell work • Determine the oxidation state of sulfur in the following compounds: S8 SO2 SO42- Na2S

7. Agenda • Bell work • Ultimate overview • Practice exercises in Ultimate • HW: Student made chapter 9 overview (due tomorrow!)

8. Ultimate Overview • Chapters 1 & 2 • Chapter 3 • Chapter 4 • Chapter 6 • Chapter 7

9. Overview • 15 minutes to prepare an overview to share with your classmates • Must include: topic, ways to calculate/perform said topic, at least 5 examples

10. Exercises • 7.1 • 7.2 • 6.1 • 6.2 • 4.1 • 4.2 • 4.3

11. Homework • Student-made chapter 9 overview due tomorrow!

13. Bell work • Turn in your chapter 9 overview • What sections of chapter 9 did you find easy to understand? What sections were most difficult?

14. Agenda • Bell work • Overview of chapter 9 • You should already be familiar with hybridization, geometries, and bond angles • Work on practice problems/homework • HW: work on practice problems (for your own good and in class quizzes) • If you were absent yesterday, make sure you review Ultimate chap 1-6 on your own!

15. Molecular Shapes • The shape of a molecule plays an important role in its reactivity. • # of bonding and non-bonding electron pairs determine shape

16. What Determines the Shape of a Molecule? • Electron pairs repel each other, so they are placed as far apart from each other as possible

17. Electron Domains • We can refer to the electron pairs as electrondomains (steric groups). • 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. • How many electron domains does A have?

18. 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.”

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

20. 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.

21. 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.

22. Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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

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

29. Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. Why?

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

31. Octahedral Electron Domain • All positions are equivalent in the octahedral domain. • There are three molecular geometries: • Octahedral • Square pyramidal • Square planar

32. 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.

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

34. 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.

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

36. .. .. .. F N O H Cl H H H B H H F F Cl Cl F F F H Cl C C Xe Cl F F Cl H F F Cl H Is it polar? Polar Polar Nonpolar Polar HCl H2O BF3 NH3 CCl4 CH3Cl Polar Nonpolar XeF4 Nonpolar Polar

37. Overlap and Bonding • We think of covalent bonds forming through the sharing of electrons by adjacent atoms. • In such an approach this can only occur when orbitals on the two atoms overlap.

38. Overlap and Bonding • Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion. • However, if atoms get too close, the internuclear repulsion greatly raises the energy.

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

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

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

42. Hybrid Orbitals • Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. • 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.

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

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

45. Hybrid Orbitals Using a similar model for boron leads to…

46. Hybrid Orbitals …three degenerate sp2 orbitals.

47. Hybrid Orbitals With carbon we get…

48. Hybrid Orbitals …four degenerate sp3 orbitals.

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

50. Hybrid Orbitals This leads to five degenerate sp3d orbitals… …or six degenerate sp3d2 orbitals.