Chapter 9 Molecular Geometries and Bonding Theories

1. Chapter 9Molecular Geometriesand Bonding Theories

2. A. Shape of geometrical structure B. Bond angles between atoms C. Ionization energy of atoms D. Atomic radii

3. 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.

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.

5. A. Octahedral B. Tetrahedral C. Trigonal bipyramidal D. Trigonal planar

6. 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. • The central atom in this molecule, A, has four electron domains.

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

8. A. trigonal planar B. tetrahedral C. trigonal bipyramidal D. octahedral

10. 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.”

11. Electron-Domain Geometries Table 9.1 contains the electron-domain geometries for two through six electron domains around a central atom.

12. 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 the number of electron domains.

13. 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.

14. There are no common features. • Each occurs about the central atom only. • Each represents a single electron domain. • All exist when a particular Lewis structure is drawn.

15. Linear Electron Domain • In the linear 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.

16. 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.

17. 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.

18. A. No, the domain with the double bond should “push” the other two electron domains resulting in the bond angle between single bonds to less than 120°. B. Yes, the existence of resonance with three resonance structures equalizes repulsions between electron domains resulting in all bond angles equaling 120°.

19. 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.

20. A. The bonding electron pair domain is “squeezed” by the repulsion experienced with two nuclei versus the nonbonding electron pair domain is repelled by only one nearby nucleus. B. The bonding electron pair domain is attracted to two nuclei versus the nonbonding electron pair domain is attracted to only one nearby nucleus . C. Bonding electron pair electrons move more slowly near two nuclei and thus expand less in volume compared to the nonbonding electron pairs that move more quickly in the presence of only one nucleus. D. Bonding electron pair electrons move more quickly near two nuclei and thus expand less in volume compared to the nonbonding electron pairs that move more slowly in the presence of only one nucleus.

21. 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.

22. Sample Exercise 9.1Using the VSPER Model Use the VSEPR model to predict the molecular geometry of (a) O3, (b) SnCl3– . Practice Exercise Predict the electron-domain and molecular geometries for (a) SeCl2, (b) CO32– .

23. Bond angles are not determined by a particular arrangement of electron domains. • A tetrahedral arrangement of electron domains results in greater electron repulsions and a less favorable geometry than electron domains in a square planar geometry. • A tetrahedral arrangement of electron domains results in smaller electron repulsions and a more favorable geometry than electron domains in a square planar geometry. • A tetrahedral arrangement of electron domains results in no greater electron repulsions and no greater favorable geometry than electron domains in a square planar geometry.

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

25. A. 45° B. 90° C. 109.5° D. 120°

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

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

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

29. Sample Exercise 9.2Molecular Geometries of Molecules with Expanded Valence Shells Use the VSEPR model to predict the molecular geometry of (a) SF4, (b) IF5. Practice Exercise Predict the electron-domain and molecular geometries of (a) BrF3, (b) ICI4– .

30. 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.

31. A. The nonbonding electron domains exert a greater repulsive force than the bonding electron domains and this results in the distortion in the C—O—H bond angle. B. The bonding electron domains exert a greater repulsive force than the nonbonding electron domains and this results in the distortion in the C—O—H bond angle. C. The hydrogen atom attached to the carbon atom in the C—O—H interacts with the nonbonding electron domain and this results inthe distortion in bond angle. D. The CH3 group is large and interacts with the C—O—H group and causes the distortion in C—O—H bond angle.

32. Sample Exercise 9.3Predicting Bond Angles Eyedrops for dry eyes usually contain a water-soluble polymer called poly(vinyl alcohol), which is based on the unstable organic molecule vinyl alcohol: Predict the approximate values for the H — O — C and O — C — C bond angles in vinyl alcohol. Practice Exercise Predict the H — C — H and C — C — C bond angles in propyne:

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

34. A. The tails of the arrows point toward regions of highest electron density as indicated by the blue color in the electron density picture. B. The tails of the arrows point toward regions of lowest electron density as indicated by the red color in the electron density picture. C. The heads of the arrows point toward regions of highest electron density as indicated by the red color in the electron density picture. D. The heads of the arrows point toward regions of lowest electron density as indicated by the yellow color in the electron density picture.

35. Yes, because COS has different elements than in CO2. • No, because COS is linear. • Yes, because O and S have different electronegativities, and CO and CS bond dipoles do not cancel each other. • No, because O and S have similar electronegativities, and CO and CS bond dipoles cancel each other.

36. Polarity

37. Sample Exercise 9.4Polarity of Molecules Predict whether these molecules are polar or nonpolar: (a) BrCl, (b) SO2, (c) SF6. Solution Practice Exercise Determine whether the following molecules are polar or nonpolar: (a) NF3, (b) BCl3.

38. 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.

39. 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.

40. A. As the internuclear bonding distance decreases the nucleus-nucleus repulsion increases and the potential energy rises. B. As the internuclear bonding distance decreases the nucleus-nucleus repulsion decreases and the potential energy rises. C. As the internuclear bonding distance decreases the electron-nucleus attraction increases and the potential energy rises. D. As the internuclear bonding distance decreases the electron nucleus repulsion increases and the potential energy rises.

41. Hybrid Orbitals • Consider beryllium: • In its ground electronic state, beryllium would not be able to form bonds, because it has no singly occupied 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. A. The small lobes of the sp hybrid orbitals of Be can not overlap the lobes of the F orbitals even if oriented properly and cannot form a bond. B. Only large hybrid orbitals of one atom can overlap with large hybrid orbitals of another atom. C. The small lobes of the sp hybrid orbitals of Be can not hold electrons and therefore can not form bonds to F whereas the large lobes can hold electrons and form bonds. D. The small lobes of the sp hybrid orbitals of Be are considerably smaller in spatial extent and form a less effective bond with the lobes of the F orbitals than do the large lobes.

45. Both p orbitals are parallel to the Be-F bonds. • Both p orbitals intersect the Be-F bonds. • Both p orbitals are at an angle of 120o to the Be-F bonds. • Both p orbitals are perpendicular to the Be-F bonds.

46. Hybrid Orbitals • With hybrid orbitals, the orbital diagram for beryllium would look like this (Fig. 9.15). • The sp orbitals are higher in energy than the 1s orbital, but lower than the 2p.

47. Hybrid Orbitals Using a similar model for boron leads to three degenerate sp2 orbitals.

48. A. 2 B. 3 C. 4 D. 5

49. A. The unhybridized p-orbital is 180° from the plane of the sp2 orbitals. B. The unhybridized p-orbital is coplanar with the plane of the sp2 orbitals. C. The unhybridized p-orbital is 109.5° from the plane of the sp2 orbitals. D. The unhybridized p-orbital is perpendicular to the plane of the sp2 orbitals.

50. Hybrid Orbitals With carbon, we get four degenerate sp3 orbitals.