Chapter 9 Molecular Geometries and Bonding Theories

1. Lecture Presentation Chapter 9 Molecular Geometriesand Bonding Theories John D. Bookstaver St. Charles Community College Cottleville, MO

2. 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.

3. 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. Electron Groups (Domains) • We can refer to the electron pairs as electron groups or 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 group (domain). • The central atom in this molecule, A, has four electron groups (domains).

6. • • • • • • • • O N O • • • • Electron Groups (Domains) • The Lewis structure predicts the number of valence electron pairs around the central atom(s) • Each lone pair of electrons constitutes one electron group on a central atom • Each bond constitutes one electron group on a central atom • regardless of whether it is single, double, or triple there are three electron groups on N three lone pair one single bond one double bond

7. VSEPR Theory • Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsiontheory • because electrons are negatively charged, they should be most stable when they are separated as much as possible • The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

8. 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.”

9. VSEPR Theory Using valence-shell electron-pair repulsion (VSEPR) theory, you should be able to predict the following for a molecule: Electron group (domain) geometry with proper name, shape and proper name, bond angles, and polarity/dipole moment.

10. Although Lewis dot structures can help us predict how a species is connected, it really can’t predict the 3-D shape (even though we sometimes draw to imply shape). We use VSEPR theory to predict the 3-D shape. In a nut-shell, this method tells us to put as much space between groups as possible.

11. VSEPR What are the bond angles? What do the shapes of molecules depend on? What constitutes a group (domain)? Are all groups treated the same? No, lone pairs take up more space than electrons in a single, double or triple bond.

12. So, how do we start? • Draw the Lewis dot structure. • Count the number of groups present, remember that’s all electrons around the atom whether in bonds or not! • Arrange the groups “equally” in space around the atom so that we have as much space as possible between groups. Note: lone pairs are put in the places with the most “space.” • Determine name of shape • If shape name includes all groups, it’s called electron group (domain) geometry or orbital geometry. • If shape name includes only those electrons in bonds, it’s called molecular geometry or just plain shape.

13. Two groups Three groups Four groups Five groups Six groups We use balloons to build something with the following number of groups(domains). So, what shape can be constructed with…

14. Two Groups (Domains) System(AB2 or AX2)

15. Three Groups (Domains) System (AB3 or AX3)

16. Four Groups (Domains) System(AB4 or AX4)

17. Five Groups (Domains) (AB5 or AX5)

18. Six Groups (Domains) (AB6 or AX6)

19. VSEPR using balloons! Remember, we use VSEPR theory to predict the 3-D shape; we are trying to put as much space between groups as possible.

20. So what are the names of these shapes? So, these names are always good when giving the electron-domain geometriesor simply orbital geometry. But, if we have any lone pairs (or a single lone electron), these names won’t be the name of the shape. Let’s continue to see what those names would be …

21. Two Electron Groups (Domains)- Linear • When there are two electron groups (domains) around the central atom, they will occupy positions on opposite sides of the central atom • This results in the electron groups taking a linear geometry • The bond angle is 180° • In the linear domain, there is only one molecular geometry: linear

22. 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.

23. Three Groups (Domain)– trigonal-planar Bent, or V-shaped Trigonal-planar boron trifluoride, BF3 3 bond pairs no lone pairs sulfur dioxide, SO2 2 bond pairs 1 lone pairs Molecular geometry (Shape):(2 possibilities) 1-trigonal-planar 2- bent

24. Derivative of Trigonal Planar Electron Geometry • When there are three electron groups around the central atom, and one of them is a lone pair, the resulting shape of the molecule is called a trigonal planar — bent shape • The bond angle is less than 120° • because the lone pair takes up more space

25. Trigonal Planar Electron Geometry • When there are three electron groups (domains) around the central atom, they will occupy positions in the shape of a triangle around the central atom • This results in the electron groups taking a trigonal planar geometry • The bond angle is 120°

26. Trigonal Planar Electron GeometryMultiple 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. Four Groups (Domains) • The electron-domain geometry is often not the shape of the molecule, however. • The molecular geometry is that defined by the positions of onlythe atoms in the molecules, not the nonbonding pairs.

28. Four Groups (Domain) Within each electron domain, then, there might be more than one molecular geometry.

29. Four Groups (Domain)– tetrahedral Molecular geometry (Shape):(3 possibilities) 1- tetrahedral (AB4 or AX4) 2- trigonal-pyramidal (AB3E1 or AX3E1) 3- bent (AB2E2 or AX2E2)

30. Derivatives of Tetrahedral Electron Geometry • When there are four electron groups around the central atom, and one is a lone pair, the result is called a pyramidal shape • because it is a triangular-base pyramid with the central atom at the apex • When there are four electron groups around the central atom, and two are lone pairs, the result is called a tetrahedral—bent shape • it is planar • it looks similar to the trigonal planar—bent shape, except the angles are smaller • For both shapes, the bond angle is less than 109.5°

31. 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.

32. Methane(AB4 or AX4)

33. Pyramidal Shape(AB3E1 or AX3E1)

34. Pyramidal Shape(AB3E1 or AX3E1)

35. Tetrahedral–Bent Shape(AB2E2 or AX2E2)

36. Tetrahedral–Bent Shape(AB2E2 or AX2E2)

37. Five Groups – trigonal-bipyramidal Molecular geometry (Shape):(4 possibilities) 1- trigonal-bipyramidal (AB5or AX5) 2- seesaw (AB4E1 or AX4E1) 3- T-shaped (AB3E2 or AX3E2) 4- linear (AB2E3 or AX2E3)

38. Derivatives of theTrigonal Bipyramidal Electron Geometry • When there are five electron groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room • When there are five electron groups around the central atom, and one is a lone pair, the result is called the seesaw shape • aka distorted tetrahedron • When there are five electron groups around the central atom, and two are lone pairs, the result is called the T-shaped • When there are five electron groups around the central atom, and three are lone pairs, the result is a linear shape • The bond angles between equatorial positions are less than 120° • The bond angles between axial and equatorial positions are less than 90° • linear = 180° axial–to–axial

39. Trigonal Bipyramidal Electron Domain (AB5or AX5) • There are two distinct positions in this geometry: • Axial • Equatorial

40. Trigonal Bipyramidal Geometry

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

42. Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System

43. Seesaw Shape(AB4E1 or AX4E1)

44. T–Shape(AB3E2 or AX3E2)

45. T–Shape

46. Linear Shape(AB2E3 or AX2E3)

47. Six Groups (Domain)– octahedral Molecular geometry (Shape):(3 possibilities) 1- octahedral (AB6 or AX6) 2- square-pyramidal (AB5E1 or AX5E1) 3- square-planar (AB4E2 or AX4E2)

48. Derivatives of theOctahedral Geometry • When there are six electron groups around the central atom, and some are lone pairs, each even number lone pair will take a position opposite the previous lone pair • When there are six electron groups around the central atom, and one is a lone pair, the result is called a square pyramid shape • the bond angles between axial and equatorial positions is less than 90° • When there are six electron groups around the central atom, and two are lone pairs, the result is called a square planar shape • the bond angles between equatorial positions is 90°

49. F F F S F F F Octahedral Shape- SF6(AB6 or AX6)

50. Square Pyramidal Shape(AB5E1 or AX5E1)