Chapter 10 Chemical Bonding II

1. Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro Chapter 10Chemical Bonding II

2. VSEPR Theory • e- groups (lone pairs and bonds) are most stable when they are as far apart as possible – valence shell electron pair repulsion theory • Maximum separation • the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule • 3-D representation Tro, Chemistry: A Molecular Approach

3. Electron Groups • the Lewis structure predicts the arrangement of valence e- around the central atom(s) • each lone pair of e- constitutes one e- group on a central atom • each type of bond constitutes one electron group on a central atom e.g. NO2 there are 3 e- groups on N 1 lone pair 1 single bond 1 double bond (counted as 1 group) Tro, Chemistry: A Molecular Approach

4. •• •• •• •• O S O •• •• O S O •• •• • • •• •• • • • • • • •• •• • • • • 5 Basic Molecular Geometries • 5 arrangements of e- groups • for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same Tro, Chemistry: A Molecular Approach

5. 2 e- Groups: Linear Geometry • occupy positions opposite, around the central atom linear geometry- bond angle is 180° e.g. CO2 Tro, Chemistry: A Molecular Approach

6. 3 e- Groups: Trigonal Geometry • occupy triangular positions trigonal planar geometry- bond angle is 120° e.g. BF3 Tro, Chemistry: A Molecular Approach

7. Not Quite Perfect Geometry 3 e– groups around central atom – why not 120° ? Because the bonds are not identical, the observed angles are slightly different from ideal. Tro, Chemistry: A Molecular Approach

8. 4 e- Groups: Tetrahedral Geometry • occupy tetrahedron positions around the central atom tetrahedral geometry - bond angle is 109.5° e.g. CH4 Tro, Chemistry: A Molecular Approach

9. 5 e- Groups: Trigonal Bipyramidal Geometry • occupy positions in the shape of a two tetrahedra that are base-to-base trigonal bipyramidal geometry e.g. PCl5 Tro, Chemistry: A Molecular Approach

10. 6 e- Groups: Octahedral Geometry • occupy positions in the shape of two square-base pyramids that are base-to-base octahedral geometry e.g. SF6 Tro, Chemistry: A Molecular Approach

11. Effect of Lone Pairs: Derivative Shapes • the molecule’s shape will be one of basic molecular geometries if all the e- groups are bonds and all the bonds are equivalent • molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapes Tro, Chemistry: A Molecular Approach

12. 3 e- Groups with Lone PairsDerivative of Trigonal Geometry • when there are 3 e- groups around central atom, and 1 of them is a lone pairtrigonal planar - bent shape- bond angle < 120°e.g. SO2 Tro, Chemistry: A Molecular Approach

13. 4 e- Groups with Lone Pairs Derivatives of Tetrahedral Geometry • when there are 4 e- groups around the central atom, and 1 is a lone pairtrigonal pyramidal shape –bond angle is 107 °e.g. NH3 Tro, Chemistry: A Molecular Approach

14. 4 e- Groups with Lone Pairs Derivatives of Tetrahedral Geometry • when there are 4 e- groups around the central atom, and 2 are lone pairstetrahedral-bent shapee.g. H2O • it looks similar to the trigonal planar-bent shape, except the angles are smaller 104.5° Tro, Chemistry: A Molecular Approach

15. Tetrahedral-Bent Shape Tro, Chemistry: A Molecular Approach

16. Tro, Chemistry: A Molecular Approach

17. 5 e- Groups with Lone Pairs Derivatives of Trigonal Bipyramidal Geometry • when there are 5 e- groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room • when there are 5 e- groups around the central atom, and 1 is a lone pair, the result is called see-saw shape • aka distorted tetrahedron • when there are 5 e- groups around the central atom, and 2 are lone pairs, the result is called T-shaped • when there are 5 e- groups around the central atom, and 3 are lone pairs, the result is called a linear shape • the bond angles between equatorial positions is < 120° • the bond angles between axial and equatorial positions is < 90° • linear = 180° axial-to-axial Tro, Chemistry: A Molecular Approach

18. Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System Tro, Chemistry: A Molecular Approach

19. See-Saw Shape Tro, Chemistry: A Molecular Approach

20. T-Shape Tro, Chemistry: A Molecular Approach

21. Linear Shape Tro, Chemistry: A Molecular Approach

22. 6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry • when there are 6 e- groups around the central atom, and 1 is a lone pair, the result is called a square pyramid shape • the bond angles between axial and equatorial positions is < 90° Tro, Chemistry: A Molecular Approach

23. 6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry • when there are 6 e- groups around the central atom, and 2 are lone pairs, the result is called a square planar shape • the bond angles between equatorial positions is 90° Tro, Chemistry: A Molecular Approach

24. Tro, Chemistry: A Molecular Approach

25. Predicting the Shapes Around Central Atoms • Draw the Lewis Structure • Determine the Number of Electron Groups around the Central Atom • Classify Each Electron Group as Bonding or Lone pair, and Count each type • remember, multiple bonds count as 1 group • Use Table 10.1 to Determine the Shape and Bond Angles Tro, Chemistry: A Molecular Approach

26. Practice – Predict the Molecular Geometry and Bond Angles in ClO2F (Chloryl Fluoride) Tro, Chemistry: A Molecular Approach

27. Practice – Predict the Molecular Geometry and Bond Angles in ClO2F Cl Least Electronegative 4 Electron Groups on Cl Cl Is Central Atom 3 Bonding Groups 1 Lone Pair Cl = 7e─ O2 = 2(6e─) = 12e─ F = 7e─ Total = 26e─ Shape = Trigonal Pyramidal Bond Angles O-Cl-O < 109.5° O-Cl-F < 109.5° Tro, Chemistry: A Molecular Approach

28. Representing 3-Dimensional Shapes on a 2-Dimensional Surface • one of the problems with drawing molecules is trying to show their dimensionality • by convention, the central atom is put in the plane of the paper • put as many other atoms as possible in the same plane and indicate with a straight line • for atoms in front of the plane, use a solid wedge • for atoms behind the plane, use a hashed wedge Tro, Chemistry: A Molecular Approach

29. Tro, Chemistry: A Molecular Approach

30. F F F S F F F SF6 Tro, Chemistry: A Molecular Approach

31. Tro, Chemistry: A Molecular Approach

32. Multiple Central Atoms • many molecules have larger structures with many interior atoms • we can think of them as having multiple central atoms • when this occurs, we describe the shape around each central atom in sequencee.g. acetic acid shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent Tro, Chemistry: A Molecular Approach

33. Describing the Geometryof Methanol Tro, Chemistry: A Molecular Approach

34. Describing the Geometryof Glycine Tro, Chemistry: A Molecular Approach

35. Practice – Predict the Molecular Geometries in H3BO3 Tro, Chemistry: A Molecular Approach

36. Practice – Predict the Molecular Geometries in H3BO3 oxyacid, so H attached to O 3 Electron Groups on B 4 Electron Groups on O B Least Electronegative B has 3 Bonding Groups 0 Lone Pairs O has 2 Bonding Groups 2 Lone Pairs B Is Central Atom B = 3e─ O3 = 3(6e─) = 18e─ H3 = 3(1e─) = 3e─ Total = 24e─ Shape on B = Trigonal Planar Shape on O = Bent

37. Practice – Predict the Molecular Geometries in C2H4 Tro, Chemistry: A Molecular Approach

38. Practice – Predict the Molecular Geometries in C2H4 3 Electron Groups on C C = 2(4e─) = 8e ─ H = 4(1e─) = 4e─ Total = 12e─ 0 Lone Pairs Shape on each C = Trigonal Planar

39. Practice – Predict the Molecular Geometries in CH3OCH3 Tro, Chemistry: A Molecular Approach

40. Practice – Predict the Molecular Geometries in Dimethyl Ether (CH3OCH3) 4 Electron Groups on C C = 2(4e─) = 8e ─ H = 6(1e─) = 6e─ O = 6(1e─) = 6e─ Total = 20e─ 2 Lone Pairs on O Shape on each C = Tetrahedral Shape on O = Bent

41. Reminder about Eletronegativity! • Electronegativity, is a chemical property that describes the tendency of an atom to e- towards itself Tro, Chemistry: A Molecular Approach

42. Polarity of Molecules • in order for a molecule to be polar it must • have polar bonds • electronegativity difference • dipole moments (charge x distance) • have an unsymmetrical shape • vector addition • polarity affects the intermolecular forces of attraction • therefore boiling points and solubilities • like dissolves like • nonbonding pairs strongly affect molecular polarity Tro, Chemistry: A Molecular Approach

43. Molecule Polarity The H-Cl bond is polar Bonding e- are pulled toward the Cl end of the molecule Net result is a polar molecule. Tro, Chemistry: A Molecular Approach

44. Vector Addition Tro, Chemistry: A Molecular Approach

45. Tro, Chemistry: A Molecular Approach

46. Molecule Polarity The O-C bond is polar The bonding e- are pulled equally toward both O’s Symmetrical molecule Net result is a nonpolar molecule Tro, Chemistry: A Molecular Approach

47. Molecule Polarity The H-O bond is polarBoth sets of bonding e- are pulled toward the O Net result is a polar molecule Tro, Chemistry: A Molecular Approach

48. Molecule Polarity Tro, Chemistry: A Molecular Approach

49. Molecule Polarity The H-N bond is polar All the sets of bonding electrons are pulled toward the N Not symmetrical Net result is a polar molecule Tro, Chemistry: A Molecular Approach

50. Molecule Polarity The C-H bond is polar Four equal dipoles cancel each other out due to symmetry Net result is a non-polar molecule Tro, Chemistry: A Molecular Approach