Structure Determines Properties!

1. Chapter 10 Chemical Bonding II Structure Determines Properties! • properties of molecular substances depend on the structure of the molecule • the structure includes many factors, including: • the skeletal arrangement of the atoms • the kind of bonding between the atoms • ionic, polar covalent, or covalent • the shape of the molecule • bonding theory should allow you to predict the shapes of molecules Tro, Chemistry: A Molecular Approach

2. Molecular Geometry • Molecules are 3-dimensional objects • We often describe the shape of a molecule with terms that relate to geometric figures • These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure • The geometric figures also have characteristic angles that we call bond angles Tro, Chemistry: A Molecular Approach

3. Using Lewis Theory to PredictMolecular Shapes • Lewis theory predicts there are regions of electrons in an atom based on placing shared pairs of valence electrons between bonding nuclei and unshared valence electrons located on single nuclei • this idea can then be extended to predict the shapes of molecules by realizing these regions are all negatively charged and should repel Tro, Chemistry: A Molecular Approach

4. 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 repulsion theory • since 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 Tro, Chemistry: A Molecular Approach

5. there are 3 electron groups on N 1 lone pair 1 single bond 1 double bond Electron Groups • the Lewis structure predicts the arrangement of valence electrons 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 Tro, Chemistry: A Molecular Approach

6. Molecular Geometries • there are 5 basic arrangements of electron groups around a central atom • based on a maximum of 6 bonding electron groups • though there may be more than 6 on very large atoms, it is very rare • each of these 5 basic arrangements results in 5 different basic molecular shapes • in order for the molecular shape and bond angles to be a “perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent • 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

7. Linear Geometry • when there are 2 electron groups around the central atom, they will occupy positions opposite each other around the central atom • this results in the molecule taking a linear geometry • the bond angle is 180° Tro, Chemistry: A Molecular Approach

8. Trigonal Geometry • when there are 3 electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom • this results in the molecule taking a trigonal planar geometry • the bond angle is 120° Tro, Chemistry: A Molecular Approach

9. Not Quite Perfect Geometry Because the bonds are not identical, the observed angles are slightly different from ideal. Tro, Chemistry: A Molecular Approach

10. Tetrahedral Geometry • when there are 4 electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom • this results in the molecule taking a tetrahedral geometry • the bond angle is 109.5° Tro, Chemistry: A Molecular Approach

11. Methane Tro, Chemistry: A Molecular Approach

12. Trigonal Bipyramidal Geometry • when there are 5 electron groups around the central atom, they will occupy positions in the shape of a two tetrahedra that are base-to-base with the central atom in the center of the shared bases • this results in the molecule taking a trigonal bipyramidal geometry • the positions above and below the central atom are called the axial positions • the positions in the same base plane as the central atom are called the equatorial positions • the bond angle between equatorial positions is 120° • the bond angle between axial and equatorial positions is 90° Tro, Chemistry: A Molecular Approach

13. Trigonal Bipyramidal Geometry Tro, Chemistry: A Molecular Approach

14. Octahedral Geometry • when there are 6 electron groups around the central atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases • this results in the molecule taking an octahedral geometry • it is called octahedral because the geometric figure has 8 sides • all positions are equivalent • the bond angle is 90° Tro, Chemistry: A Molecular Approach

15. Octahedral Geometry Tro, Chemistry: A Molecular Approach

16. The Effect of Lone Pairs • lone pair groups “occupy more space” on the central atom • because their electron density is exclusively on the central atom rather than shared like bonding electron groups • relative sizes of repulsive force interactions is: Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair • this effects the bond angles, making them smaller than expected Tro, Chemistry: A Molecular Approach

17. 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 Derivative of Trigonal Geometry • when there are 3 electron groups around the central atom, and 1 of them is a lone pair, the resulting shape of the molecule is called a trigonal planar - bent shape • the bond angle is < 120° Tro, Chemistry: A Molecular Approach

18. Derivatives of Tetrahedral Geometry • when there are 4 electron groups around the central atom, and 1 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 4 electron groups around the central atom, and 2 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 < 109.5° Tro, Chemistry: A Molecular Approach

19. Bond Angle Distortion from Lone Pairs Tro, Chemistry: A Molecular Approach

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

21. Derivatives of theTrigonal Bipyramidal Geometry • when there are 5 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 5 electron 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 electron groups around the central atom, and 2 are lone pairs, the result is called T-shaped • when there are 5 electron 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

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

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

24. Linear Shape Tro, Chemistry: A Molecular Approach

25. Derivatives of theOctahedral Geometry • when there are 6 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 6 electron 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° • when there are 6 electron 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

26. Square Pyramidal Shape Tro, Chemistry: A Molecular Approach

27. Tro, Chemistry: A Molecular Approach

28. -1 Practice – Predict the Molecular Geometry and Bond Angles in SiF5─ Si Least Electronegative 5 Electron Groups on Si Si Is Central Atom 5 Bonding Groups 0 Lone Pairs Si = 4e─ F5 = 5(7e─) = 35e─ (─) = 1e─ total = 40e─ Shape = Trigonal Bipyramid Bond Angles Feq-Si-Feq = 120° Feq-Si-Fax = 90° Tro, Chemistry: A Molecular Approach

29. 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

30. 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

31. Tro, Chemistry: A Molecular Approach

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

33. 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 sequence 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

34. Describing the Geometry of Methanol Describing the Geometry of Glycine Tro, Chemistry: A Molecular Approach

35. 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 = Tetrahedral Bent

36. Polarity of Molecules • in order for a molecule to be polar it must • have polar bonds • electronegativity difference - theory • bond dipole moments - measured • have an unsymmetrical shape • vector addition • polarity affects the intermolecular forces of attraction • therefore boiling points and solubilities • like dissolves like • nonbonding pairs affect molecular polarity, strong pull in its direction Tro, Chemistry: A Molecular Approach

37. Molecule Polarity The H-Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule. Tro, Chemistry: A Molecular Approach

38. Tro, Chemistry: A Molecular Approach

39. Molecule Polarity The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule. Tro, Chemistry: A Molecular Approach

40. Molecule Polarity The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. Tro, Chemistry: A Molecular Approach

41. Molecule Polarity The H-N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule. Tro, Chemistry: A Molecular Approach

42. Molecular Polarity Affects Solubility in Water • polar molecules are attracted to other polar molecules • since water is a polar molecule, other polar molecules dissolve well in water • and ionic compounds as well • some molecules have both polar and nonpolar parts Tro, Chemistry: A Molecular Approach

43. A Soap MoleculeSodium Stearate Tro, Chemistry: A Molecular Approach

44. 3.0 N 3.5 3.0 O O Cl 3.5 S 3.5 3.5 O O Practice - Decide Whether the Following Are Polar Trigonal Bent Trigonal Planar 2.5 1) polar bonds, N-O 2) asymmetrical shape 1) polar bonds, all S-O 2) symmetrical shape polar nonpolar Tro, Chemistry: A Molecular Approach

45. Problems with Lewis Theory • Lewis theory gives good first approximations of the bond angles in molecules, but usually cannot be used to get the actual angle • Lewis theory cannot write one correct structure for many molecules where resonance is important • Lewis theory often does not predict the correct magnetic behavior of molecules • e.g., O2 is paramagnetic, though the Lewis structure predicts it is diamagnetic Tro, Chemistry: A Molecular Approach