1. Ch. 6: Chemical Bonding I: Drawing Lewis Structures and Determining Molecular Shapes Dr. Namphol Sinkaset Chem 200: General Chemistry I

2. I. Chapter Outline I.Introduction II.Electronegativity III.Lewis Structures IV.Resonance V.Exceptions VI.Bond Energies and Bond Lengths VII.VSEPR Theory VIII.Molecular Polarity

3. I. Bonding Theories Chemistry revolves around compounds, so how these are held together is an important topic. How they are bonded predicts many of their properties. We will cover 3 bonding theories. In this chapter, we expand on Lewis theory.

4. I. Importance of Shape In condensed phases (liquids/solids), molecules are in close proximity, so they interact constantly. The 3-D shape of a molecule determines many of its physical properties. We want to be able to predict 3-D shape starting from just a formula of a covalent compound.

5. I. Binding Sites

6. II. Lewis Theory Simple interpretation of Lewis theory implies that e-’s are equally shared.

7. II. Reality Shows Otherwise

8. II. Electronegativity Atoms don’t share e-’s equally. Electronegativity is the relative ability of a bonded atom to attract shared e-.  It can be thought of as how greedy an atom is for e- when it is sharing them.

9. II. Unequal Sharing of e- More electronegative atoms will pull shared e- towards them. This results in a partial charge separation which can be indicated in one of two ways. This is known as a polar covalent bond.

10. II. Electronegativity Values

11. II. Using ΔEN Differences in electronegativity can be used to determine the bond type.

12. II. Ionic Character of Polar Bonds

13. III. Lewis Structures The first step to getting the 3-D shape of a molecule is getting the correct 2-D structure. The 2-D structure will be the basis of our 3-D shape assignment. We outline the general steps for drawing Lewis structures.

14. III. Steps for Drawing Lewis Structures 1)Determine total # of valence e-. 2)Place atom w/ lower Group # (lower electronegativity) as the central atom. 3)Attach other atoms to central atom with single bonds. 4)Fill octet of outer atoms. (Why?) 5)Count # of e- used so far. Place remaining e- on central atom in pairs. 6)If necessary, form higher order bonds to satisfy octet rule of central atom. 7)Allow expanded octet for central atoms from Period 3 or lower.

15. III. Lewis Structure Practice Draw correct Lewis structures for NF 3, CO 2, SeCl 2, PI 5, IF 2 -, IF 6 +, and H 2 CO.

16. IV. Multiple Valid Lewis Structures Sometimes more than one Lewis structure can be drawn for the same molecule. For example, ozone (O 3 ).

17. IV. Resonance Structures Resonance structures are also known as resonance forms. A resonance structure is one of two or more Lewis structures that have the same skeletal structure (atoms in same place), but different electron arrangements.

18. IV. Resonance Hybrid Neither resonance form is a true picture of the molecule. The molecule exists as a resonance hybrid, which is an average of all resonance forms. In a resonance hybrid, e- are delocalized over the entire molecule.

19. IV. Sample Problem Draw the resonance structures of the carbonate anion.

20. IV. Important Resonance Forms If all resonance forms have the same surrounding atoms, then each contributes equally to the resonance hybrid. If this is not the case, then one or more resonance forms will dominate the resonance hybrid. How can we determine which forms will dominate?

21. IV. Formal Charge formal charge: the charge an atom would have if bonding e- were shared equally formal charge = (# valence e-) – (unshared e- + ½ shared e-)

22. IV. Formal Charges in O 3 We calculate formal charge for each atom in the molecule. For oxygen atom A (on the left), there are 6 valence e-, 4 unshared e-, and 4 shared e-. The formal charge for this O atom is 0. NOTE: sum of all formal charges must equal the overall charge of the molecule!

23. IV. Using Formal Charges Formal charges help us decide the most important resonance forms when we consider to the following guidelines: 1)Small f.c.’s are better than larger f.c.’s. 2)Same sign f.c.’s on adjacent atoms is undesirable. 3)Electronegative atoms should carry higher negative f.c.’s.

24. IV. Sample Problem Find the dominant resonance structures for the sulfate anion.

25. V. Exceptions to the Octet Rule We’ve already discussed expanded valence cases, but there are other exceptions as well.  Compounds w/ odd # of e-’s: free radicals. Examples include NO and NO 2.  Incomplete octets: e- deficient atoms like Be and B, e.g. BeCl 2 and BF 3.  Expanded octets – when d orbitals are used to accommodate more than an octet.

26. VI. Bonding and Energy Lewis theory shows a bond as sharing two electrons, but not all bonds are identical. Bonds can vary in their strength and in their length. Bond energy is the energy needed to break 1 mole of the bond in the gas phase.

27. VI. Average Bond Energies

28. VI. Bond Length Bond length is the distance between bonded atoms. In general, as the bond weakens, the bond length increases. As with bond energies, we can list average bond lengths.

29. VI. Average Bond Lengths

30. VII. VSEPR Theory From a correct Lewis structure, we can get to the 3-D shape using this theory. VSEPR stands for valence shell electron pair repulsion. The theory is based on the idea that e- pairs want to get as far away from each other as possible!

31. VII. VSEPR Categories There are 5 electron geometries from which all molecular shapes derive.

32. VII. Drawing w/ Perspective We use the conventions below to depict a 3-D object on a 2-D surface.

33. VII. Determining 3-D Shape The 5 electron geometries (EG) are a starting point. To determine the molecular geometry (MG), we consider the # of atoms and the # of e- pairs that are associated w/ the central atom. All the possibilities for molecular geometry can be listed in a classification chart.

34. VII. Linear/Trigonal Planar Geometries First, we have the linear and trigonal planar EG’s. EGBondsLone PairsMG Linear20linear Trigonal planar 30trigonal planar 21bent

35. VII. Tetrahedral Geometries EGBondsLone PairsMG Tetrahedral40tetrahedral 31pyramidal 22bent 13linear

36. VII. Trigonal Bipyramidal Geometries EGBondsLone PairsMG Trigonal Bipyramidal 50trigonal bipyramidal 41see-saw 32T-shaped 23linear 14

37. VII. Octahedral Geometries EGBondsLone PairsMG Octahedral60octahedral 51square pyramidal 42square planar 33T-shaped 24linear 15

38. VII. Steps to Determine Molecular Geometry 1)Draw Lewis structure. 2)Count # of bonds and lone pair e-’s on the central atom. 3)Select electronic geometry. 4)Place e-’s and atoms that lead to most stable arrangement (minimize e- repulsions). 5)Determine molecular geometry.

39. VII. Trig Bipy is Special In other EG’s, all positions are equivalent. In trig bipy, lone pairs always choose to go equatorial first. Why?

40. VII. Lone Pairs Take Up Space Lone pair e-’s don’t have another nucleus to “anchor” them.

41. VII. Distortion of Angles Lone pair e-’s take up a lot of room, and they distort the optimum angles seen in the EG’s.

42. VII. Some Practice Draw the molecular geometries for SF 4, BeCl 2, ClO 2 -, TeF 5 -, ClF 3, and NF 3.

43. VII. Larger Molecules

44. VIII. Molecular Polarity Individual bonds tend to be polar, but that doesn’t mean that a molecule will be polar overall. To determine molecular polarity, you need to consider the 3-D shape and see if polarity arrows cancel or not.

45. VIII. Sample Problem Determine the molecular geometry of IF 2 - and state whether it is polar or nonpolar.

46. VIII. Polarity and Properties Polarity is the result of a compound’s composition and structure. Knowing that a compound is polar/nonpolar allows us to explain its properties.