2. V alence S hell E lectron P air R epulsion Theory Planar triangular Tetrahedral Trigonal pyramidal Bent

3. VSEPR Theory Based on Electron Dot (Lewis structures) Theory predicts shapes of compounds abbreviated VSEPR VSEPR (pronounced “vesper”) stands for: Valence Shell Electron Pair Repulsion VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves) Electrons around central nucleus repel each other. So, structures have atoms maximally spread out

4. VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs. Molecular Shape

5. Vocabulary: “domain” = any electron pair, or any double or triple bond is considered one domain. “lone pair” = “non-bonding pair” = “unshared pair” = any electron pair that is not involved in bonding “bonding pair” = “shared pair” = any electron pair that is involved in bonding Need help: VSEPR introduction video VSEPR practice problems

6. How to determine shapes of molecules 1. Determine the central atom (usually the atom with the lowest subscript and/or the atom capable of forming the most bonds). 2. Draw the electron dot structure and bar diagram 3. Determine the molecular geometry using ALL electron pairs AND atoms around the central atom 4. Modify the geometry to determine the molecular shape if non- bonding electron pairs exist by ignoring them, BUT LEAVE THE ATOMS OF BONDED PAIRS WHERE THEY ARE. This is done because even if the electrons have no atom attached, these unbonded electron pairs still affect the shape of the structure NOTE- when counting the total number of electron pairs around the central atom, include both the bonding pairs and lone pairs. Double and triple bonds are counted as a bonding pair. 6. Use the VSEPR geometry table to predict the geometry of the molecule (it can only be one of 5 shapes!!).

7. VSEPR THEORY: Example: BeH 2 1) Central Atom?  Be (only 1 atom)

8. VSEPR THEORY: Example: BeH 2 2) Electron Dot? 2) Bar Diagram? H Be H H—Be—H Note that Be violates the octet rule—this is an exception!

9. VSEPR THEORY: Example: BeH 2 3) Geometry? Hint: What is the furthest apart you can spread two atoms attached to a central atom? H H Be

10. VSEPR THEORY: Example: BeH 2 4) Shape? Ignore any unbonded pairs of electrons — not necessary in this case.  LINEAR H H Be

11. LINEAR They push each other to opposite sides of center (180  apart). BeH 2

12. VSEPR THEORY: Example: BF 3 1) Central Atom?  B (only 1 atom)

13. VSEPR THEORY: Example: BF 3 2) Electron Dot? 2) Bar Diagram? F—B—F F Note that B violates the octet rule— this is an exception! F B F F

14. VSEPR THEORY: Example: BF 3 3) Geometry? Hint: What is the furthest apart you can spread three atoms attached to a central atom? B F F F

15. VSEPR THEORY: Example: BF 3 4) Shape? Ignore any unbonded pairs of electrons — not necessary in this case.  trigonal planar B F F F

16. TRIGONAL PLANAR They push each other apart equally at 120  degrees. BF 3

17. VSEPR THEORY: Example: CH 4 1) Central Atom?  C (only 1 atom)

18. VSEPR THEORY: Example: CH 4 2) Electron Dot? 2) Bar Diagram? H C H H H H—C—H H H

19. VSEPR THEORY: Example: CH 4 3) Geometry? Hint: What is the furthest apart you can spread four atoms attached to a central atom? Think in 3D! C H H H H

20. VSEPR THEORY: Example: CH 4 4) Shape? Ignore any unbonded pairs of electrons —not necessary in this case.  tetrahedral C H H H H

21. TETRAHEDRAL Each repels the other equally - 109.5  - not the expected 90 . Think in 3D. CH 4

22. VSEPR THEORY: Example: NH 3 1) Central Atom?  N (only 1 atom)

23. VSEPR THEORY: Example: NH 3 2) Electron Dot? 2) Bar Diagram? H N H H H—N—H H

24. VSEPR THEORY: Example: NH 3 3) Geometry? Hint: What is the furthest apart you can spread three atoms plus one unbonded pair of electrons attached to a central atom? Think in 3D! H N H H ~109.5 o

25. VSEPR THEORY: Example: NH 3 4) Shape? Ignore any unbonded pairs of electrons —it IS necessary in this case.  trigonal pyramidal H N H H ~109.5 o

26. TRIGONAL PYRAMIDAL The thicker, lone pair forces the others a little bit closer together (~107.3  ) NH 3

27. VSEPR THEORY: Example: H 2 O 1) Central Atom?  O (only 1 atom)

28. VSEPR THEORY: Example: H 2 O 2) Electron Dot? 2) Bar Diagram? O H H O—H H

29. VSEPR THEORY: Example: H 2 O 3) Geometry? Hint: What is the furthest apart you can spread two atoms plus two unbonded pairs of electrons attached to a central atom? Think in 3D! H O H ~109.5 o

30. VSEPR THEORY: Example: H 2 O 4) Shape? Ignore any unbonded pairs of electrons —it IS necessary in this case.  bent H O H ~109.5 o

31. BENT The 2 bonding pairs are pushed apart by 3 rd and 4 th pair (not seen) H20H20

32. NOTE: The geometry around the central atom of a water molecule is tetrahedral The molecular shape is bent. H20H20

33. What do you mean by the molecular shape vs the geometry??? The geometry takes into account ALL the bonds and unbonded electrons The molecular shape “ignores” the unbonded pairs of electrons

34. Comparing the 2 “bents”… Both bent molecules are affected by unshared pairs – 1 pair on the left, 2 on the right.

35. Tetrahedral vs. Trigonal pyramidal Tetrahedral geometry around the central atom Tetrahedral Molecular Shape Tetrahedral geometry around the central atom Trigonal Pyramidal Molecular Shape

36. Tetrahedral vs. Trigonal pyramidal On the right, the 4 th lone pair, is not seen as part of the actual molecule, yet affects shape. If another one of the bonding pairs on “trigonal pyramidal” were a lone pair, what is the result?

37. VSEPR overview Each shape has a name (you will have to know these ) Names of Shapes: tetrahedral trigonal pyramidal Bent Linear trigonal planar

38. Triangular PlanarTetrahedral Trigonal pyramidal Linear Bent or V Models

39. Another way to draw structures in “3D”

40. MoleculeLewis StructureNumber of electron pairs CH 4 NH 3 SHAPE Tetrahedral Trigonal Pyramidal 4 4 (3 shared 1 lone pair)

41. MoleculeLewis StructureNumber of electron pairs H2OH2O CO 2 MOLECULAR SHAPE Bent 4 (2 shared 2 lone pairs) 2 Linear

42. MoleculeLewis StructureNumber of electron pairs BeCl 2 BF 3 SHAPE 2 3 Linear Trigonal Planar

43. Back to Electronegativy You must also figure our if there are polar or nonpolar bonds between the atoms AND if the overall molecule is polar or nonpolar What????????

44. What is the difference between a polar bond and a polar molecule? A polar bond is a charged bond. There is a positive and negative end that do not cancel each other out A polar molecule contains polar bonds that do not cancel each other out.

45. Fluorine is the Base of Comparison 0.0 to 0.2non-polar covalent 0.3 to 1.4polar covalent 1.5 ionic

46. Which atom attracts e- more? H ― Cl δ+δ+δ+δ+ δ-δ-δ-δ- electronegativities 2.1 3.0 C = O ― ― H H 2.53.5 2.1 2.1 O = C = O

47. POLAR MOLECULES POLAR MOLECULES = uneven distribution of charge. negativeslightly positive Creating poles. 1 side of molecule is negative ; one side is slightly positive. * Creating poles. NON-POLAR MOLECULES NON-POLAR MOLECULES = no difference in charge on outside of molecule. Electrons are evenly distributed. Uniform charge on outside of molecule. Recall polarity and electronegativity

48. Determine if OF 2 is polar or non polar The electronegativities of oxygen and fluorine, 3.44 and 3.98, respectively, produce a 0.54 difference that leads us to predict that the O-F bonds are polar. The molecular geometry of OF2 is bent. Such an asymmetrical distribution of polar bonds would produce a polar molecule.

49. Predict the polarity of CH 4 (methane) bonds Step 1: Determine polarity of bonds Bonds Bonds are evenly spaced out. If bonds making up a molecule are non-polar, then the molecule is non-polar. Therefore, CH 4 is a non-polar molecule. molecule Step 2: Determine polarity of molecule When there are no polar bonds in a molecule, there is no permanent charge difference between one part of the molecule and another, and the molecule is nonpolar.

50. Carbon dioxide. bonds Step 1: Determine polarity of bonds If bonds making up a molecule are polar, then the molecule may be polar or non-polar, depending on its shape. Which atom attracts electrons more? Molecule (due to the shape) In this case the shape is linear Step 2: Determine polarity of Molecule (due to the shape) In this case the shape is linear O = e- pulled toward

51. The center of the positive charges in located on the carbon atom The center of the negative charge is also located on the carbon atom. non-polar. Since center of both the positive and negative charge are located in the same spot in the molecule, there is no difference in overall charge) so the molecule is non-polar.

52. Look at sulfur dioxide. Step 1: Determine polarity of bonds Center of positive charge is on the sulfur atom. While the center of negative charge is located ½ way between the two oxygen atoms. Since polarity of the bonds and shape of the molecule result in an uneven distribution of charge – SO 2 is a polar molecule. Which atom attracts more e-(s)?

53. Determine if BF 3 is polar or non polar B-F bonds are polar but they are symmetrically arranged around the central fluorine atom. No side of the molecule has more negative or positive charge than another side, and so the molecule is nonpolar.

54. Is CCl 4, a polar molecule? Or nonpolar? In CCl 4, there are 4 bonds arranged symmetrically around the central carbon atom. Each bond is polar, you can tell due to the electronegativity differences between carbon and chlorine. Because the 4 atoms of chlorine are arranged symmetrically, the polarity of the bonds cancel each other out, making a non-polar molecule.

55. Predict whether the following molecules are polar or nonpolar: (a) BrCl; (b) SO 2 ; SOLUTION (a) Chlorine is more electronegative than bromine. Consequently, BrCl will be polar with chlorine carrying the partial negative charge: (b) Because oxygen is more electronegative than sulfur, the molecule has polar bonds. For each of these, the VSEPR model predicts a bent geometry. Because the molecule is bent, the bond dipoles do not cancel and the molecule is polar

56. Determine if OF 2 is polar or non polar The electronegativities of oxygen and fluorine, 3.44 and 3.98, respectively, produce a 0.54 difference that leads us to predict that the O-F bonds are polar. The molecular geometry of OF2 is bent. Such an asymmetrical distribution of polar bonds would produce a polar molecule.

57. Now that you have seen how to apply the two steps to determine the polarity of molecules, see if you can predict the polarity of the following: H 2 O Ammonia (NH 3 ) SO 3 CH 3 Cl PH 3 CCl 4

58. H 2 O (Water) Step 1: Polarity of bonds Based on electronegativity difference between H and O, bond is polar Step 2: Shape of molecule Based on VSEPR theory, water is bent. Center of positive charge is between the two hydrogen, and center of negative charge on oxygen. WATER is a POLAR molecule.

59. PH 3 Step 1: Polarity of bonds Based on electronegativity difference between H and P, bonds are polar Step 2: Shape of Molecule Trigonal Pyramidal 1 unshared pair around central Atom 3 shared bond Polar Molecule…

60. NH 3 (Ammonia) Step 1: Polarity of bonds Based on electronegativity difference between H and N, bond is polar Step 2: Shape of molecule Based on VSEPR theory, ammonia has a trigonal pyramidal shape. Center of positive charge is between hydrogen atoms, and center of negative charge on oxygen. AMMONIA is a POLAR molecule.

61. CCl 4 (carbon tetrachloride) Step 1: Polarity of bonds Based on electronegativity difference between C and Cl, bonds are polar Step 2: Shape of molecule Based on VSEPR theory, CCl 4 has a tetrahedral shape. Center of positive charge is on carbon, and center of negative is also on the carbon. No separation of charge. Carbon tetrachloride is a NON- POLAR molecule.

62. SO 3 (Sulfur trioxide) Step 1: Polarity of bonds Based on electronegativity difference between S and O, bond is polar Step 2: Shape of molecule Based on VSEPR theory, SO 3 is trigonal planar. Center of positive charge is on the sulfur, and center of negative charge is between the oxygen atoms (also on S). SO 3 a NON- POLAR molecule.

63. CH 3 Cl (Chloromethane) Step 1: Polarity of bonds C-H bonds are non-polar, C-Cl bon is polar Step 2: Shape of molecule Based on VSEPR theory, CH 3 Cl is tetrahedral. Cl end of bond is negative, while C end of bond is positive. There is a net separation of charge so molecule is POLAR.

64. QUIZ Which of the following molecules contains polar bonds but is nonpolar? a) H 2 O b) NH 3 c) F 2 d) CCl 4 e) CH 2 Cl 2

65. Answer In CCl 4, there are 4 bonds arranged symmetrically around the central carbon atom. Each bond is polar, you can tell due to the electronegativity differences between carbon and chlorine. Because the 4 atoms of chlorine are arranged symmetrically, the polarity of the bonds cancel each other out, making a non-polar molecule.