1. 32A Lab “Molecular Shapes” Introduction to Lewis Structures –Graphical variation on the “Octet Rule” Molecular Shapes –Electronic versus Atomic –Mutual electrostatic repulsion, VSEPR idea Materials of modern interest –Hydrocarbons, graphene, nanotubes Lab experiment –Chime Tutorial, Cabrillo exercises 1

2. 2 Electron Dot Diagrams G.N. Lewis idea (UC Berkeley) –Elegantly simple idea, but very instructive –Show each bonding electron as a dot As elements brought together, dots merge Most stable configuration is filled shell –2 dots for Hydrogen (2s 2 or [He] configuration) –8 dots for most others (s 2 +p 6, Octet rule) Methane example C(4dot) + 4*H (1dot) –Can have more electron pairs than bonds “lone pairs” are non-bonding electrons Lone pairs occupy a geometrical position –Are part of molecular shape consideration

3. 3 Lewis Structure (electron dot diagram) for ammonia Each of the 3 hydrogen atoms will share its electron with nitrogen to form a bonding pair of electrons (covalent bond) so that each hydrogen atom has a share in 2 valence electrons (electronic configuration of helium) and the nitrogen has a share in 8 valence electrons (electron configuration of neon)

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5. 5 Each oxygen will share 2 of its valence electrons in order to form 2 bonding pairs of electrons (a double covalent bond) so that each oxygen will have a share in 8 valence electrons (electronic configuration of neon). Lewis Structure (electron dot diagram) for the oxygen molecule

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7. 7 Electron Dot Diagrams Lines between atoms are 2-electrons –One line equivalent to 2 dots 2 lines (double bond) equivalent to 4 dots 3 lines (triple bond) equivalent to 6 dots –Can rotate around one line (no interference) 2 lines (double bond) restricts rotation 3 lines (triple bond), no rotation, always planar

8. 8 Each of 4 carbon valence electrons shares a bond with 1 from Chlorine

9. Fluorine most electronegative, Francium the least negative. Pauling’s scale maximum is 4, also indicated by column height

10. 10 Arrow Convention Head of arrow points to negative end(s) of molecule

11. 3 representations for water

12. 12 What about Shapes? Paintings versus Sculptures Lewis Diagrams are 2-D on paper –Do not provide shapes in nature Need 3-D models to handle geometry –Molecular shapes define behavior Right hand and left hand molecules in biology “Hydrogen Bonding” a key attribute of water –Relies on angles and lone pair electrons Macro-molecules, chains, gels, drugs

13. 13 2D versus 3D in Art Soyer painting versus Rodin Sculpture … which is more realistic?

14. 14 Escher illusions 3-D exploration in 2-D space

15. 15 Escher 2-D illustration, executed in 3-D with Legos

16. 16 2-D illusion and 3-D reality How can we apply this to Chemistry?

17. 17 How to represent 3D molecules? Pictures and physical models –Drawings of “ball & stick” or “balloon” models –3-D constructions using tinker-toys, legos, etc. Other forms of analogies and models –Computer simulations –Objects from other fields –Mathematical shapes

18. 18 Another 3-D representation in 2D Distortion inevitable, like Mercator Projection of globe

19. Mathematical Shapes Geometry provides a number of convenient shapes we can use Many molecules are examples from mathematical ideas There are 5 “Platonic Solids”, we will consider many of these as molecules

20. 20 Tetrahedron A regular tetrahedron is composed of four triangular faces, three of which meet at each vertex. This example with = four "equilateral triangles, " is one of the 5 Platonic solids.vertex Platonic solids

21. 21 Shape of Ammonium is Tetrahedral 4 hydrogen ions repel each other, most equidistant arrangement is Tetrahedral, 109 degree angles Same geometry applies for Methane, CH 4

22. 22 Electronic vs Mechanical shapes Electronic configuration define spatial positions –Electrons avoid each other, max. separation –Common electronic shapes Linear, planar, tetrahedral, pyramidal, octahedral Mechanical (molecular), is where atoms are –Not all electronic positions have atoms –Lone pairs define shape, but no atom there –Common molecular shapes Linear, planar, tetrahedral, pyramidal, octahedral missing atoms  bent (water), see-saw, square pyramid

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24. 24 VSEPR is a useful model (Valence Shell Electron-Pair Repulsion) Like-charge atoms push each other away –Repelling atoms try for maximum separation Max. separation varies by number of items 2 items repelling => 2-D linear shape, 180 o 3 items repelling => 2-D trigonal shape, 120 o 4 items repelling => 3-D tetrahedral shape, 109 o 5 items repelling => trigonal bi-pyramid, 120 o & 90 o 6 items repelling => 3-D octahedral shape, 90 o (same as “square bi-pyramid”)

25. 25 Electron Avoidance Like Charges repel (electrons are all negative) … so electrons put maximum distance between each other

26. 26 VSEPR Model “Valence Shell Electron Pair Repulsion” = VSEPR –Basic idea is that electrons within orbitals repel each other –Orbital repulsion gives rise to specific directions for bonding –Specific bonding directions lead to 3-D shapes of molecules 2-Cloud case –Repulsion forces in-line orientation, 180 degree separation –Minimum influence from most distant arrangement –Examples include O=C=O, H-C≡N 3-Cloud case –Maximum separation achieved in planar arrangement –Approximately 120 degrees between atoms, depends on charge –Example includes formaldehyde H 2 -C=O

27. 27 Balloon Illustration 2 objects must have linear (1-D) arrangement 3 objects must have triangular (2-D) arrangement 4 object often have Tetrahedral (3-D) arrangement

28. 28 3-cloud case

29. 29 Triangular (planar) Symmetry 3 single bonds equivalent (in shape) to a mix of 2 single + 1 double

30. 30 VSEPR Models 4-Cloud case –Repulsion forces maximum separation into tetrahedron –Minimum influence between most distant electrons –Angle between any 2 is 109.5 degrees –Many examples, such as Methane CH 4 –Lone pairs help define shape, but do not involve other atoms

31. 31 4-cloud case

32. 32 Methane, a 4-cloud case

33. 33 VSEPR Model 5-Cloud case –Maximum separation achieved in 5 directions –3 direction outward in a plane, plus above and below Called a “Trigonal Bi-pyramid” shape –Most common with elements having 5 valence electrons PCl 5 is a typical example

34. 34 An example of trigonal bi-pyramid molecular geometry that results from five electron pair geometry is PCl 5. The phosphorus has 5 valence electrons and thus needs 3 more electrons to complete its octet. However this is an example where five chlorine atoms are present and the octet is expanded to 10. The Lewis diagram is as follows: Cl = 7 e- x 5 = 35 e- P = 5 e- = 5 e- Total = 40 e- The Chlorine atoms are as far apart as possible at nearly 90 o and 120 o bond angle. This is trigonal bi-pyramid

35. 35 VSEPR Model 6-Cloud case –Maximum separation in 6 directions is an Octahedron 4 clouds in a plane (as a square), plus above and below 90 degree angle between any two faces or cloud directions –Another very common configuration Uranium hexaflouride for isotope separation by gas diffusion

36. 36 6-Cloud Case Octahedral shape

37. 37 Octahedron A regular octahedron is a Platonic solid composed of eight equilateral triangles as faces, four of which meet at each vertex.Platonic solidequilateral trianglesvertex The regular octahedron is a special kind of triangular antiprism and of square bipyramid. The regular octahedron has 6 vertices and 12 edges antiprismbipyramid

38. 38 Uranium Hexafluoride a gas used for separating isotopes via diffusion example of octahedral shape, 6 clouds

39. 39 6 Cloud case (less one) example is Bromine pentafluoride Square Pyramidal has 6 lobes, but one is non-bonding and unseen in BrF 5

40. 40 Can have word combinations … such as “trigonal planar” Trigonal around each carbon Double bond denies rotation, forces planar shape

41. 41 Summary on Shapes Electronic Structures & Shapes –Dominated by electron behavior –Explains how repulsion dictates geometry –“Lone Pairs” contribute to shapes, geometry Molecular Structures –Shapes from “Real Atoms”, not electrons –What a atom-sized person would see –Shapes with missing parts You see the atoms in a molecule You can’t observe electrons

42. 42 Summary on Shapes Electronic Structure of water is Tetrahedral –Oxygen has 4 electron “arms” as SP 3 bonds –Two arms attached to hydrogen + 2 lone pairs Molecular Structure of water is Bent –2 hydrogens engage two arms of tetrahedron –2 lone pairs occupy other 2 arms … felt but unseen

43. 43 Organic Molecules Organic ≡ Carbon based –Most numerous class of materials –Countless combinations are possible –Carbon bonds with almost any other element Carbon to carbon bonds very common –Gases (acetylene, propane) –Linear chain liquids (gasoline, oil) –Ring structures (benzene, cyclopentane) –Crystalline structures (diamond, graphite)

44. 44 Basic shapes for Organics Carbon is electronically tetrahedral –Implication is NOT a straight line formation Real hydrocarbons zig-zag at 109 o Ring structures are common –Benzene, cyclohexane Recently discovered new shapes –Buckeyballs (found in soot) –Graphene sheets (in pencil lead) –Carbon Nanotubes, fibers

45. 45 We draw straight lines for convenience, it’s easier Real molecules hve tetrahedral carbon, zig-zag shaped Fig. 13-12, p. 382

46. 46 Hydrocarbons Alkane series, linear progression of carbon –Methane = 1 carbon (natural gas) –Ethane = 2 carbon (ripening fruit) –Propane = 3 carbon chain (gas BBQ) –Butane = 4 carbon chain (cigarette lighters) –Pentane = 5 carbon chain –Hexane = 6 carbon chain (Naptha) –Heptane = 7 carbon chain –Octane = 8 carbon chain (gasoline reference)

47. 47 Octane and Iso-Octane Zig-Zag shape from tetrahedral bonding

48. “Buckeyballs” named for Buckminster Fuller 60 carbons in sockerball shape, “C60” a closed surface hexagon & pentagon mix 48

49. Graphene, a 2-D array of carbon excellent electrical conductor, very strong 49

50. Carbon Nanotubes, graphene rolled up like “chicken wire”, some are semiconductors 50

51. 51 Computer Modeling CABRILLO COLLEGE has an interactive molecule display, allowing users to twist and turn common molecules to investigate their structures. Almost like a video game, but more educational than killing aliens. We will use this in lab class, lots of options to investigate, plus some answers http://c4.cabrillo.edu/chem30a/exercises/Exer_1/index.html http://c4.cabrillo.edu/chem30b/exercises/chpt_11_060700/index.html

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53. 53 Cabrillo website interactive exercises, part 1 is on Lewis Structures, draw them out Answers to selected items also provided

54. 54 Section 2, shapes of ions, You draw Lewis dot structures & name the shape

55. 55 Section 3, Polar & Non-Polar materials You will identify which is which

56. 56 Section 4: show structure, charge distribution, and if polar

57. 57 Section 5, naming compounds

58. 58 Section 6, naming ions

59. 59 Section 7, involves nomenclature combine anion in picture with cations in the list

60. 60 Section 8 (last one) Match the ion with the name and give its charge

61. Computer Lab reserved for us Do the Chime Tutorial first –Use Internet Explorer to load it Google Chrome, other browsers do NOT work –Answer questions in lab supplement –Can finish this at home … but with extra effort Requires JAVA program, free download Requires Chime program, copy available here Do Cabrillo College exercises after tutorial –Start each section, ask questions –Can finish at home, need only JAVA –See Jaguar posting for details

62. More instructions are on Jaguar 62