Molecular Geometry and Bonding Theories

1. Molecular Geometry and Bonding Theories

2. Two Simple Theories of Covalent Bonding • Valence Shell Electron Pair Repulsion Theory • VSEPR • R. J. Gillespie - 1950’s • Valence Bond Theory • Hybridized orbitals • L. Pauling - 1930’s & 40’s

3. Stereochemistry • Study of the 3 dimensional shapes of molecules • TWO MODELS • VSEPR Theory • Valence Bond Theory • Some questions to examine: Why are we interested in shapes? What role does molecular shape play in life? How do we determine molecular shapes? How do we predict molecular shapes?

4. Determining Molecular Structure • Draw the Lewis dot structure identify central atom • Count # of regions of high electron density on central atom • VSEPR tells the geometry around central atom

5. Determining Molecular Structure • Identify lone pair effect on ideal molecular geometry • Repeat procedure for more than one central atom • Determine polarity from entire molecular geometry • electronegativity differences

6. VSEPR Theory • regions of high electron density around the central atom go as far apart as possible to minimize repulsions • five basic shapes • based on # of regions of high electron density • several modifications of these five basic shapes will also be examined

7. VSEPR Theory Two regions of high electron density

8. VSEPR Theory Three regions of high electron density

9. VSEPR Theory Four regions of high electron density

10. VSEPR Theory Five regions of high electron density

11. VSEPR Theory Six regions of high electron density

12. VSEPR Theory • electronic geometry (electron domain) • determined by the locations of regions of high electron density around the central atom(s) • molecular geometry • determined by the arrangement of atoms around the central atom(s) • electron pairs are not used in the molecular geometry determination

13. VSEPR Theory • CH4 - methane • electronic geometry tetrahedral • molecular geometry tetrahedral bond angles = 109.5o

14. VSEPR Theory • H2O - water • electronic geometry tetrahedral • molecular geometry bent or angular bond angle = 104.50

15. VSEPR Theory • lone pairs of electrons (unshared pairs) require more volume than shared pairs • there is an ordering of repulsions of electrons around central atom

16. VSEPR Theory • lone pair to lone pair repulsion is strongest • lone pair to bonding pair repulsion is intermediate • bonding pair to bonding pair repulsion is weakest • mnemonic for repulsion strengths lp/lp > lp/bp > bp/bp • lone pair to lone pair repulsion is why bond angles in water are less than 109.50

17. Valence Bond Theory • covalent bonds are formed by overlap of atomic orbitals • atomic orbitals on the central atom can mix and exchange their character - hybridization • common hybrids pink flowers, mules, corn, grass

18. Valence Bond Theory • hybridizedorbitals describe same shapes as VSEPR • Name of orbital Shape of orbital sp3 tetrahedral sp2 trigonal planar sp linear sp3d trigonal bipyramidal sp3d2 octahedral

19. Hybrid Orbitals • Electronic StructuresLewis Formulas 1s2s2p Be ­¯­¯ 3s3p Cl [Ne] ­¯­¯­¯­

20. Hybrid Orbitals • Dot Formula Electronic Geometry

21. Hybrid Orbitals • VSEPRPolarity

22. Hybrid Orbitals • VSEPRPolarity

23. Hybrid Orbitals • Molecular Geometry same as electronic geometry symmetrical & nonpolar

24. Hybrid Orbitals • Valence Bond Theory (Hybridization) 1s2s2p1ssp hyb2p Be ­¯ ­¯ Þ­¯ ­ ­ 3s3p Cl [Ne] ­¯­¯­¯­

25. Hybrid Orbitals Linear

26. Hybrid Orbitals • examples BF3, BCl3 • all are trigonal planar, nonpolar molecules

27. Hybrid Orbitals • Electronic StructuresLewis Formulas 1s2s2p B ­¯­¯­ 3s3p Cl [Ne] ­¯­¯­¯­

28. Hybrid Orbitals • Dot FormulaElectronic Geometry

29. Hybrid Orbitals • VSEPRPolarity

30. Hybrid Orbitals • Molecular Geometry

31. Hybrid Orbitals • Valence Bond Theory (Hybridization) 1s2s2p1ssp2 hybrid B ­¯ ­¯­ Þ ­¯­ ­ ­ 5s5p Cl [Ne]­¯­¯­¯­

32. Hybrid Orbitals Trigonal Planar

33. Hybrid Orbitals • examples CH4, CF4, CCl4, SiH4, SiF4 • all are tetrahedral, nonpolar molecules • as long as they have the same 4 substituents

34. Hybrid Orbitals • Electronic StructuresLewis Formulas 2s2p C [He] ­¯­ ­

35. Hybrid Orbitals • Electronic StructuresLewis Formulas 2s2p C [He] ­¯­ ­ 1s H ­

36. Hybrid Orbitals • Dot FormulaElectronic Geometry

37. Hybrid Orbitals • VSEPRPolarity

38. Hybrid Orbitals • Molecular Geometry

39. Hybrid Orbitals • Valence Bond 2s2pfour sp3 hybrid orbitals C [He] ­¯­ ­ Þ C [He]­ ­ ­ ­ 1s H ­

40. Hybrid Orbitals Tetrahedron

41. Hybrid Orbitals • Examples PF5, AsF5, PCl5, etc. • All are trigonal bipyramidal, nonpolar molecules.

42. Hybrid Orbitals • Electronic StructuresLewis Formulas 4s4p As [Ar] 3d10­¯­ ­ ­ 2s2p F [He] ­¯­¯­¯­

43. Hybrid Orbitals • Dot FormulaElectronic Geometry

44. Hybrid Orbitals • VSEPRPolarity

45. Hybrid Orbitals • VSEPRPolarity

46. Hybrid Orbitals • Molecular Geometry

47. Hybrid Orbitals • Valence Bond (Hybridization) 4s4p4d As [Ar] 3d10­¯­ ­ ­ ___ ___ ___ ___ ___ ß five sp3 d hybrids ­ ­ ­ ­ ­

48. Hybrid Orbitals Trigonal Bipyramidal Molecules • Valence Bond (Hybridization)

49. Hybrid Orbitals Trigonal Bipyramid Molecules • Valence Bond (Hybridization)

50. Variations of Trigonal Bipyramidal Shape • If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes. • One lone pair - seesaw shape • Two lone pairs - T-shape • Three lone pairs - linear