Molecular Structure & Covalent Bonding Theories

1. Chapter 8 Molecular Structure & Covalent 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. VSEPR Theory • regions of high electron density around the central atom are 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

4. VSEPR Theory • Two regions of high electron density

5. VSEPR Theory • Three regions of high electron density

6. VSEPR Theory • Four regions of high electron density

7. VSEPR Theory • Five regions of high electron density

8. VSEPR Theory • Six regions of high electron density

9. VSEPR Theory • electronic geometry– the arrangement of the valence shell electrons around the central atom • 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) • What the molecule really looks like electron pairs are not used in the molecular geometry determination

10. VSEPR Theory • CH4 vs. H2O • CH4- methane • electronic geometry tetrahedral • molecular geometry tetrahedral bond angles = 109.5o

11. 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 • H2O - water • electronic geometry Tetrahedral • molecular geometry bent or angular bond angle = 104.50

12. VSEPR Theory 1. lone pair to lone pair repulsion is strongest 2. lone pair to bonding pair repulsion is intermediate 3. bonding pair to bonding pair repulsion is weakest • lone pair to lone pair repulsion is why bond angles in water are less than 109.50

13. 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

14. Hybrid orbitals helps describe the same shapes as VSEPR – (hybridization – mixing of orbitals) sp - 2 sp2 - 3 sp3 - 4 sp3d - 5 sp3d2 - 6 s + p orbitals s + p + p orbitals s + p + p + p orbitals  s + p + p + p + d orbitals  s + p + p + p + d + d orbitals  From orbital diagrams Hybridization - CP

15. sp - 2 sp2 - 3 sp3 - 4 sp3d - 5 sp3d2 - 6 • Linear • Trigonal planar • Tetrahedral • Trigonal bipyramidal • Octahedral Name of orbital – number of pairs on central atom (Regions of high e- density around the central atom) Shape (name) of orbital

16. Molecular Geometry Terminology • In the next sections the following terminology will be used A = central atom B = bonding pairs around central atom U = lone pairs around central atom For example: AB3U designates that there are 3 bonding pairs and 1 lone pair around the central atom (4 central pairs total)

17. AB2 Molecules - No Lone Pairs on A - Linear Molecules • Ex. BeCl2, BeBr2,BeI2, HgCl2, CdCl2 ~ all are linear, nonpolar molecules • Dot FormulaElectronic Geometry

18. AB2 Molecules - No Lone Pairs on A - Linear Molecules • VSEPRPolarity

19. AB2 Molecules - No Lone Pairs on A - Linear Molecules Molecular Geometry same as electronic geometry symmetrical & nonpolar

20. AB3 Molecules - No Lone Pairs on A - Trigonal Planar Molecules • Examples: BF3, BCl3 • all are trigonal planar, nonpolar molecules • Dot FormulaElectronic Geometry

21. AB3 Molecules - No Lone Pairs on A - Trigonal Planar Molecules • VSEPRPolarity

22. AB3 Molecules - No Lone Pairs on A - Trigonal Planar Molecules Molecular Geometry

23. AB4 Molecules - No Lone Pairs on A - Tetrahedral Molecules • Ex. CH4, CF4, CCl4, SiH4, SiF4 • all are tetrahedral, nonpolar molecules ~ as long as they have the same 4 substituents • Dot FormulaElectronic Geometry

24. AB4 Molecules - No Lone Pairs on A - Tetrahedral Molecules • VSEPRPolarity

25. AB4 Molecules - No Lone Pairs on A - Tetrahedral Molecules Molecular Geometry

26. AB3U Molecules - One Lone Pair - Pyramidal Molecules • examples NH3,NF3 • first example of lone pairs on the central atom • electronic and molecular geometry are different • all 3 substituents the same but molecule is polar • NH3 andNF3 are pyramidal, polar molecules

27. AB3U Molecules - One Lone Pair - Pyramidal Molecules • Dot FormulasElectronic Geometry Molecular Geometry

28. AB3U Molecules - One Lone Pair - Pyramidal Molecules • VSEPRPolarity

29. AB3U Molecules - One Lone Pair - Pyramidal Molecules • VSEPRPolarity

30. AB2U2 - Two Lone Pairs - V-Shaped Molecules • Example H2O • water is an angular, bent or V-shaped, polar molecule • Dot FormulaElectronic Geometry Molecular Geometry

31. AB2U2 - Two Lone Pairs - V-Shaped Molecules • VSEPRPolarity

32. ABU3 - Three Lone Pairs - Linear Molecules • Hydrogen halides - HF, HCl, HBr, HI • Dot Formula Electronic Geometry

33. ABU3 - Three Lone Pairs - Linear Molecules • VSEPR Polarity HF is a polar molecule. Molecular Geometry

34. AB5- No Lone Pairs - TrigonalBipyramidal Molecules • Ex. PF5, AsF5, PCl5, etc. All are trigonalbipyramidal, nonpolar molecules. • Dot FormulaElectronic Geometry

35. AB5- No Lone Pairs - Trigonal Bipyramidal Molecules • VSEPRPolarity

36. AB5- No Lone Pairs - Trigonal Bipyramidal Molecules • PolarityMolecular Geometry

37. AB5- No Lone Pairs - Trigonal Bipyramidal Molecules • Valence Bond (Hybridization)

38. AB4U- One Lone Pair - Seesaw Molecules • For one lone pair an AB4U molecule results. • AB4U molecules have a seesaw shaped molecular geometry and are polar. SF4is anAB4U molecule lone pair occupies an equatorial position

39. AB4U- One Lone Pair - Seesaw Molecules • VSEPRMolecular Geometry

40. AB3U2 - Two Lone Pairs - T-shaped Molecules • For two lone pairs an AB3U2 molecule results • AB3U2 molecules have a T-shaped molecular geometry and are polar IF3is anAB3U2molecule two lone pairs occupy equatorial positions

41. AB3U2 - Two Lone Pairs - T-shaped Molecules • VSEPR Molecular Geometry

42. AB2U3 - Three Lone Pairs - Linear Molecules • For three lone pairs an AB2U3 molecule results • AB2U3 molecules have a linear molecular geometry and are nonpolar XeF2is anAB2U3molecule three lone pairs occupy equatorial positions

43. AB2U3 - Three Lone Pairs - Linear Molecules • VSEPR Molecular Geometry

44. AB6- No Lone Pairs - Octahedral Molecules • Ex. SF6, SeF6, SCl6, etc. • These are octahedral and nonpolarmolecules if all 6 substituents are the same • Dot FormulaElectronic Geometry Molecular Geometry

45. AB6- No Lone Pairs - Octahedral Molecules • VSEPRPolarity

46. AB6- No Lone Pairs - Octahedral Molecules • Valence Bond (Hybridization)

47. AB5U- One Lone Pair - Square Pyramidal Molecules • For one lone pair an AB5U molecule results. • AB5U molecules have a square pyramidal molecular geometry and are polar. IF5is anAB5U molecule lone pair occupies an axial position

48. AB5U- One Lone Pair - Square Pyramidal Molecules • VSEPR Molecular Geometry

49. AB4U2 - Two Lone Pairs - Square Planar Molecules • For two lone pairs an AB4U2 molecule results. • AB4U2 molecules have a square planar molecular geometry and are nonpolar. XeF4is anAB4U2 molecule lone pairs occupy axial positions

50. AB5U- One Lone Pair - Square Pyramidal Molecules • VSEPRPolarity