Molecular Geometry and Bonding Theories

1. Molecular Geometry and Bonding Theories Chemistry: The Central Science Chapter 9 Created by Ray Guest and Stacey Dobrosky Cool

2. Bond Length and Strength • All bonds are not created equal • Which is the strongest bond: C – C, C = C, or CC ? From Ch 8.8 in Chemistry: The Central Science

3. Bond Length and Strength From Ch 8.8 in Chemistry: The Central Science

4. Molecular Shapes

5. Molecular Shapes • The shape for 109.5º is called tetrahedral, with carbon being the central atom

6. How do we determine the shapes of molecules? • We assume that the valence electrons are going to repel one another

7. Molecular Shapes • This process of repulsion we call Valence Shell Electron Repulsion Theory or VSEPR for short

8. Molecular Shapes Bond Angles?

10. How exactly do we determine which shape? • We distinguish between lone pairs (non-bonding electrons) and bonding pairs of electrons

11. VSEPR Model • Electron domain geometry is determined by ALL electron pairs (both bonding and nonbonding) Demo

12. VSEPR Model • Electron pair geometry is not the same as your molecular geometry (molecule shape) • What’s confusing is they use the same names

13. VSEPR Model

14. VSEPR Model – So how do you get the shapes? • Draw the Lewis structure • Determine the electron-domain geometry by arranging the electron minimizing repulsion • Ignore the non-bonded electrons to determine the molecular shape Page 369 & 375 in the Chang book

15. VSEPR Model Try some – first do the Lewis dot structure, then give the electron geometry and finally, give the molecular geometry and polarity

16. Practice What is the electron domain geometry for AsH3? • Bent • Tetrahedral • Trigonal pyramidal • Trigonal planar

17. Practice What is the molecular geometry for AsH3? • Bent • Tetrahedral • Trigonal pyramidal • Trigonal planar

18. Practice Is AsH3 polar or nonpolar? • Polar • Nonpolar

19. Practice What is the electron domain geometry for H2O? • Bent • Tetrahedral • Trigonal pyramidal • Trigonal planar

20. Practice What is the molecular geometry for H2O? • Bent • Tetrahedral • Trigonal pyramidal • Trigonal planar

21. Practice Is H2O polar or nonpolar? • Polar • Nonpolar

22. Practice What is the electron domain geometry for CO2? • Bent • Tetrahedral • Linear • Trigonal planar

23. Practice What is the molecular geometry for CO2? • Bent • Tetrahedral • Linear • Trigonal planar

24. Practice Is CO2 polar or nonpolar? • Polar • Nonpolar

25. Hybridization • We blend the s and p orbitals of the valence electrons and end up with the proper geometry. • We combine the s orbital and p orbitals to get sphybridization.

28. sp Hybridization • Allows for Linear geometry (180o) • Combines one s and one p orbital

29. Hybrid Orbitals • Consider beryllium: • In its ground electronic state, it would not be able to form bonds.

30. Hybrid Orbitals But if it absorbs the small amount of energy it can form two bonds.

31. Hybrid Orbitals • With hybrid orbitals the orbital diagram for beryllium would look like this. • The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.

32. 2p sp Hybridization In terms of energy 2p Energy 2s

33. Breaking the octet • PCl5 • The model predicts that we must use the d orbitals. • sp3d hybridization

34. Including the d orbital • Orbitals create the geometric shapes: • trigonal bipyramid (sp3d) • octahedral (sp3d2). • Mix s, p, and d orbitals in the hybridization.

35. In Short… • To figure out hybridization, you count all of the things attached to the central atom (of atom of interest) including the lone pairs • sp2 – 3 things • sp3 – 4 things • sp3d – • Note: The hybridization matches the electron domain geometry

36. Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom.

37. Hybridization of P in PF5

38. Hybridization of S in SF6

40. Square Planar

41. Valence Bond Theory • Hybridization is a major player in this approach to bonding. • There are two ways orbitals can overlap to form bonds between atoms.

42. Sigma () Bonds • Sigma bonds are characterized by • Head-to-head overlap. • Cylindrical symmetry of electron density about the internuclear axis.

43. Pi () Bonds • Pi bonds are characterized by • Side-to-side overlap. • Electron density above and below the internuclear axis.

44. Single Bonds Single bonds are always  bonds, because  overlap is greater, resulting in a stronger bond and more energy lowering.

45. Multiple Bonds In a multiple bond one of the bonds is a  bond and the rest are  bonds.

46. Multiple Bonds • In a molecule like formaldehyde (shown at left) an sp2 orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen.

47. Multiple Bonds • The unhybridized p orbitals overlap in  fashion.

48. Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a  bond, and two pairs of p orbitals overlap in  fashion to form the two  bonds.

49. Ethylene:C2H4 H H • C has two s bonds and one p C C H H

50. Acetylene:C2H2 • C can make two s and two p H C C H