1. Chapter 9 Bonding and Molecular Structure: Orbital Hybridization and Molecular Orbitals

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3. Chemical Bonding Chapter 9
Advanced Theories of
Chemical Bonding Chapter 9
Atomic Orbitals
Molecules

4. Two Theories of Bonding
MOLECULAR ORBITAL THEORY — Robert Mullikan ( )
valence electrons are delocalized
valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

5. Two Theories of Bonding
VALENCE BOND THEORY — Linus Pauling
valence electrons are localized between atoms (or are lone pairs).
half-filled atomic orbitals overlap to form bonds.
See Figures 9.1 and 9.2.
Linus Pauling,

6. Sigma Bond Formation by Orbital Overlap
Two s orbitals overlap

7. Sigma Bond Formation
Two s orbitals overlap
Two p orbitals overlap

8. Using VB Theory
Bonding in BF3
planar triangle
angle = 120o

9. Bonding in BF3
How to account for 3 bonds 120o apart using a spherical s orbital and p orbitals that are 90o apart?
Pauling said to modify VB approach with ORBITAL HYBRIDIZATION
— mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry.

10. Bonding in BF3 rearrange electrons hydridize orbs. unused p orbital
three sp 2
hybrid orbitals 2p 2s
See Figure 9.8

11. Bonding in BF3
The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp2 hybrids.
Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.

12. Bonding in BF3
An orbital from each F overlaps one of the sp2 hybrids to form a B-F  bond.

13. BF3, Planar Trigonal

14. Bonding in CH4 How do we account for 4 C—H sigma bonds 109o apart?
Need to use 4 atomic orbitals — s, px, py, and pz — to form 4 new hybrid orbitals pointing in the correct direction.

15. Bonding in a Tetrahedron Formation of Hybrid Atomic Orbitals
4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals.
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16. Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals
4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals.
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17. Bonding in CH4, See Figure 9.6

19. Bonding in Glycine

20. Bonding in Glycine

21. Bonding in Glycine

22. Bonding in Glycine

23. Bonding in Glycine

25. Orbital Hybridization See Figure 9.5
BONDS SHAPE HYBRID REMAIN
2 linear sp 2 p’s
3 trigonal sp2 1 p planar
4 tetrahedral sp3 none

26. Multiple Bonds
Consider ethylene, C2H4

27. Sigma Bonds in C2H4
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28. π Bonding in C2H4
The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond. (See Fig. 9.8)

29. π Bonding in C2H4
The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond. (See Fig. 9.10)
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30. Multiple Bonding in C2H4
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31. s and π Bonding in C2H4
See Figure 9.10

32. s and π Bonding in CH2O
See Figure 9.11

33. s and π Bonding in C2H2
See Figure 9.12

34. s and π Bonding in C2H2
See Figure 9.12

35. Consequences of Multiple Bonding
There is restricted rotation around C=C bond.
See Figure 9.13

36. Consequences of Multiple Bonding
Restricted rotation around C=C bond.
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37. Double Bonds and Vision
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See Screen 9.13, Molecular Orbitals and Vision

38. Molecular Orbital Theory
Valence electrons are delocalized
Valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

39. The Paramagnetism of O2
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40. Molecular Orbital Theory
Bonding and antibonding sigma MO’s are formed from 1s orbitals on adjacent orbitals.
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41. Molecular Orbital Theory
1. No. of MO’s = no. of atomic orbitals used.
2. Bonding MO is lower in energy than atomic orbitals. Antibonding MO is higher.
3. Electrons assigned to MO’s of higher and higher energy.
See Figure 9.16

42. Dihelium Molecule Bond order = 1/2 [# e- in bonding MOs
- # e- in antibonding MOs]

43. Sigma Bonding from p Orbitals

44. π Bonding from p Orbitals
Sideways overlap of atomic 2p orbitals that lie in the same direction in space give π bonding and antibonding MOs.

45. s & π Bonding from p Orbitals

46. s & π Bonding from p Orbitals

47. Note change in order of energy of sigma and pi MOs on going from N2 to O2.