Two Theories of Bonding

1. Two Theories of Bonding • MOLECULAR ORBITAL THEORY — Robert Mullikan (1896-1986) • valence electrons are delocalized • valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

2. 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 Screen 10.3 and Figures 10.1 and 10.2.

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

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

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

6. 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. (See Screen 10.6)

7. 2p 2s hydridize orbs. rearrange electrons 2 unused p three sp orbital hybrid orbitals Bonding in BF3 See Figure 10.9 and Screen 10.6

8. 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.

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

10. BF3, Planar Trigonal

11. 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.

12. Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals.

13. Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals.

14. Bonding in CH4, Figure 10.6 Figure 10.6

16. Bonding in Glycine

17. Bonding in Glycine

18. Bonding in Glycine

19. Bonding in Glycine

20. Bonding in Glycine

22. Orbital HybridizationFigure 10.5 BONDS SHAPE HYBRID REMAIN 2 linear sp 2 p’s 3 trigonal sp2 1 p planar 4 tetrahedral sp3 none

23. Multiple Bonds Consider ethylene, C2H4

24. Sigma Bonds in C2H4

25. 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. 10.8)

26. 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. 10.10)

27. Multiple Bondingin C2H4

28. sand π Bonding inC2H4 Figure 10.10

29. sand π Bonding inCH2O Figure 10.11

30. sand π Bonding inC2H2 Figure 10.12

31. sand π Bonding inC2H2 Figure 10.12

32. Consequences of Multiple Bonding There is restricted rotation around C=C bond. Figure 10.13

33. Consequences of Multiple Bonding Restricted rotation around C=C bond.

34. Double Bonds and Vision See Screen 10.13, Molecular Orbitals and Vision