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Chapter 13 I: Valence Bond Theory

Chapter 13 I: Valence Bond Theory. Valence Bond Theory. Bonding theories. Hybridization. Bonding Theories. Lewis structures & VSEPR do not explain: -how a bond forms -what orbitals involved in bonding A bonding theory can answer these questions Two bonding theories widely used:

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Chapter 13 I: Valence Bond Theory

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  1. Chapter 13 I: Valence Bond Theory • Valence Bond Theory • Bonding theories • Hybridization

  2. Bonding Theories • Lewis structures & VSEPR do not explain: -how a bond forms -what orbitals involved in bonding • A bonding theory can answer these questions • Two bonding theories widely used: • Valence Bond Theory (VBT) • Molecular Orbital Theory (MOT)

  3. Valence bond theory (VBT) • Major assumptions of VBT: • Bonds form when orbitals on atoms overlap • Two electrons of opposite spin in the orbital overlap region 1s orbital of H atom with 1 e- 1s orbital of H atom with 1 e- 2 e-s in the overlap region- 3

  4. 74 pm Valence bond theory • According to this model  1s orbitals of each atom overlap to form the H-H bond Electrons with opposite spins 4

  5. Valence bond theory (VBT) Problem: Simple overlap of atomic orbitals do not produce the observed shape of molecules

  6. 1s orbitals of H py orbital of C px orbital of C Valence bond theory • According to the e- configuration  C can form 2 bonds Doesn’t work!! Cannot get the shape of CH4 molecule out of this! 6

  7. Valence bond theory • Example – CH4 • C is the central atom • Valence e- configuration of C  2s22px12py1 2pz0 Experiments show 4 equivalent C-H bonds • How can these 4 bonds be obtained? 7

  8. Valence bond theory • How can C get 4 orbitals to overlap with 4 H 1s orbitals &give a tetrahedral shape? • Answer: Consider hybrid orbitals for C • What are hybrid orbitals? • 4 pure atomic orbitals (2s, 2px, 2py, & 2pz) of C are mixed  4 equivalent hybridatomic orbitals are produced 8

  9. Valence bond theory s + px + py + pz  4 equivalent hybrid orbitals  Each is called sp3 hybridorbital 9

  10. Hybridization s + px + py + pz  4 equivalent hybrid orbitals  Each is calledsp3 hybridorbital  VSEPR  EGG for 4 orbitals  tetrahedral 10

  11. + 1 4 s p sp3 Hybridization • Each sp3 hybrid orbital: • is an atomic orbital • has mixed s & p character • has 25% s & 75% p character 11

  12. 2p 2sp3 energy 2s Unhybridized Hybridized Hybridization • To explain why carbon forms four identical single bonds  assume the original orbitals will blend together 12

  13. Hybridization Four sp3 hybrid orbitals of carbon Overlap of sp3 Orbitals of carbon With 1s orbitals of H One sp3 hybrid orbital 13

  14. Hybridization: Ammonia • Example – Ammonia (NH3) • 4 e- groups4 sp3 orbitalstetrahedral EGG • 3 bonding & 1 unshared e- groups • trigonal pyramidal geometry 14

  15. Hybridization: Water 4 e- groups  tetrahedral EGG  4 sp3 orbitals Two lone pairs & two bond pairs 15

  16. sp3 hybrids sbond 1s orbital of H C-C single bonds Example: Ethane (CH3CH3) s bond - formed by an endwise (head-on) overlap 16

  17. Double bond (bond order of 2) • Example: Ethene (CH2CH2) H H C C H H • Each C has 3 e- groups  trigonal planar EGG  Need 3 equivalent orbitals 17

  18. Double bond • To get 3 equivalent hybrid orbitals: s + px + pz  sp2 + sp2 + sp2 • Best EGG for three orbitals  trigonal planar • Each sp2 hybrid orbital overlaps with one H 1s orbital 18

  19. 2p 2p 2sp2 energy 2s Unhybridized Hybridized Double bond • For trigonal planar EGG  three sp2 hybrids • One p orbital remains 19

  20. C C Double bond • What happens to the remaining unhybridized p orbitals on each C? • The two p orbitals overlap side-to- side • Result  formation of a second bond  (pi) bond p orbitals A p bond is a sideways overlap that occurs both above and below the plane of the molecule H H H H 20

  21.  bond formation 21

  22.  &  bonds -bond -bond 22

  23. Bonding in ethene 1s orbital p overlap sp2 hybrids s bond sp2 hybrids p overlap p bond 23

  24. Triple bond (bond order of 3 ) • Example: Ethyne (CHCH) H C C H • Each C has 2 e- groups  linear EGG  Need 2 equivalent orbitals 24

  25. Triple bond • To get 3 equivalent hybrid orbitals: s + px  sp + sp • Best EGG for two orbitals  linear • Each sp hybrid orbital overlaps with one H 1s orbital 25

  26. 2p 2p energy 2sp 2s Unhybridized Hybridized sp hybrid orbital • For linear EGG  need two sp hybrids • Two p orbitals remain unhybridized 26

  27. sp hybrid orbital Now two p orbitals are available to form p bonds. H H 27

  28. Triple bonds (bond order of 3 ) 28

  29. Bonding in ethyne sp hybrid p overlaps 29

  30. Multiple bonds: a summary • -Bonds: electron density lies on the axis between the nuclei • All single bonds are -bonds • -Bonds: electron density lies above & below the plane of the nuclei • A double bond consists of one -bond & one -bond • A triple bond has one -bond & two -bonds • The p-orbitals involved in -bonding come from unhybridized orbitals 30

  31. Summary:  &  bonds 31

  32. Hybrid orbitals: a summary • d orbitals can also be involved in the formation of hybrid orbitals • Hybrid Shape • sp Linear • sp2 Trigonal planar • sp3 Tetrahedral • sp3d Trigonal bipyramidal • sp3d2 Octahedral 32

  33. Hybrid orbitals: a summary sp for linear sp2 for trigonal planar sp3 for tetrahedral sp3d for trigonal bipyramidal sp3d2 for octahedral 33

  34. Hybrid orbitals: a summary s + p s + p + p s + p + p + p 34

  35. Hybrid orbitals: a summary s + p + p + p + d s + p + p + p + d + d 35

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