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Bonding Theories: Valence Bond Theory Molecular Orbital Theory

Bonding Theories: Valence Bond Theory Molecular Orbital Theory. Lewis Theory: Electron Groups and Molecular Shapes. There are Regions of electrons in an atom: Some from placing shared pairs of valence electrons between bonding nuclei;

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Bonding Theories: Valence Bond Theory Molecular Orbital Theory

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  1. Bonding Theories:Valence Bond TheoryMolecular Orbital Theory

  2. Lewis Theory: Electron Groups and Molecular Shapes There are Regions of electrons in an atom: • Some from placing shared pairs of valence electrons between bonding nuclei; • Other from placing lone pairs valence electrons on a single nuclei Regions of electron groups should repel each other (VSEPR) and determines the molecular shape and molecular polarity Tro: Chemistry: A Molecular Approach, 2/e

  3. Tro: Chemistry: A Molecular Approach, 2/e

  4. Problems with Lewis Theory • Useful predicting trends, but not quantitative e.g. bond angles, bond strength, bond length • Can not give one correct structure for many molecules where resonance is important • Often does not predict the correct magnetic behavior of molecules • e.g. O2 is paramagnetic, though the Lewis structure predicts it is diamagnetic Tro: Chemistry: A Molecular Approach, 2/e

  5. Valence Bond Theory. I Linus Pauling et al: • Chemical bonds form when the orbitals (wave functions of electrons in the atoms) on those atoms interact (overlap) • The kind of interaction depends on whether the orbitals align along the axis between the nuclei, or outside the axis Tro: Chemistry: A Molecular Approach, 2/e

  6. Orbital Interaction As two atoms approach, the half-filledvalence atomic orbitals on each atom would interact and become more stable: covalent bonds Covalent bonds are regions of high probability of finding the shared electrons in the molecule Tro: Chemistry: A Molecular Approach, 2/e

  7. Orbital interaction Stabilizes the molecule • Forming covalent bond would stabilize the molecule because they would contain paired electrons shared by both atoms • Attraction between the shared electrons and the nuclei > Repulsion between the nuclei Tro: Chemistry: A Molecular Approach, 2/e

  8. H ↑↓ H─S bond ↑ 1s ↑↓ ↑ ↑ ↑↓ S 3s 3p ↑ 1s ↑↓ H─S bond H Atomic Orbital in the Bonding of H2S + Predicts bond angle = 90° Actual bond angle = 92° Tro: Chemistry: A Molecular Approach, 2/e

  9. Valence Bond Theory I – Problem Many molecules such as methane (tetrahedral, bond angle = 109.5°) can not be explained by half-filled atomic orbitals • C = 2s22px12py12pz0 would predict two or three bonds that are 90° apart Tro: Chemistry: A Molecular Approach, 2/e

  10. Valence Bond Theory II: Orbital Hybridization • When forming covalent bonds, each valence electron can reside in either standard s, p, d, and f orbitals, or inhybridized orbitals of these atomic orbitals. • Covalent bond forms via half-filled atomic orbitals interacting, ONE pair of electrons (↑↓) in the new bonding orbital • Shape of the molecule determined by the geometry of the interacting orbitals Tro: Chemistry: A Molecular Approach, 2/e

  11. Hybridization + = • Some atoms hybridize their orbitals to maximize bonding • Hybridization: mixing different types of orbitals in the valence shell to make a new set of degenerate (equal energy) orbitals • 1 x s orbital + 2 x p orbital = 3 x sp2 hybrid orbital • Same type of atom can have different types of hybridization • C = sp, sp2, sp3 Tro: Chemistry: A Molecular Approach, 2/e

  12. Hybrid Orbitals • The number of standard atomic orbitals combined = the number of hybrid orbitals formed • combining a 2s with a 2p gives 2 x sp hybrid orbitals • H cannot hybridize!! • its valence shell only has one orbital • The number and type of standard atomic orbitals combined determines the shape of the hybrid orbitals

  13. Orbital Diagram of sp3 Hybridization in C

  14. Orientation of sp3 Hybridized Orbitals Tro: Chemistry: A Molecular Approach, 2/e

  15. Methane: Hydrogen bonded with sp3-hybridized Carbon Tro: Chemistry: A Molecular Approach, 2/e

  16. Various Carbon Hybridizations Unhybridized    2p 2s sp hybridized     2sp 2p sp2 hybridized     2sp2 2p sp3 hybridized     2sp3 Tro: Chemistry: A Molecular Approach, 2/e

  17. sp3 Hybridization • Atom with _____ electron groups around it • tetrahedral geometry • 109.5° angles between hybrid orbitals • Atom uses hybrid orbitals for all bonds and lone pairs Tro: Chemistry: A Molecular Approach, 2/e

  18.  sp3 Hybridized AtomsOrbital Diagrams • Place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy • Lone pairs generally occupy hybrid orbitals Unhybridized atom sp3 hybridized atom C        2p 2sp3 2s N        2p 2sp3 2s Tro: Chemistry: A Molecular Approach, 2/e

  19. Bonding with Valence Bond Theory According to valence bond theory, bonding takes place between atoms when their atomic or hybrid orbitals interact (“overlap”). To interact, • The orbitals must either be aligned along the axis between the atoms. or • The orbitals must be parallel to each other and perpendicular to the interatomic axis Tro: Chemistry: A Molecular Approach, 2/e

  20. Ammonia Formation with sp3 N Tro: Chemistry: A Molecular Approach, 2/e

  21. Sigma (s) bond • The interacting atomic orbitals point along the axis connecting the two bonding nuclei • either standard atomic orbitals or hybrids • s–to–s, p–to–p, hybrid–to–hybrid, s–to–hybrid, etc. Tro: Chemistry: A Molecular Approach, 2/e

  22. pi (p) bond • Definition: The bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei • between unhybridized parallel p orbitals • Double bond = one s bond + one p bond • Triple bond = one s bond + two p bonds • Strength of orbital overlap: s bonds > p bonds Tro: Chemistry: A Molecular Approach, 2/e

  23. sp2 • Atom with _____ electron groups around it • trigonal planar system • C = trigonal planar • N = trigonal bent • O = “linear” • 120° bond angles • flat • Atom uses hybrid orbitals for s bonds and lone pairs, uses nonhybridized p orbital for p bond Tro: Chemistry: A Molecular Approach, 2/e

  24.  sp2 Hybridized AtomsOrbital Diagrams Unhybridized atom sp2 hybridized atom C 3 s 1 p        2p 2sp2 2p 2s N 2 s 1 p        2p 2sp2 2p 2s Tro: Chemistry: A Molecular Approach, 2/e

  25. Tro: Chemistry: A Molecular Approach, 2/e

  26. Formaldehyde, CH2O Orbital Diagram p   p C p O s sp2 C sp2 O       s s   1s H 1s H Tro: Chemistry: A Molecular Approach, 2/e

  27. Hybrid orbitals overlap to form a s bond. Unhybridized p orbitals overlap to form a p bond. Tro: Chemistry: A Molecular Approach, 2/e

  28. H ･ ･ C N H H CH2NH Orbital Diagram p   p C p N s sp2 C sp2 N       s s s    1s H 1s H 1s H Tro: Chemistry: A Molecular Approach, 2/e

  29. Does Bond Rotation Affect Bonding? • s bond forms along the internuclear axis, rotation around that bond does not affect the interaction between the orbitals. • p bond interacts above and below the internuclear axis, so rotation around the axis requires the breaking of the interaction between the orbitals Tro: Chemistry: A Molecular Approach, 2/e

  30. Bond Rotation: s bond vs. p bond Tro: Chemistry: A Molecular Approach, 2/e

  31. Rigidity of double bonds leads to TWO different compounds Different Polarity (dipole moment), Density, melting points, boilding points: Tro: Chemistry: A Molecular Approach, 2/e

  32. p s p sp hybridization • Atom with _____ electron groups • linear shape • 180° bond angle • Atom uses hybrid orbitals for s bonds or lone pairs, uses nonhybridized p orbitals for p bonds Tro: Chemistry: A Molecular Approach, 2/e

  33. Tro: Chemistry: A Molecular Approach, 2/e

  34. Tro: Chemistry: A Molecular Approach, 2/e

  35. HCN Orbital Diagram 2p     p C p N s sp C sp N     s  1s H Tro: Chemistry: A Molecular Approach, 2/e

  36.  sp Hybridized AtomsOrbital Diagrams Unhybridized atom sp hybridized atom C 2s 2p        2p 2sp 2p 2s N 1s 2p        2p 2sp 2p 2s Tro: Chemistry: A Molecular Approach, 2/e

  37. sp3d • Atom with five electron groups • trigonal bipyramid electron geometry • Seesaw, T–Shape, Linear • 120° & 90° bond angles • Use empty d orbitals from valence shell • d orbitals can be used to make p bonds Tro: Chemistry: A Molecular Approach, 2/e

  38. Tro: Chemistry: A Molecular Approach, 2/e

  39. sp3d Hybridized AtomsOrbital Diagrams Unhybridized atom sp3d hybridized atom     P      3p 3sp3d 3s 3d     S      3p 3s 3sp3d 3d (non-hybridizing dorbitals not shown) Tro: Chemistry: A Molecular Approach, 2/e

  40. SOF4 Orbital Diagram p   d S p O s sp2 O sp3d S         s s s s     2p F 2p F 2p F 2p F Tro: Chemistry: A Molecular Approach, 2/e

  41. sp3d2 • Atom with six electron groups around it • octahedral electron geometry • Square Pyramid, Square Planar • 90° bond angles • Use empty d orbitals from valence shell to form hybrid • d orbitals can be used to make p bonds Tro: Chemistry: A Molecular Approach, 2/e

  42. Tro: Chemistry: A Molecular Approach, 2/e

  43. ↑↓ ↑↓ ↑↓ ↑ 5p 5s 5d sp3d2 Hybridized AtomsOrbital Diagrams Unhybridized atom sp3d2 hybridized atom ↑↓ ↑↓ ↑ ↑ S ↑ ↑ ↑ ↑ ↑ ↑ 3p 3s 3d 3sp3d2 I ↑↓ ↑ ↑ ↑ ↑ ↑ 5sp3d2 (non-hybridizing d orbitals not shown) Tro: Chemistry: A Molecular Approach, 2/e

  44. Tro: Chemistry: A Molecular Approach, 2/e

  45. Predicting Hybridization and Bonding Scheme 1. Start by drawing the Lewis structure 2. Use VSEPR Theory to predict the electron group geometry around each central atom 3. Select the hybridization scheme based on the electron group geometry 4. Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals 5. Label the bonds as s or p Tro: Chemistry: A Molecular Approach, 2/e

  46. Example: Predict the hybridization and bonding scheme for CH3CHO Tro: Chemistry: A Molecular Approach, 2/e

  47. Predict the hybridization and bonding scheme for CH3CHO Tro: Chemistry: A Molecular Approach, 2/e

  48. Predict the hybridization and bonding scheme for CH3CHO Tro: Chemistry: A Molecular Approach, 2/e

  49. Practice – Predict the hybridization of all the atoms in H3BO3 H = can’t hybridize B = 3 electron groups = sp2 O = 4 electron groups = sp3 Tro: Chemistry: A Molecular Approach, 2/e

  50. • • Practice – Predict the hybridization and bonding scheme of all the atoms in NClO • • • • • • O N C l • • • • s:Osp2─Nsp2 ↑↓ ↑↓ ↑↓ N = 3 electron groups = sp2 O = 3 electron groups = sp2 Cl = 4 electron groups = sp3 ↑↓ s:Nsp2─Clp ↑↓ ↑↓ p:Op─Np Tro: Chemistry: A Molecular Approach, 2/e

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