General Chemistry

1. General Chemistry M. R. Naimi-Jamal Faculty of Chemistry Iran University of Science & Technology

2. فصل نهم: شکل هندسی‌ مولکول اوربیتال مولکولی

3. The Shapes of Molecules H O H

4. Terminology VSEPR Theory Valence Shell Electron Pair Repulsion theory Electron pairs repel each other whether they are in chemical bonds (bond pairs) or unshared (lone pairs). Electron pairs assume orientations about an atom to minimize repulsions.

5. Molecular Geometry • Electron group geometry – distribution of e- pairs. • Molecular geometry – distribution of nuclei. • Most important factor in determining geometry is relative repulsion between electron pairs.

6. Balloon Analogy

7. No. of e- Pairs Around Central Atom planar trigonal Example Geometry linear 2 F—Be—F 180° F 3 B F F 120° 109° H 4 tetrahedral C H H H

8. Structure Determination by VSEPR Ammonia, NH3 1. Draw electron dot structure 2. Count BP’s and LP’s of the central atom: 4 pairs 3. The 4 electron pairs are at the corners of a tetrahedron.

9. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL.

10. Structure Determination by VSEPR Water, H2O 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. The electron pair geometry is TETRAHEDRAL.

11. Structure Determination by VSEPR Water, H2O The electron pair geometry is TETRAHEDRAL. The molecular geometry is bent.

12. Structure Determination by VSEPR Formaldehyde, CH2O 1. Draw electron dot structure 2. Count BP’s and LP’s = 3 3. There are 3 electron pairs are at the corners of a planar triangle. The electron pair geometry is PLANAR TRIGONAL with 120o bond angles.

13. Structure Determination by VSEPR Formaldehyde, CH2O The electron pair geometry is PLANAR TRIGONAL The molecular geometry is also planar trigonal.

14. Structure Determination by VSEPR Methanol, CH3OH 1. Draw electron dot structure 2. Define bond angles 1 and 2

15. H •• H—C—O—H •• 109° H 109° Structure Determination by VSEPR Methanol, CH3OH Define bond angles 1 and 2 Angle 1 = 109o Angle 2 = 109o In both cases the atom is surrounded by 4 electron pairs.

16. Structure Determination by VSEPR Acetonitrile, CH3CN Draw the electron dot structure

17. H •• H—C—C N 180° 109° H Structure Determination by VSEPR Acetonitrile, CH3CN Define bond angles 1 and 2 Angle 1 = 109o Angle 2 = 180o One C is surrounded by 4 electron “lumps” and the other by 2 “lumps”

18. Phenylalanine, an amino acid

19. Phenylalanine

20. STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE

21. F F F P F F 90° F F F S 90° F F Compounds with 5 or More Pairs Around the Central Atom Trigonal bipyramid 90° 5 electron pairs 120° Octahedron 6 electron pairs F

22. •• •• • • F • •• •• •• • • • • S F F • • •• •• • • • • F • • •• Sulfur Tetrafluoride, SF4 Number of valence electrons = 34 Central atom = S Dot structure Electron pair geometry = ?

23. •• •• • • F • •• •• •• • • • • S F F • • •• •• • • • • F • • •• 90° F F • 120° S • F F Sulfur Tetrafluoride, SF4 Number of valence electrons = 34 Central atom = S Dot structure Electron pair geometry = trigonal bipyramid (because there are 5 pairs around the S)

24. •• •• • • F • •• •• •• • • • • S F F • • •• •• • • • • F • • •• F F • 120° S • F F Sulfur Tetrafluoride, SF4 Lone pair is in the equator because it requires more room. 90°

25. Hybridization of Atomic Orbitals

26. sp3 Hybridization

27. Bonding in Methane

28. sp3 Hybridization in Nitrogen

29. Bonding in Nitrogen

30. sp2 Hybridization in Boron

31. Orbitals in Boron

32. sp Hybridization in Be

33. Orbitals in Beryllium

34. sp3d and sp3d2 Hybridization

35. Hybrid Orbitals and VSEPR • Write a plausible Lewis structure. • Use VSEPR to predict electron geometry. • Select the appropriate hybridization.

36. Multiple Covalent Bonds • Ethylene (H2C=CH2) has a double bond in its Lewis structure. • VSEPR says trigonal planar at carbon.

37. Ethylene

38. Acetylene • Acetylene, C2H2, has a triple bond. • VSEPR says linear at carbon.

39. Applying VSEPR Theory • Draw a plausible Lewis structure. • Determine the number of e- groups and identify them as bond or lone pairs. • Establish the e- group geometry. • Determine the molecular geometry. • Multiple bonds count as one group of electrons. • More than one central atom can be handled individually.

40. Molecular Orbital Theory • Atomic orbitals are isolated on atoms. • Molecular orbitals span two or more atoms. • LCAO: • Linear Combination of Atomic Orbitals. Ψ1 = φ1 + φ2 Ψ2 = φ1 - φ2

41. Combining Atomic Orbitals

42. Molecular Orbitals of Hydrogen

43. Basic Ideas Concerning MOs • Number of MOs = Number of AOs. • Bonding and antibonding MOs formed from AOs. • e-fill the lowest energy MO first. • Pauli exclusion principle is followed. • Hund’s rule is followed

44. #e- in bonding MOs - # e- in antibonding MOs Bond Order = 2 Bond Order • Stable species have more electrons in bonding orbitals than antibonding.

45. BO = (1-0)/2 = ½ BO = (e-bond - e-antibond)/2 H2+ BO = (2-0)/2 = 1 H2 BO = (2-1)/2 = ½ He2+ BO = (2-2)/2 = 0 He2 Diatomic Molecules of the First-Period

46. Molecular Orbitals of the Second Period • First period use only 1s orbitals. • Second period have 2s and 2p orbitals available. • p orbital overlap: • End-on overlap is best – sigma bond (σ). • Side-on overlap is good – pi bond (π).

47. Molecular Orbitals of the Second Period

48. Combining p orbitals

49. Expected MO Diagram of A2

50. MO Diagram of A2 , Z ≥ 8