1. Chemical Bonding

2. Chemical bond: attractive force holding two or more atoms together. Covalent bond results from sharing electrons between the atoms. Usually found between nonmetals. Ionic bond results from the transfer of electrons from a metal to a nonmetal. Metallic bond: attractive force holding pure metals together. Chemical Bonds, Lewis Symbols, and the Octet Rule

3. Figure 8.3: Ionic Bonding

4. Figure 8.5: Covalent Bonding

5. Chemical Bonds Bond Type Single Double Triple # of e’s 2 4 6 Notation — =  Bond order 1 2 3 Bond strength Increases from Single to Triple Bond lengthDecreases from Single to Triple

6. Strengths of Covalent Bonds

7. Bonding Atoms  Why do atoms bond? - each atom wants a full outermost energy level - gain, lose, and share valence electrons to achieve the duet or octet rule aka: “being happy” - gives each atom an electron configuration similar to that of a noble gas ex. Group 18: He, Ne, Ar

8. Chemical Structure/Models  Chemical Structure/Molecular Models - arrangement of bonded atoms or ions - bond length: the average distance between the nuclei of two bonded atoms - bond angles: the angle formed by two bonds to the same atom

9. 9 Multiple bonding in Lewis structures Two atoms can share one pair of electrons (single bond) 2 pairs of electrons (double bond) or 3 pairs of electrons (triplet bonds) C 2 H 6 (ethane) C 2 H 4 (ethylene) C 2 H 2 (acetylene) Rule: More bonds, shorter bonds: triple bond shorter than double bond; double bond shorter than single bond.

10. Multiple Bonds Double and triple bonds can form between atoms in order to fill the outer energy level This occurs when two atoms share more than one pair of electrons

11. Multiple Lewis Structures Some molecules can have more than one possible Lewis structure, usually when one single bond and one double bond can be exchanged within the rules of drawing Lewis structures Example of SO 2 (g)

12. 12 Resonance structures: two or more Lewis electronic structures for the same atomic composition Example: ozone: atomic composition = O 3 Atomic constitution: Electronic constitution: Neither Lewis structure is an accurate representation of the actual molecule: an equal mixture of the two structures is required: Both O-O bonds of O 3 are equal in length.

13. 13 Molecular Structure Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space. Physical and chemical properties depend on the geometry of a molecule.

14. 14 Molecular Structures 3-D Model 3-D Drawing

15. 15 Does it matter? The Thalidomide Story  The chemical structure of thalidomide. “The Same and Not the Same”, by Roald Hoffmann 1995, Columbia University Press models – enantiomers (mirror image)

16. 16 Does it matter? Fatty Acids ‘trans’ fatty acid ‘cis’ fatty acid

17. 17 VSEPR Model The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them.

18. 18 VSEPR Model Electron Pair Geometry – is determined by the number and arrangement of all electron pairs (bonding and lone) around the central atom. Molecular geometry – is determined by the arrangement of atoms (or bonding electron pairs only) around the central atom. In molecules with no lone pairs, Electron Pair Geometry = Molecular Geometry N H H : H

19. 19 Fig. 9-4, p.383 AXE shorthand notation: A - central atom X - terminal atoms E - lone pair electrons AX 3 E 0

20. 20 1.Draw the Lewis structure. 2.Determine how many electron pairs (bonded and non-bonded) are around the central atom. **Treat a multiple bond like a single bond when determining a shape. 3.Write the AXE shorthand notation. 4.Determine the electron pair geometry (**one of the five basic shapes). 5.If the molecule has lone pairs around the central atom, then determine the molecular geometry. (This is a subset of the electron geometry.) Predicting Molecular Geometry: VSEPR

21. 21 Linear (Electron Geometry) Two e- pairs about central atom bond loneMolecular pairs pairsGeometry 2 0 linear.. 1 1-3 linear The molecular geometry here is the same as the electronic geometry even though there is a lone pair. ‘Two points make a line.’

22. 22 electron pair geometry = molecular geometry Geometry is Linear. Bond angle is 180 o. AX 2 E 0 Example 1: BeCl 2 Cl Be Cl 1. Draw the Lewis structure 2. Two electron pairs around the central atom. Two bonded and Zero lone pairs. Predicting Molecular Geometry

23. 23 bond loneMolecular pairs pairsGeometry.. 3 0 triangular planar 2 1 angular (bent) 1 2 linear Trigonal Planar (Electron Geometry) Three e- pairs about central atom Model

24. 24 Predicting Molecular Geometry triangular planar (or trigonal planar) AX 3 E 0.. F: :F B :F:.. Example 2: BF 3 Three electron pairs around the central atom. Three bonded and Zero lone pairs.

25. 25 Predicting Molecular Geometry AX 3 E 0 AX 2 E 1 electron geometry = triangular planar. molecular geometry = bent or angular. Example 3: SO 2 O S O S OO Three electron pairs around the central atom. Two bonded and One lone pairs.

26. 26 bond lone pairs 4 0 tetrahedral.. 3 1 triangular pyramidal 2 2 angular (bent).. Tetrahedral (Electron Geometry) Four e- pairs about central atom Model

27. 27 Predicting Molecular Geometry CH 4 electron pair geometry = molecular geometry AX 4 E 0 Example 4: H H H C H Four electron pairs around the central atom. Zero lone pairs. tetrahedral

28. 28 Predicting Molecular Geometry NH 3 AX 4 E 0 AX 3 E 1 electron geometry = tetrahedral. molecular geometry = triangular pyramidal Example 5: H H N H H N H H Four electron pairs around the central atom. Three bonded and One lone pair.

29. 29 Predicting Molecular Geometry H2OH2O AX 4 E 0 AX 2 E 2 electron geometry = tetrahedral molecular geometry = angular or bent Example 6: H O H Four electron pairs around the central atom. Two bonded and Two lone pairs. O H H

30. The Effect of Nonbonding Electrons By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. Therefore, the bond angle decreases as the number of lone pairs increases VSEPR Model HyperChem

31. 31 Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion : bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair

32. 32 The atoms are non-equivalent. axial Green atoms are axial; equatorial blue atoms are equatorial. 120° 90° Triangular bipyramidal.. Seesaw.. T-shaped.. Linear Trigonal Bipyramidal (Electron Geometry) Five e- pairs about central atom **Put lone pairs in the equatorial positions.

33. 33 Predicting Molecular Geometry : F : : : F : : : : F : : P F : : : Example 7: PF 5 AX 5 E 0 electron and molecular geometry= trigonal bipyramidal Five electron pairs around the central atom. Zero lone pairs.

34. 34 Predicting Molecular Geometry Example 8: SF 4 AX 5 E 0 AX 4 E 1 S F F F F : : : : ::: : : : :: electron geometry = trigonal bipyramidal molecular geometry = seesaw Five electron pairs around the central atom. Four bonded and One lone pair.

35. 35 Predicting Molecular Geometry AX 5 E 0 AX 3 E 2 Br F F : : F : : : : : : : :: electron geometry = trigonal bipyramidal molecular geometry = T-shaped Five electron pairs around the central atom. Three bonded and Two lone pairs. Example 9: BrF 3

36. 36 Predicting Molecular Geometry Example 10: XeF 2 F F Xe : : : : :: : : : AX 5 E 0 AX 2 E 3 electron geometry = trigonal bipyramidal molecular geometry = linear Five electron pairs around the central atom. Two bonded and Three lone pairs.

37. 37 Octahedral 90°.. Square pyramid.. Square planar Equivalent atoms Octahedral (Electron Geometry) Six e- pairs about central atom

38. 38 Predicting Molecular Geometry F: : : :F : : S F: : : :F : : :F: : : Example 11: SF 6 AX 6 E 0 electron geometry = octahedral molecular geometry = octahedral Six electron pairs around the central atom. Six bonded and Zero lone pairs.

39. 39 Predicting Molecular Geometry Example 12: IF 5 AX 6 E 0 AX 5 E 1 I F F F : F F : : : : : : : : : : : : : : : electron geometry = octahedral molecular geometry = square pyramidal Six electron pairs around the central atom. Five bonded and Two lone pairs.

40. 40 Predicting Molecular Geometry Example 13: XeF 4 AX 6 E 0 AX 5 E 1 Xe F F : F F : : : :: : : : : : : : : electron geometry = octahedral molecular geometry = square planar Six electron pairs around the central atom. Four bonded and Two lone pairs.

41. VSEPR Model

44. Intermolecular Forces What holds molecules to each other Pgs. 203-213 Pg. 207, #5 Pg. 209-213 # 62-63, Pg. 215 #8, 12

45. Intermolecular Forces They are what make solid and liquid molecular compounds possible. The weakest are called van der Waal’s forces - there are two kinds Dispersion forces Dipole Interactions depend on the number of electrons more electrons stronger forces Bigger molecules

46. 46 Types of Intermolecular Forces Intermolecular Interactions London ForcesDipole-Dipole ForcesHydrogen Bonding (0.05 – 40 kJ/mol) (5 – 25 kJ/mol) (10 – 40 kJ/mol) (Intramolecular Covalent Bond 150 – 1000 kJ/mol) δ+δ+ δ-δ- δ+δ+ δ-δ-

47. 47 London Forces (dispersion forces) Induced Dipole When electrons are momentarily unevenly distributed in the molecule, polarization occurs. All molecules, EVEN nonpolar ones experience London Forces! (Nonpolar molecules do not experience any other intermolecular interaction) Types of Intermolecular Forces

48. 48 To boil (l g), molecules must have enough energy to overcome their intermolecular forces. The higher the intermolecular force …the higher the boiling point! Types of Intermolecular Forces Dispersion Forces increase with increased number of electrons. increased polarizability

49. 49 A polar molecule is a Permanent Dipole that creates …. Dipole-Dipole forces Types of Intermolecular Forces

50. 50 The more polar the molecule (at a given size) … … the higher the boiling point! Types of Intermolecular Forces

51. 51 X—H - - - :Z— X = N, O, F Z = N, O, F Hydrogen bond …is established by the attraction between hydrogen and an electron pair on a small, very electronegative atom. This bond is responsible of determining the three dimensional structure of large proteins molecules Types of Intermolecular Forces +δ+δ

52. 52 Because of the hydrogen bond, water has a boiling point 200 C higher than if the bond were not present. Water: One molecule can participate in four H bonds with other molecules. Types of Intermolecular Forces