Intermolecular Attractions

1. Intermolecular Attractions Bonding and VSEPR Theory Structures of Solids and liquids

2. Electron Dot (Lewis) Diagrams Explain Chemical Bonding Chemical bonds – occur when electrons are transferred or shared by elements so that they each become more stable

3. Drawing Electron Dot Diagrams Electrons usually stay in pairs when bonded. Bonding pairs – pair of electrons that form the bond - can be represented as a line segment Lone (or unbonded) pairs – pairs of electrons that are not involved in bonds and are shown as dots

4. How many bonding pairs and lone pairs are in the following compound?

5. How many bonding pairs and lone pairs are in the following compound? • 6 bonding pairs, 18 lone pairs • 3 bonding pairs, 18 lone pairs • 6 bonding pairs, 9 lone pairs • 3 bonding pairs, 9 lone pairs

6. How many bonding pairs and lone pairs are in the following compound? • 6 bonding pairs, 18 lone pairs • 12 bonding pairs, 18 lone pairs • 12 bonding pairs, 36 lone pairs • 6bonding pairs, 6 lone pairs

7. Drawing Electron Dot Formulas for Compounds Exceptions: Hydrogen only needs 2 electrons (1 bond) Boron tends to need only 6 electrons (3 bonds) Single atoms go in the center If more than one single atom, middle atom central atom

8. Draw the electron dot formula A) NBr3 B) Water C) Chlorite ion D) CF2Cl2

9. Multiple Bonds If there are not enough electrons to form full octets, multiple bonds may need to be formed.

10. Draw the electron dot formula E) O2 F) CO2

11. Resonance Structures If there are more than one possibility, resonance structures are drawn. Resonance structures show possible locations of the bonds. In reality the electrons exist as an average of the two structures – splitting time equally between them.

12. Resonance Example • Each resonance structure is shown followed by the combination with the double bonds shown with a dotted line as one of the bonds.

13. Draw the electron dot formulas including resonance structures G) SO2 H) N2O

14. Classifying Bond Types • Chemical bonds can be classified by how much the bonded electrons are shared or are not shared by the elements involved. • Electronegativity: The ability of one atoms in a molecule to attract electrons to itself. • Wolfgang Pauling set electronegativities on a scale from 0.7 (Cs) to 4.0 (F). • Electronegativity increases • across a period and • down a group.

15. Electronegativitiesof Elements Electronegativity

17. Bond Classification based on Electronegativity Difference As the difference in electronegativity increases, electrons are less equally shared and become more polar. Nonpolar covalent bond - electrone

18. Bond Classification based on Electronegativity Difference Type of Bonds Electronegativity Difference Nonpolar covalent Polar covalent Ionic 0 - 0.4 0.4 – 1.8 > 1.8

20. Classify the bond between the following elements: Cl and Cs • Ionic • Polar Covalent • Nonpolar Covalent

21. Classify the bond between the following elements: C and H • Ionic • Polar Covalent • Nonpolar Covalent

22. Classify the bond between the following elements: N and O • Ionic • Polar Covalent • Nonpolar Covalent

23. Interactions between Molecules Intermolecular Forces

24. Polarity of a Compound • Like bonds, compounds can also be classified as polar or nonpolar. • Polarity is based on: • Difference in electronegativity of atoms within a compound • Symmetry of the compound

25. Nonpolar Compounds Diatomic molecules are always nonpolar. Also, compounds that are totally symmetric may be nonpolar as well.

26. Nonpolar Compound – the bonds are polar but the dipoles cancel out since the compound is symmetrical (tetrahedral)

27. Nonpolar Compound – the bonds are polar but the dipoles cancel out since the compound is symmetrical (linear)

28. Polar Compounds Polar compounds have one side of the compound that is more positive and another side that is more negative.

29. Polar Compounds

30. Polar Compounds

31. Polar Compounds

32. BF3 = Polar or Nonpolar • Polar • Nonpolar

34. CH3F = Polar or Nonpolar? • Polar • Nonpolar

36. CF4 = Polar or Nonpolar • Polar • Nonpolar

37. Br2 = Polar or Nonpolar • Polar • Nonpolar

38. PBr3 = Polar or Nonpolar • Polar • Nonpolar

39. Intermolecular Forces Intermolecular Forces are forces that exist between two molecules that hold them together. Intermolecular Forces are caused by charge differences and polarity (because positive and negatives attract) The stronger the polarity, the stronger the attraction between molecules.

40. Intermolecular Forces • The stronger the polarity, the stronger the attraction between molecules. • The strength of the attraction between molecules determines properties such as: • Boiling point • Melting point • Surface tension • Cohesion • Capillary action

41. Types of Intermolecular Forces • Three major types of intermolecular forces: • Dipole-Dipole Interactions • Hydrogen Bonds • Dispersion Forces

42. Dipole-Dipole Interaction Occurs in polar molecules. Positive pole of one molecule is attracted to the negative pole of the next molecule.

43. Hydrogen Bonds Occurs in polar molecules when the hydrogen atom is attracted to the more electronegative nitrogen, oxygen, or fluorine atom of another molecule.

45. Dispersion Forces Dispersion forces are the weakest type of intermolecular forces because they exist between nonpolar molecules. Usually, the electrons are shared equally. But because electrons are constantly moving, sometimes a temporary dipole forms when all the electrons are on one side of the molecule. This temporary dipole would cause an attraction with another temporary dipole.

46. Summary of Intermolecular Forces (from strongest to weakest)

47. What kind of intermolecular force would exist in H2O? • Hydrogen bonding • Dipole-Dipole • Dispersion

48. What kind of intermolecular force would exist in PCl3? • Hydrogen bonding • Dipole-Dipole • Dispersion

49. What kind of intermolecular force would exist in Br2? • Hydrogen bonding • Dipole-Dipole • Dispersion

50. What kind of intermolecular force would exist in NH3? • Hydrogen bonding • Dipole-Dipole • Dispersion