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Intermolecular Attractions

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

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Intermolecular Attractions

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  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. How many electrons do most atoms want in their outer energy level to be stable? • 0 • 1 • 2 • 6 • 8 • 18

  4. Bonds that form between two nonmetals are which type? • Ionic • Covalent • Metallic

  5. 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

  6. How many bonding pairs are in the following compound? • 1 • 2 • 3 • 4 • 6 • 9 • 18

  7. How many lone pairs are in the following compound? • 1 • 2 • 3 • 4 • 6 • 9 • 18

  8. 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

  9. 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

  10. Draw the electron dot formula. Then state how many bonding and unbonding pairs are present. A) NBr3 B) Water C) Chlorite ion (ClO2- ) D) CF2Cl2

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

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

  13. 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.

  14. 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.

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

  16. What is the name of the property that describes the tendency of an atom to attract electrons when bonded to another atom? • Ionization energy • Conductivity • Electronegativity • Metallic Character • Bond length

  17. 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.

  18. Electronegativitiesof Elements Electronegativity

  19. Bond Classification based on Electronegativity Difference As the difference in electronegativity increases, electrons are less equally shared and become more polar.

  20. Bond Classification based on Electronegativity Difference Type of Bonds Electronegativity Difference Nonpolar covalent Polar covalent Ionic x ≤ 0.4 0.4 < x < 1.8 x ≥ 1.8

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

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

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

  24. Interactions between Molecules Intermolecular Forces

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

  26. Nonpolar Compounds - Diatomic molecules are always nonpolar. (ex. F2) • Also, compounds that are totally symmetric may be nonpolar as well. (ex. CCl4)

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

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

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

  30. Polar Compounds

  31. Polar Compounds

  32. Polar Compounds

  33. BF3 = Polar or Nonpolar • Polar • Nonpolar

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

  35. CF4 = Polar or Nonpolar? • Polar • Nonpolar

  36. Br2 = Polar or Nonpolar • Polar • Nonpolar

  37. PBr3 = Polar or Nonpolar • Polar • Nonpolar

  38. 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.

  39. 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

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

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

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

  43. 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.

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

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

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