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1. Introduction to Polarity and Intermolecular Forces

  • Covalent bonds form as pairs of electrons are shared between nonmetal atoms.

  • Sharing of electrons can be equal or unequal.

2. Electronegativity

  • Definitions:

    • Electronegativity: Ability of an atom to attract shared electrons.

  • Variation in Electronegativity:

    • Increases from left to right across a period and from bottom to top within a group.

    • Fluorine has the highest electronegativity; noble gases do not form bonds.

  • Examples:

    • In HCl, chlorine's higher electronegativity causes a polar bond.

    • Resulting partial charges:

      • Chlorine has a partial negative charge (δ-), hydrogen has a partial positive charge (δ+).

3. Characteristics of Polar and Nonpolar Bonds

  • Polar Bonds:

    • Occurs between different nonmetals with varying electronegativities.

  • Nonpolar Bonds:

    • Found in identical nonmetal atoms (e.g., H2) where electrons are shared equally.

4. Molecular Polarity

  • A molecule with polar bonds is not always polar.

    • Example: Carbon dioxide (CO2) has polar C=O bonds but is symmetrical and nonpolar.

  • Symmetrical arrangements lead to cancellation of polarities.

  • Symmetrical Geometries: Linear, trigonal planar, tetrahedral.

  • Asymmetrical Geometries: Bent, trigonal pyramidal do not cancel out dipoles.

5. Dipoles

  • Definition: A dipole has both a positive and negative end; molecules remain neutral overall.

  • Representation:

    • Dipole: + - (polar molecule).

    • Nonpolar: + + + - (nonpolar molecule).

6. Measuring Charge Distribution

  • Formal Charge:

    • Based on equal sharing of electrons and does not consider electronegativity.

    • Example: For cyanide ion (CN-): Carbon shares 3 electrons with nitrogen, which has a formal charge of -1.

  • Oxidation Numbers:

    • Based on assigning all shared electrons to the atom with higher electronegativity.

  • Dipole Moment:

    • Measures the separation of charges in a molecule; greater separation leads to a larger dipole moment.

7. Intermolecular Forces

  • Definition: Forces of attraction between different molecules, weaker than covalent bonds.

    • Dipole-Dipole Attraction: Attraction between the positive end of one dipole and the negative end of another.

    • Hydrogen Bonding:

      • Strong intermolecular force between H and highly electronegative atoms (O, N, F).

      • Responsible for unique properties of water (e.g., ice being less dense than water).

8. Attractions in Nonpolar Molecules

  • London Dispersion Forces:

    • Occur in all substances, even nonpolar.

    • Caused by temporary dipoles when electron distribution is uneven.

    • Weaker than dipole-dipole interactions; strength increases with molecular mass.

    • Affect how gases can be liquefied due to intermolecular attractions.

9. Interaction of Polar Molecules and Ions

  • Polar molecules (like water) can interact with ions (e.g., Na+ and Cl-) through ion-dipole interactions when dissolved.

10. Chromatography

  • Method to separate mixtures based on polarity.

    • Paper chromatography uses water as the mobile phase and paper as stationary; different polarities lead to varying migration rates of components.