Chemistry Notes

Molecular Polarity and Intermolecular Forces

Introduction

  • Molecular polarity relates to how charges are distributed in a molecule due to differences in electronegativity between atoms within a covalent bond.

Electronegativity

  • Electronegativity is defined as how much an atom in a covalent bond wants or pulls on the bonding electrons.
    • It varies across the periodic table:
    • Cl (Chlorine) is more electronegative than H (Hydrogen).
    • This leads to a situation where Cl tends to gain electrons (becoming negatively charged), while H tends to lose electrons (becoming positively charged).

Polar and Non-Polar Molecules

  • Any molecule that has a dipole moment is considered polar.
    • Example: Water (H₂O) is a polar molecule.
    • In H₂O:
    • H-O-H structure exhibits single bonds between H and O.
    • Oxygen is more electronegative than Hydrogen, leading to a partial negative charge on the O and partial positive charges on the H.
  • Non-polar molecules exhibit either equal sharing of electrons or symmetrical arrangement of polar bonds that cancel each other out.

Types of Intermolecular Forces

  1. Dipole-Dipole Forces

    • Occur between polar molecules.
    • Example: CH₃-CH₃ (does not exhibit dipole-dipole interactions because it is non-polar).
    • H₂O has significant dipole-dipole bonding because of its polar nature.

  2. Hydrogen Bonds

    • A special type of dipole-dipole interaction that occurs between molecules containing H bonded to highly electronegative atoms (N, O, F).
    • Example: H-F is considered one of the strongest intermolecular bonds due to hydrogen bonding between fluorine and hydrogen.

  3. Dispersion Forces

    • Present in all molecules but are the only forces in non-polar molecules.
    • Strength increases with the number of electrons present.
    • They can induce temporary dipoles, effectively creating a weak interaction (induced dipole).

Electronegativity Values

  • The electronegativity scale guides the understanding of how polar or non-polar a molecule is:
    • Molecules with significant differences in electronegativity are more polar.
    • Most Polar Bonds:
    • H-F (Hydrogen-Fluorine)
    • Least Polar Bonds:
    • H-I (Hydrogen-Iodine)

Molecular Geometry Considerations

  • If the central atom of a molecule has lone pairs and all surrounding atoms are the same, the molecule is often non-polar despite having polar bonds due to symmetrical cancellation of dipoles.
  • Symmetrical molecules like CH₂CH₂ (ethylene) exhibit non-polar characteristics.

Summary of Intermolecular Forces

  • Ranging from strongest to weakest:
    1. Hydrogen Bonds
    2. Dipole-Dipole Bonds
    3. Dispersion Forces
  • Intermolecular forces play a crucial role in determining physical properties of substances, such as boiling points and solubility.

Applications

  • Understanding molecular polarity and intermolecular forces is essential for predicting chemical behavior, reaction mechanisms, and the properties of solvents and solutions.