INTERMOLECULAR FORCES

Intermolecular Forces (IMFs) are defined as the invisible "dotted lines" that hold different molecules together. They are distinct from intramolecular forces, which are the forces within a molecule that hold the atoms together (e.g., covalent bonds). Intermolecular forces can be classified into two categories based on the molecule's polarity:

  • Polar Molecules: Molecules with a net dipole moment due to differences in electronegativity between atoms, leading to partial positive (extδ+ext{δ}^+) and partial negative (extδ−ext{δ}^-) charges.

    Polar vs. Non-Polar Bonds & Molecules | ChemTalk

  • Non-Polar Molecules: Molecules that do not have a net dipole moment; the charge distribution is even, and they typically include molecules with identical atoms or symmetrical molecules.

Types of Intermolecular Forces

  1. London Dispersion Forces (LDF)

    • Strength Level: The weakest of all intermolecular forces.

    • Real-life Example: Mineral Oil.

    • Mechanism of Formation: Instantly formed dipoles (a pair of equal and opposite charges) in non-polar atoms create temporary attractions between them.

      London Dispersion Forces Definition

  2. Dipole-Dipole Forces

    • Strength Level: Medium strength.

    • Real-life Example: Acetone.

    • Mechanism: Occur between the positive end of one polar molecule and the negative end of a neighboring polar molecule

      Dipole-dipole Forces: Definition and Examples

  3. Ion-Dipole Forces

    • Strength Level: Very strong forces.

    • Real-life Example: When table salt (NaClNaCl) dissolves in water, the sodium ions (Na+Na^+) and chloride ions (Cl−Cl^-) interact strongly with polar water molecules.

    • Mechanism: An electrostatic attraction between a fully charged ion (either a positive cation or negative anion) and a polar molecule

      Example of ion- dipole interaction

  4. Hydrogen Bonding

    • Strength Level: The strongest of the standard IMFs.

    • Real-life Example: Water and Alcohol.

    • Mechanism: Occurs when hydrogen is bonded to highly electronegative atoms like Nitrogen, Oxygen, or Fluorine, allowing for strong attractions between them.

      Hydrogen Bond: Definition, Types, and Examples

Concepts Demonstrating Intermolecular Forces

  • Volatility (Evaporation)

    • Definition: The measure of how easily a liquid converts into a gas.

    • Real-life Example: Perfume evaporating quickly due to weaker IMFs allowing molecules to escape into the air.

    • Mechanism: weaker IMFs = evaporting quickly

      What is the Definition of Evaporation in Chemistry?

  • Surface Tension (The Penny Test)

    • Definition: How well a liquid holds itself together in a "dome" shape due to cohesive forces.

    • Real-life Example: The ability of water to form droplets on a penny, demonstrating strong IMFs at the surface.

    • Mechanism: stronger IMFs = hold itself better

      8.1.3: Properties of Liquids - Chemistry LibreTexts

  • Solubility (The Salt Test)

    • Rule: "Like Dissolves Like."

    • Real-life Example: Salt dissolving in water due to ion-dipole attractions between water and salt ions. Non-polar liquids like oil do not dissolve salt because they cannot form ion-dipole attractions.