Intermolecular forces (IMFs) are forces of attraction or repulsion that act between molecules—basically, they are the “stickiness” that molecules have toward each other. They are weaker than the intramolecular forces (like covalent or ionic bonds that hold atoms together within a molecule) but are crucial in determining the physical properties of substances.
There are several main types of intermolecular forces:
- London Dispersion Forces (Van der Waals forces)
- Present in all molecules, but dominant in nonpolar molecules.
- Caused by temporary fluctuations in electron density that create temporary dipoles.
- Weakest IMF, but strength increases with larger, heavier molecules.
- Dipole-Dipole Interactions
- Occur in polar molecules.
- Positive end of one molecule is attracted to the negative end of another.
- Stronger than London dispersion but weaker than hydrogen bonding.
- Hydrogen Bonding
- A special type of strong dipole-dipole interaction.
- Happens when H is covalently bonded to N, O, or F, and is attracted to a lone pair on another N, O, or F.
- Responsible for water’s unusually high boiling point and surface tension.
- Ion-Dipole Forces
- Occur between an ion and a polar molecule (important in solutions like salt in water).
- Stronger than hydrogen bonds.
How IMFs Affect States of Matter
Intermolecular forces largely determine whether a substance is a solid, liquid, or gas at a given temperature:
- Strong IMFs → higher melting and boiling points because more energy is required to overcome the attractions between molecules.
- Weak IMFs → lower melting and boiling points; molecules escape more easily into the gas phase.
Examples:
- Water (H₂O) has strong hydrogen bonds → liquid at room temperature, high boiling point (100°C).
- Oxygen (O₂) has only London dispersion forces → gas at room temperature, low boiling point (−183°C).
- Iodine (I₂) has stronger London forces because of its large size → solid at room temperature.
In short, the stronger the intermolecular forces, the more “stuck together” the molecules are, which affects density, boiling/melting points, and viscosity.