Intermolecular Forces

What are IMF’s? INTERmolecular forces—attractions between molecules that can vary from strong to weak. IMF’s are always weaker than INTRA molecular forces, the forces that hold atoms together in a compound (Ionic, Polar Covalent, Nonpolar Covalent).

What properties do IMF’s affect & WHY? High Melting Points/ High Boiling Points High Viscosity High Surface Tension Low Vapor Pressure = Strong IMF’s like Hydrogen Bonding WHY? Strong IMF’s mean molecules like to stay close. So they won’t melt or boil easily; liquids will be thick and “sticky”; liquids will NOT turn into vapors/gases easily.

Types of IMF’s Bond IMF Characteristics Results in Ionic Strongest. Holds ionic crystals together. High MP/BP. Solid Covalent Hydrogen Bonding polar 2nd strongest. Special D-D that happens only when H is on one end of a polar molecule and F, O, or N are on the other end. High MP/BP, surface tension, etc. Liquid, Solid Dipole-Dipole Weaker. Occur between – and + ends of polar molecules. Medium MP/BP Liquid, Gas London Dispersion Forces nonpolar Weakest. Occur between ALL atoms and nonpolar molecules. Low MP/BP Gas usually

Types of IMF’s Bond IMF Characteristics Examples Ionic Strongest. Holds ionic crystals together. Covalent Hydrogen Bonding polar 2nd strongest. Special D-D that happens only when H is on one end of a polar molecule and F, O, or N are on the other end. Dipole-Dipole Weaker. Occur between – and + ends of polar molecules. London Dispersion Forces nonpolar Weakest. Occur between ALL atoms and nonpolar molecules.