Intermolecular forces Intermolecular forces are the forces of attractions that exist between molecules in a compound. These cause the compound to exist in a certain state of matter: solid, liquid, or gas; and affect the melting and boiling points of compounds as well as the solubilities of one substance in another.

Ion dipole interaction

States of matter Solid: A state of matter in which the matter is not compressible nor does it flow. Liquid: A state of matter in which the matter is not compressible but can flow. Gas: A state of matter in which the matter is compressible and can flow.

The stronger the attractions between particles (molecules or ions), the more difficult it will be to separate the particles. When substances melt, the particles are still close to one another but the forces of attraction that held the particles rigidly together in the solid state have been sufficiently overcome to allow the particles to move. When substances boil, the particles are completely separated from one another and the attractions between molecules are completely overcome

As the size of the halogens increases, the melting and boiling points increase. Melting Points and Boiling Points of Similar Substances with Increasing Formula WeightsSubstanceFW (g/mol)mp (°C)bp (°C)F238-220-188Cl271-100.98-34.6Br2160-7.258.78I2254113.5184.35

Ionic Compounds and Metals Electrostatic forces - these forces occur between charged species and are responsible for the extremely high melting and boiling points of ionic compounds and metals.

Covalent Compounds London forces - all molecules have the capability to form London forces. These are solely dependent on the surface area and the polarizability of the surface of the molecule. These are the only types of forces that non-polar covalent molecules can form. They result from the movement of the electrons in the molecule which generates temporary positive and negative regions in the molecule.

London dispersion forces

Dipole-dipole forces - only polar covalent molecules have the ability to form dipole-dipole attractions between molecules. Polar covalent molecules act as little magnets, they have positive ends and negative ends which attract each other.

Dipole dipole forces

Hydrogen bonding - these occur between polar covalent molecules that possess a hydrogen bonded to an extremely electronegative element, specifically - N, O, and F.