As we have seen, the atoms that make up a molecule are linked by intramolecular bonds. For a chemical change to occur, it is necessary to break these bonds, which are what determine the chemical properties of substances.
But there are other intermolecular bonds that make molecules or ions attract or repel each other. These bonds are what determine the physical properties of substances, such as melting point, boiling point, solubility, etc.
Intermolecular forces are the forces of attraction between the molecules of covalent substances. These forces are present in covalent substances in solid or liquid state. In gases, they appear when the temperature of the gas is reduced, the substance changing first to a liquid state and then to a solid state.
The melting and boiling points of covalent substances provide good information on the magnitude of intermolecular forces. Thus, the lower the melting (boiling) points, the lower the forces that hold the molecules together.
These links are weak, but important. For example, these intermolecular forces are the cause that not all molecules are gases.
Intermolecular forces can be of two types: