The force that holds molecules of a single material together is called intermolecular forces. These forces are weaker than the intramolecular forces that hold atoms together within a molecule, but they are still important in determining the physical properties of a substance.

There are three main types of intermolecular forces:

* Van der Waals forces: These are the weakest type of intermolecular force and arise from temporary fluctuations in electron distribution around molecules. They are further classified into:

* London dispersion forces: These occur between all molecules, regardless of polarity.

* Dipole-dipole forces: These occur between polar molecules that have permanent dipoles.

* Dipole-induced dipole forces: These occur between polar and non-polar molecules, where the polar molecule induces a temporary dipole in the non-polar molecule.

* Hydrogen bonding: This is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine). It is the strongest type of intermolecular force.

* Ionic forces: These are electrostatic attractions between ions, which are charged atoms or molecules. They are stronger than van der Waals forces but weaker than covalent bonds.

The strength of intermolecular forces determines the melting point, boiling point, viscosity, and other physical properties of a substance. For example, substances with strong intermolecular forces will have higher melting and boiling points than substances with weak intermolecular forces.