The molecules in a solid are held together by intermolecular forces. These forces are weaker than the intramolecular forces that hold atoms together within a molecule, but they are strong enough to keep the molecules in a fixed, rigid arrangement.

Here are the main types of intermolecular forces:

* Van der Waals forces: These are weak, temporary forces that arise from temporary fluctuations in electron distribution within molecules. They are present between all molecules, but are especially important in nonpolar molecules.

* London dispersion forces: These are the weakest type of Van der Waals forces, and are present in all molecules.

* Dipole-dipole forces: These forces occur between polar molecules, where the positive end of one molecule attracts the negative end of another.

* 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 forces occur between oppositely charged ions. They are present in ionic compounds.

The specific type of intermolecular force that dominates in a solid depends on the type of molecules present. For example, water is a solid (ice) at low temperatures because of strong hydrogen bonding between water molecules.

Key points:

* Intermolecular forces are responsible for the physical properties of solids, such as their melting point, hardness, and density.

* The stronger the intermolecular forces, the more tightly the molecules are held together, and the higher the melting point of the solid.

* In general, solids with strong intermolecular forces are more rigid and have higher melting points than solids with weak intermolecular forces.