The states of matter depend on how the average distance between their molecules is compared with the size of the molecules themselves. In a gas, the molecules are separated in average by distances much larger than the molecules sizes. There are, as we know, substances in solid state at normal temperature and pressure. Their molecules are not individual isolated units like in the gaseous state. The separation of the molecules in a solid state can be considered in the order of the size of the molecule itself, and the force that keep the molecules together are of the same magnitude as the strength of the forces that keep the atom in the molecule together. This is typical os solids that present a crystal lattice formation.