Molecular solids and liquids
The molecular substances' condensation into solid and liquid forms is a manifestation of intermolecular forces. The enthalpies of mixture (that is melting) and vaporization provide a direct measure of the energy needed to overcome such type of forces.
We speak of molecular solids when molecules keep their identity, with the geometries identical to those in the gas phase. The molecular solids' structures sometimes look like those formed by close-packing of spheres, even though with highly unsymmetrical and polar molecules the directional nature of intermolecular forces may play a vital role. The Molecular liquids are more disorganized but the structural changes among solid and liquid can be subtle and the melting point of a molecular solid is not in usual a good guide to the strength of intermolecular forces. A improved correlation is found with the normal boiling point, like molecules become isolated in the vapor and the affect of intermolecular interactions is lost.
The vaporization's enthalpy ΔHvap divided by the normal boiling point in kelvin (Tb) gives the standard entropy of vaporization and as per the Trouton's rule its magnitude is generally around 90 J K-1 mol-1. Trouton's rule is not quantitatively trustworthy and stop working when molecules have an not usual degree of organization in either the liquid or vapor phase (example due to hydrogen bonding); Though, it does express a helpful qualitative relationship among the boiling point and the strength of intermolecular forces. Diagram 1 depicts the normal boiling points for noble gas elements and some molecular hydrides.