A crystal formed by van der Waals force interaction between molecules. Most nonmetallic elements and their compounds, such as dry ice (CO2), I2 and most organic substances, are solid molecular crystals. Molecular crystals are composed of molecules, which can be polar or nonpolar. The intermolecular force is very weak. Molecular crystals have low melting point and boiling point, low hardness and are volatile. Many substances are gaseous or liquid at room temperature. For example, O2 and CO2 are gases, while ethanol and glacial acetic acid are liquids. The melting point and boiling point of crystals of the same type of molecules increase with the increase of molecular weight, for example, the melting point and boiling point of halogen elements increase in the order of F2, Cl2, Br2 and I2; Hydrides of nonmetallic elements melt with the main group from top to bottom according to the periodic law, and the boiling point rises; The melting point of homologues of organic compounds increases with the increase of carbon atoms. However, besides Van der Waals force, there are also hydrogen bonding forces among HF, H2O, NH3 and CH3CH2OH, and their melting points are high.

The solubility of substances composed of molecules follows the principle of "similarity and compatibility". Polar molecules are easy to dissolve in polar solvents, while nonpolar molecules are easy to dissolve in nonpolar organic solvents, such as NH3 and HCl, which are easy to dissolve in water and difficult to dissolve in CCl4 and benzene. Br2 and I2 are insoluble in water, but soluble in organic solvents such as carbon tetrachloride and benzene. According to this property, Br2 and I2 can be extracted and separated from their aqueous solutions with solvents such as CCl4 and benzene.

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