1. crystal molecules (Molecular crystal) crystal particles of this type may be atoms or molecules. Gravitational attraction between the particles may be between polar molecules. Or a van der Waals force. Type the London such as naphthalene solid ice for the polar molecule is anchored by gravity between the terminals or hydrogen bonds. Solid crystal molecules are usually mild or moderate hardness, with relatively low melting point non-conductive. In the case of crystalline molecular polarity Some are easy to sublimate, like naphthalene.