Up to this point we have confined our discussion to covalently bonded substances consisting of small discrete molecules. The forces between these molecules are quite weak compared with covalent bonds. There are some covalently bonded substances, however, in which no small individual molecules exist. These substances contain extremely large molecules, and so they are called macromolecular substances. They are invariably solids at room temperatureA physical property that indicates whether one object can transfer thermal energy to another object.. Quite often we can regard each crystalA solid with a regular polyhedral shape; for example, in sodium chloride (table salt) the crystal faces are all at 90° angles. A solid in which the atoms, molecules, or ions are arranged in a regular, repeating lattice structure. of the substance as a single molecule.