Solids can be classified on the basis of the bonds that hold the atoms or molecules together. This approach categorizes solids as either molecular, covalent, ionic, or metallic. Molecular solids are characterized by relatively strong intramolecular bonds between the atoms that form the molecules and much weaker intermolecular bonds between these molecules. Because the intermolecular bonds are relatively weak, molecular solids are often soft substances with low melting points.