The van der Waals forces holding CO 2 molecules together in dry ice, for example, are so weak that dry ice sublimes it passes directly from the solid to the gas phase at -78 C. Network covalent solids are a unique class of materials that can be viewed as a single giant molecule made up of an almost endless number of covalent bonds, such as the structure of diamond shown in the figure below. Because all of the bonds in this structure are equally strong, covalent solids are often very hard and they are notoriously difficult to melt. The structure of a material is determined by the way in which the simplest repeating units are arranged in space. When these simple repeating units occur in an regular fashion from one end of the solid to the other the material is said to be crystalline . The relationship between the physical properties of a material and its structure is best illustrated by the different elemental forms of carbon.
Carbon occurs as a variety of allotropes.
It was not until 1797, however, that Smithson Tennant was able to show that diamonds consist solely of carbon. The properties of diamond are remarkable. It is also the hardest natural substance and it expands less on heating than any other material. The characteristic properties of diamond are a direct result of its structure. Carbon, with four valence electrons, forms covalent bonds to four neighboring carbon atoms arranged toward the corners of a tetrahedron, as shown in the figure below.