Diamond and Graphite
The simplest example of a macromolecular solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. is diamond. Crystals of diamond contain only carbon atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., and these are linked to each other by covalent bonds in a giant three-dimensional network, as shown below. Note how each carbon atom is surrounded tetrahedrally by four bonds.
Such a network of carbon atoms extends throughout the crystal so that the whole diamond is one extremely large covalently bonded entity, i.e., a macromolecule.
Because strong covalent bonds, rather than London forces or dipole forces, hold the carbon atoms together in this crystal, it takes a great deal of energyA system's capacity to do work. to separate them. Accordingly, diamond has an extremely high melting pointThe temperature at which a solid becomes a liquid. Also called freezing point., 3550°C—much higher than any ionic solid. Diamond is also the hardest substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. known. Each carbon atom is held firmly in its place from all sides and is thus very difficult to displace or remove.
Carbon also exists in a second, more familiar, crystalline form called graphite, whose crystal structure is also shown in part b of the figure. You use graphite every time you write with a pencil. (Pencil leads consist of C, not Pb!) The structure of graphite consists of flat layers. In each layer the carbon atoms are arranged in a regular hexagonal array. We can regard each layer as a large number of benzene rings fused together to form a gigantic honeycomb. All carbon-carbon bonds in this honeycomb are equivalent and intermediateIn chemical kinetics, a species that is formed in an early step in a reaction mechanism and then consumed in a later step; evidence of existence of an intermediate may be important for the interpretation of a rate law. in character between a single and a double bondAttraction between two atoms (nuclei and core electrons) that results from sharing two pairs of electrons between the atoms; a bond with bond order = 2..
While there are strong covalent bonds between the carbon atoms in a given plane, only weak London forces attract the planes together. The various layers can therefore slide past each other quite easily. When a pencil lead rubs across paper, the planes slide past each other and thin plates of crystal are left behind on the paper. These sliding plates also make graphite useful as a lubricant.
When an elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. can exist in more than one crystalline form, as carbon can in diamond and graphite, each form is said to be an allotropeOne of two or more different structural forms for an element that exist in the same physical state at the same temperature and pressure.. Other elements, such as sulfur and phosphorus, also form allotropes.