One element, many structures
Carbon is a single element, yet the materials made from it range from a soft, dark lubricant to the hardest common natural substance, from a flexible fiber to a nearly transparent crystal. The different structural forms of a single element are called allotropes, and the carbon allotropes are unusually varied because carbon atoms can bond to their neighbors in several distinct ways.
What separates one form from another is almost never the chemistry. It is the geometry: how many neighbors each atom has, how the bonds are oriented, and whether the atoms repeat in an orderly pattern or sit in a jumble. Once you know that, most of the properties follow.
Two ways to bond: the root of the family
Most of the family comes from two bonding arrangements.
In the first, each carbon atom bonds to four neighbors in a three-dimensional tetrahedral pattern. This is the arrangement in diamond. Every bond is strong and the network extends in all directions, which is the basis of diamond’s extreme hardness and its electrical insulation. The details of that lattice are covered under diamond crystal structure.
In the second, each atom bonds to three neighbors in a flat, honeycomb-like sheet. The fourth electron is shared across the sheet, which lets electricity move along it. This sheet structure is the building block of graphite, graphene, nanotubes and fullerenes. The difference between those four is what the sheets do: stack, stand alone, roll up, or close into a cage.
A third group, the amorphous carbons, has no long-range order at all and often mixes both bonding types.
Diamond
Diamond is the three-dimensional network described above. It is very hard, an excellent conductor of heat, and normally an electrical insulator. It is also brittle in the sense that it can cleave along certain planes. Natural diamond forms deep in the Earth, while synthetic diamond is made in presses or by vapor growth; those routes are covered elsewhere on this site. For the property most people ask about, thermal conductivity, diamond is exceptional among non-metals.
Graphite
Graphite is a stack of honeycomb sheets. Within each sheet the bonds are strong, but the sheets are held to one another only weakly, so they slide easily. That explains why graphite is soft, marks paper, and works as a dry lubricant. It also conducts electricity reasonably well along the sheets and tolerates very high temperatures when oxygen is absent. Its role in industry is described in the article on industrial graphite.
Graphene
Graphene is one layer of that honeycomb, a sheet a single atom thick. It is strong for its thickness, conducts electricity and heat well along the sheet, and is almost transparent. Making it in large, uniform, defect-free areas at low cost remains difficult, so the gap between laboratory promise and shipped products is real. The full picture is in the discussion of graphene sheets.
Carbon nanotubes
Roll a graphene sheet into a seamless cylinder and you have a carbon nanotube. Tubes can have a single wall or several nested walls, and the angle at which the sheet is rolled affects whether a tube behaves like a metal or a semiconductor. They are used mainly as additives in polymers, coatings and electrodes. See carbon nanotubes for the structure and uses in more detail.
Fullerenes
Fullerenes are closed, hollow cages built from carbon atoms arranged in pentagons and hexagons. The best known has 60 atoms and resembles a soccer ball. These are true molecules rather than extended solids, which sets them apart from the other crystalline forms. Their relationship to tiny diamond particles is explored in the article on fullerene molecules.
Amorphous and disordered carbon
Soot, charcoal, carbon black and many hard coatings are largely disordered. Their properties depend on the mix of the two bonding types and on how much hydrogen is present. Some of these coatings are hard and slippery enough to resemble diamond in use, even though they are not crystalline; the comparison is made under diamond-like carbon.
Porous carbon belongs here as well. Activated carbon is processed to develop a huge internal surface area, which is why it is used to capture contaminants from water and air.
Carbon in fiber and composite form
Not every important carbon material is defined by a single structure. Carbon fiber consists of very thin filaments of mostly graphite-like, but imperfectly ordered, carbon, aligned along the fiber. Embedded in resin it forms polymer matrix composites, which are valued for stiffness at low weight. Embedded in carbon it forms the heat-resistant materials covered under carbon-carbon composites. In both cases the carbon is the reinforcement, and the fiber-matrix interface often decides how well the finished part performs.
A quick comparison
| Form | Atom arrangement | Typical character |
|---|---|---|
| Diamond | 3D network, four bonds each | Very hard, insulating, conducts heat well |
| Graphite | Stacked flat sheets | Soft, slippery, conducts electricity |
| Graphene | Single sheet | Thin, strong, conductive |
| Nanotube | Rolled sheet | Slender, strong along its length |
| Fullerene | Closed cage molecule | Discrete molecules, not a bulk solid |
| Amorphous | No long-range order | Varies widely with composition |
How scientists tell them apart
Because these forms can look similar to the eye (many are simply black), they are identified by measurement. Raman spectroscopy is a common technique for carbon, since the diamond-type and sheet-type bonding give distinct signatures. X-ray methods reveal how regular the atomic arrangement is.
Why the family matters
The same element supplies a gemstone-grade abrasive, a furnace lining, a battery electrode, an aircraft brake and a water filter. Choosing among them is a matter of matching structure to job: three-dimensional bonding for hardness, layered bonding for lubrication and conduction, porosity for adsorption, and fibers for light, stiff reinforcement. The articles in this section take each form in turn.