What is an Allotrope?

Allotropes are forms of a chemical element that differ at the molecular level, or in the way the atoms are arranged into molecules. Many elements occur in different allotropes, among them carbon, oxygen, phosphorus, and sulfur. These different forms can differ greatly in their physical properties, such as color, hardness and electrical conductivity, and in their chemical reactivity. There are various ways in which one allotrope can be converted to another, including by heating and cooling, high pressure or even exposure to light. An allotrope should not be confused with an isotope, which differs at the atomic, rather than molecular, level.

There are two types of allotrope. The enantiotropic type can undergo a reversible change into another allotrope under certain conditions, such as different temperature or pressure. For example, there is a form of the element tin that is stable below 55.4°F (13°C), and another that is stable above this temperature — it is possible to convert one to the other, and back again by raising or lowering the temperature. Monotropic means that one form is the most stable, and cannot easily be converted to and from another form; some examples are the graphite form of carbon, and the most common form of oxygen (O₂), as opposed to the less stable ozone (O₃).

Carbon is the element with the greatest number of allotropes, although — as of 2013 — the precise number is not clear as some have been disputed. The various accepted forms are radically different from one another, ranging from soft to hard, opaque to transparent, abrasive to smooth, and displaying many other variations and contrasts. The ability of this element to take so many different forms stems from the fact that a carbon atom can form four single bonds to other atoms. It can also form double, and occasionally triple, bonds. This allows great variety in the types of molecular and crystalline structures that are possible.

Amorphous carbon is the commonest form and is familiar to nearly everyone as coal, charcoal and soot. This black, opaque allotrope is non-crystalline, and the atoms do not form any regular structures. Coal is in fact quite an impure form in that 10% or more consists of other elements.

Graphite is the material that forms the “lead” in pencils. It consists of sheets of carbon atoms arranged into linked two-dimensional hexagons. The sheets slide off one another easily, which is why it can be used to write on paper. Although carbon is a non-metal, this allotrope has a slightly metallic appearance and conducts electricity.

Diamond is a crystalline type of carbon in which each atom has four single bonds joining it to other atoms, forming linked tetrahedra. It forms naturally deep in the Earth, at high temperatures and very high pressures. Although they are extremely hard, due to is structure and the strength of the bonds that hold the atoms together, diamonds are not forever: the structure is not completely stable at normal pressure and temperature, and it very slowly converts to graphite. The change, however, is so slow that it is not noticeable on human timescales. Diamonds can also be created artificially from graphite at high temperature and pressure.

Another crystalline allotrope is the mineral lonsdaleite. It resembles diamond and is thought to be created, in small quantities, by the impact of meteorites. The pressure created converts graphite into a three-dimensional form that retains the hexagonal structure, producing a hard, crystalline material.

Among the most fascinating forms of carbon are the fullerenes. These are hollow, three-dimensional structures with walls consisting of arrangements of atoms into hexagons, pentagons, and sometimes other shapes. One of the best known is the “buckyball,” or more correctly, buckminsterfullerene: 60 carbon atoms that form a hollow sphere, also known as C₆₀. Larger spheres are also possible, with larger numbers of carbon atoms. Buckyballs can be manufactured, but also occur naturally, and have been found on the Earth in soot and in space.

Nanotubes are another well-known form of fullerene. These consist of tiny cylinders whose walls have a similar structure to those of buckyballs. They can be up to several millimeters long and may be open or closed at the ends. Nanotubes have an extremely high strength-to-weight ratio, and are also good electrical conductors; it is thought that they may have many important technological applications, especially in the world of nanotechnology.

Carbon nanofoam is a synthetic allotrope consisting of atoms linked in a web-like structure. It is one of the most lightweight materials known, due to its extremely low density, and is only a few times heavier than air. Unusually, it is ferromagnetic — attracted to magnets — and is also a semiconductor.

The oxygen in the air that people breathe consists of molecules containing two atoms of oxygen — O₂. Atoms of this element can form single bonds with two other atoms or a double bond with one other. The normal form of oxygen has a double bond between the two atoms, but it can also exist in a molecule containing three atoms, each joined by single bonds to two others. This form is known as ozone (O₃).

Ozone is less stable, and much more reactive, than O₂, and in its pure form, it is a serious fire hazard. It is also toxic, as it damages the lungs if inhaled. Ozone can be produced by the reactions of gases produced by motor exhaust under the influence of sunlight, and can become a serious pollutant in urban areas. It is also produced in the upper atmosphere by the interaction of O₂ and ultraviolet light from the Sun, forming the “ozone layer” that shields life on the Earth’s surface from the most damaging forms of ultraviolet light.

This is another element with several strongly contrasting allotropes. When it is first isolated from its compounds, it appears as white phosphorus. This form is made of tetrahedrons of four atoms; it is very reactive, highly toxic, and glows in the dark at room temperature, due to a slow reaction with oxygen in the air. By heating it for some time in a sealed container, it can be converted to red phosphorus, a much less reactive, non-toxic form in which the tetrahedrons are linked together into chains. A third form, black phosphorus, can be obtained by heating the white form at high pressure — it has its atoms arranged in hexagons that form sheets, much like graphite.