Allotropes are different forms of an element that can vary in their physical and chemical properties. Carbon has the most allotropes, including amorphous carbon, graphite, diamond, lonsdaleite, fullerenes, and carbon nanofoam. Oxygen has two allotropes, O2 and O3 (ozone), while phosphorus has three, white, red, and black. Allotropes can be converted into each other through various methods, and they are not to be confused with isotopes, which differ at the atomic level.
Allotropes are forms of a chemical element that differ at the molecular level or in the way atoms are organized into molecules. Many elements are found in different allotropes, including 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 into another, including heating and cooling, high pressure, or even exposure to light. An allotrope is not to be confused with an isotope, which differs at the atomic, rather than the molecular level.
There are two types of allotropes. The enantiotropic type can undergo a reversible change to another allotrope under certain conditions, such as different temperature or pressure. For example, there is one form of the element tin that is stable below 55.4°F (13°C) and another that is stable above this temperature – you can convert one to the other and vice versa raising or lowering the temperature. Monotropic means that one form is the most stable and cannot be easily converted to and from another form; some examples are the graphite form of carbon and the more common form of oxygen (O2), as opposed to the less stable ozone (O3).
Carbon
Carbon is the element with the most allotropes, although – as of 2013 – the precise number is unclear as some have been disputed. The various forms accepted are radically different from each other, ranging from soft to hard, from opaque to transparent, from abrasive to smooth, and show many other variations and contrasts. The ability of this element to take on so many different forms comes from the fact that one carbon atom can form four single bonds with other atoms. It can also form double and sometimes triple bonds. This allows for a great variety in the types of possible molecular and crystal structures.
Amorphous carbon is the most common form and is as familiar to almost everyone as charcoal, charcoal, and soot. This black, opaque allotrope is non-crystalline and the atoms do not form regular structures. Coal is in fact a rather impure form as 10% or more is made up of other elements.
Graphite is the material that forms the “lead” in pencils. It consists of sheets of carbon atoms arranged in connected two-dimensional hexagons. The sheets slide over each other easily which is why it can be used to write on paper. Although carbon is a nonmetal, 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 that join it to other atoms, forming connected tetrahedra. It forms naturally deep inside the Earth, at high temperatures and very high pressures. Although they are extremely hard, due to their structure and the strength of the bonds that hold the atoms together, diamonds are not forever: the structure is not completely stable under normal pressure and temperature and converts very slowly to graphite. The change, however, is so slow that it is not perceptible on human timescales. Diamonds can also be created artificially from graphite under high temperature and pressure.
Another crystalline allotrope is the mineral lonsdaleite. It resembles diamond and is thought to have been created, in small quantities, by meteorite impacts. The pressure created converts the graphite into a three-dimensional shape that maintains 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 made up of arrangements of atoms in hexagons, pentagons, and sometimes other shapes. One of the best known is the “buckyball”, or more correctly, buckminsterfullerene: 60 carbon atoms forming a hollow sphere, also known as C60. Larger spheres, with a greater number of carbon atoms, are also possible. Buckyballs can be manufactured, but they also occur in nature and have been found on Earth in soot and in space.
Nanotubes are another well-known form of fullerene. These are made up of tiny cylinders whose walls have a structure similar to those of buckyballs. They can be up to several millimeters long and can be open or closed at the ends. Nanotubes have an extremely high strength-to-weight ratio and are also good electrical conductors; they are thought to have many important technological applications, especially in the world of nanotechnology.
Carbon nanofoam is a synthetic allotrope made up of atoms connected in a network-like structure. It is one of the lightest 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.
Oxygen
The oxygen in the air that people breathe consists of molecules containing two oxygen atoms – O2. Atoms of this element can form single bonds with two other atoms or a double bond between them. The normal form of oxygen has a double bond between 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, is a serious fire hazard. It is also toxic, as it damages the lungs when inhaled. Ozone can be produced by the reactions of engine exhaust gases 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’ which protects life on the earth’s surface from the most harmful forms of ultraviolet light.
Phosphorus
This is another element with several strongly contrasting allotropes. When first isolated from its compounds, it appears as white phosphorus. This shape is composed of tetrahedra 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 into red phosphorus, a much less reactive, non-toxic form in which the tetrahedra are linked together in chains. A third form, black phosphorus, can be obtained by heating the white form under high pressure: it has its atoms arranged in hexagons that form sheets, much like graphite.
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