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An allotrope is each of the different stable forms in which we can find or prepare a pure element . That is, allotropes are the different forms in which elemental substances occur, either naturally or synthetically. A common example of an allotrope is graphite, which is one of the forms in which the element carbon can be obtained.
Another important allotrope of carbon is diamond, an extremely hard, transparent crystalline form of the element that forms the basis of life. With the exception of synthetic (artificially synthesized) elements, each element in the periodic table has at least one allotrope, although it usually has several. While some of these allotropes may be worthless, others may be extremely valuable, as illustrated by the difference between graphite carbon and diamond carbon.
Characteristics and properties of allotropes
physical properties
The example of carbon illustrates a very important aspect of allotropes, which is that they can have radically opposite physical and chemical characteristics and properties.
Carbon graphite, for example, is an electrically conductive material, it is very soft, it has a structure in the form of layers or sheets of carbon atoms with sp 2 hybridization linked together by means of single and double bonds that are constantly exchanged . through resonance.
Instead, diamond is the hardest material we know of. It is formed by a three-dimensional crystal lattice in which each carbon atom is simultaneously linked to four other atoms by means of single covalent bonds. This characteristic makes diamond one of the best electrical insulators known (as opposed to graphite, which is a conductor).
Chemical properties
Allotropes also often have markedly different chemical properties. For example, phosphorus can be found in the form of various allotropes, among which white, red, and black phosphorus are the most common. White and red phosphorus have similar phosphorus atoms with tetrahedral geometry. However, white phosphorus is extremely toxic and highly flammable, igniting spontaneously just by coming into contact with oxygen in the air. This makes it useful as a fuse in certain explosives such as hand grenades.
Instead, red phosphorus is much more stable. It can come into contact with air without causing a fire. On the other hand, black phosphorus forms only under conditions of high pressure and a temperature of more than 200 °C, but once formed it can be cooled and is even more stable than red phosphorus.
physical state
The examples of the allotropes of phosphorus mentioned in the previous section are all solid at room temperature. However, allotropes can also exist in other states of aggregation. For example, in addition to the three solid isotopes mentioned (and at least as many more), phosphorus can also exist as a gaseous allotrope of the formula P 4 , forming a tetrahedral structure with one phosphorus at each vertex.
crystal structure
Finally, allotropes can also be differentiated from each other based on their crystal structure. We have already seen how carbon can form two very different kinds of three-dimensional structures that give rise to markedly different properties. In addition to this, some allotropes may also not have a well-defined crystal structure, in which case they are said to be amorphous allotropes.
From a macroscopic point of view, amorphous allotropes are easy to recognize because no type of facet or defined structure is observed on their surface that suggests a highly ordered internal structure.
Microscopically, however, amorphous solids are often simply a mixture of a large number of small crystalline solids of different sizes, and even different local crystal structures.
Importance of allotropes
The allotropy of an element can become extremely important from many points of view. The fact that some allotropes are more stable than others makes them preferable for the transport and handling of the respective element. On the other hand, some allotropes have desirable properties that other allotropes do not.
An example of the above is the hardness of diamond, the conductivity of graphite, and the combination of hardness and conductivity of another very important allotrope of carbon, the one that makes up carbon nanotubes.
On the other hand, transforming one allotrope into another can be essential for many industrial applications of the different elements. For example, silicon is one of the most important elements in the electronics industry. It is the semiconductor that forms the basis of all the microchips and processors that power all of our electronic devices. However, silicon can be found in two allotropic forms: amorphous silicon and crystalline silicon.
Amorphous silicon is used as a semiconductor in the manufacture of low-cost solar panels, while for the manufacture of microchips only monocrystalline silicon can be used, that is, a giant single crystal of silicon is needed in which all the atoms are perfectly ordered in order to create the patterns that are part of the circuits of each microchip.
Examples of common allotropes
Natural allotropes of carbon:
carbon graphite
diamond carbon
graphene
single-walled carbon nanotubes
double-walled carbon nanotubes
multi-walled carbon nanotubes
Fullerenes such as Buckminsterfulerene or C 60
Natural allotropes of oxygen:
Atomic oxygen (O)
Gaseous or molecular oxygen (O 2 )
Ozone ( O3 )
Tetraoxygen (O 4 )
solid oxygen O 8
Natural allotropes of nitrogen:
Gaseous molecular nitrogen (N 2 )
cubic solid nitrogen
hexagonal solid nitrogen
Natural allotropes of boron:
Amorphous Boron (brown powder)
α-rhombohedral boron
β-rhombohedral boron
boro-γ rock salt
Borophenes (structures similar to graphene but made of boron instead of carbon)
References
Bolívar, G. (2019, July 10). Boron: history, properties, structure, uses . lifer. https://www.lifeder.com/boro/
Chang, R., & Goldsby, K. (2013). Chemistry (11th ed.). McGraw-Hill Interamericana de España SL
Educaplus.org. (n.d.). Element properties . http://www.educaplus.org/elementos-quimicos/propiedades/alotropos.html
Flowers, G. (2021, June 11). What are the allotropic forms of nitrogen? The-Answer.com. https://la-respuesta.com/preguntas-comunes/cuales-son-las-formas-alotropicas-del-nitrogeno/
