The allotropy in chemistry it is the characteristic that certain chemical elements have of presenting themselves in several different forms but in the same state of aggregation of matter. The structure of the elements can vary depending on their molecular arrangement and the conditions in which they are formed, such as pressure and temperature..
Only when it comes to chemical elements is the word allotropy used, designating each of the ways in which an element can be found in the same phase as an allotrope; whereas for compounds exhibiting different crystalline structures it does not apply; in this case it is called polymorphism.
Other cases are known, such as oxygen, in which allotropy can occur as a change in the number of atoms of the substance. In this sense, there is a notion of two allotropes of this element, which are better known as oxygen (Otwo) and ozone (O3).
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As previously mentioned, allotropes are the different ways in which the same element can be found, so this variation in its structure causes these species to appear with different physical and chemical characteristics..
Likewise, the allotropic transformation between one element and another occurs by the way in which the atoms are arranged within the molecules; that is, the way in which the link originates.
This change between one allotrope and another can occur for different reasons, such as changes in the conditions of pressure, temperature, and even the incidence of electromagnetic radiation such as light..
When the structure of a chemical species is altered it can also change its behavior, modifying properties such as its electrical conductivity, hardness (in the case of solid substances), melting or boiling point and even physical qualities such as its color..
Additionally, allotropy can be of two types:
- Monotropic, when one of the element structures has greater stability than the others under all conditions.
- Enanthropic, when the different structures are stable under different conditions but can transform one into the other in a reversible way at certain pressures and temperatures.
Although there are more than a hundred known elements in the periodic table, not all have allotropic forms. The most well-known allotropes are presented below.
This element of great abundance in nature represents the fundamental basis of organic chemistry. Several allotropic species of this are known, among which diamond, graphite and others that will be exposed below..
The diamond shows a molecular arrangement in the form of tetrahedral crystals whose atoms are linked by single bonds; this means that they are arranged through hybridization sp3.
Graphite is made up of consecutive sheets of carbon, where its atoms are linked in hexagonal structures by double bonds; that is, with hybridization sptwo.
In addition to the two important allotropes mentioned above, which are the best known of carbon, there are others such as carbyne (as linear acetylenic carbon, LAC, is also known), where its atoms are arranged in a linear way by means of triple bonds; that is, with hybridization sp.
- Graphene, whose structure is very similar to graphite).
- Fullerene or buckminsterfullerene, also known as buckyball, whose structure is hexagonal but its atoms are arranged in the shape of a ring.
- Carbon nanotubes, cylindrical in shape.
- Amorphous carbon, without crystalline structure.
Sulfur also has several allotropes considered common, such as the following (it should be noted that all of these are in the solid state):
As its name implies, its crystalline structure is made up of octagonal rhombuses and is also known as α sulfur..
Known as β sulfur, it has the shape of a prism made up of eight sulfur atoms.
It produces prismatic crystals that are stable at certain temperatures, forming needles devoid of color..
Also called sulfur, it has an amorphous structure.
It has viscosity characteristics contrary to most of the elements, since in this allotrope it grows with increasing temperature.
This non-metallic element is commonly found in nature in combination with other elements and has several associated allotropic substances:
It is a solid with a tetrahedral crystalline structure and has applications in the military field, even being used as a chemical weapon..
It has the highest stability among the allotropes of this element and is very similar to graphene.
Forms an amorphous solid with reducing properties but is devoid of toxicity.
As its name indicates, it is made up of two phosphorus atoms and is a gaseous form of this element.
It is a solid with a crystalline structure with a monoclinic molecular arrangement..
Also solid amorphous structure.
Despite being one of the most common elements in the Earth's atmosphere and one of the most abundant elements in the universe, it has few known allotropes, among which dioxygen and trioxygen stand out..
Dioxygen is better known by the simple name of oxygen, a gaseous substance essential for the biological processes of this planet..
Trioxygen is better known simply as ozone, a highly reactive allotrope whose most famous function is to protect the Earth's atmosphere from external radiation sources..
Forms a solid phase of trigonal structure with characteristics of metastability.
There are also six other solid species that oxygen forms, with different crystalline structures..
In the same way, there are elements such as selenium, boron, silicon, among others, that present different allotropes and that have been studied with a lesser or greater degree of depth..
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