What is Allotropy?

The notion of allotropy is used in the field of chemistry to refer to the property that certain chemical elements have of appearing with different characteristics in terms of physics or with different molecular structures. A molecule that is made up of a single element and has various structures is called an allotrope. In its etymology we find that it is composed of the other concepts, spinning around and a suffix that indicates “quality”.

It is important to note that allotropic properties take place in elements of equivalent composition but different aspects, if they are in the solid state. In other words, the state of aggregation of matter must be the same for properties to occur.

The diversity of properties is linked to the way atoms are arranged in space. This particularity means that the same chemical element can have various conformations. The phosphorus, for example, may appear as white phosphorus or red phosphorus. In a similar sense, carbon, according to different factors, appears as diamond or graphite.

Allotropy is also present in oxygen. The O2 (environmental oxygen) that is in the atmosphere can be breathed by living beings and enables combustion. The O3 (ozone), however, is toxic and absorbs ultraviolet radiation. As you can see, oxygen is a chemical element that has allotropy.

In sulfur you can also notice allotropy. We can find structures such as plastic sulfur, the alpha sulfur, the monoclinic sulfur and molten sulfur, among other allotropes of the same element.

It should be noted that, in the case of sulfur, these are different crystalline forms, with structural units that are packaged differently. So there are experts who speak of polymorphism and not allotropy. To avoid potential confusion between the two concepts, it is recommended to understand allotropy as several forms of the same element with different molecular units. In carbon and other elements, what changes are the chemical bonds that atoms make.

Continuing with the polymorphism, sulfur is capable of producing monoclinic crystals of an intense yellow hue (in this case its shape resembles, at each end, the blade of a chisel, a manual cutting tool used to shape or wood carving) or amber rhombic crystals (the shape of these crystals can be defined as that of a parallelepiped, a geometric body made up of six parallelograms, of which only the opposites of each other are parallel and equal).

Although human beings have known sulfur since ancient times, which take us back to prehistory, it was only at the end of the 20th century that it fully understood the allotropy that this element possesses. In nature, the most common of them is the octazufre cycle, which if it does not reach a temperature of 95 °C forms crystals of relative thickness, while above it the resulting crystals are acicular (that is, they have the appearance or needle shape).

The first synthesis of a sulfur allotrope, on the other hand, was carried out in 1891, with one having rings of a size different from eight. It was the hexazufre cycle, the second of all the real allotropes of this element to be discovered.

The limits of ring measurements of sulfur allotropes that have been possible to synthesize so far are 6 and 20, although science estimates that there are some with rings that are above this last value. Of all of them, the one that has shown the greatest stability, beyond the octazufre cycle, is the dodecazufre cycle (its size is 12).

Allotropy