Sulfur Allotropy. Red and White Sulfur Allotropy

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Allotropy is the property that certain chemical elements have to form different simple substances, that is, substances formed only by a single type of element.

The different substances formed are called shapes or allotropic varieties or of allotropes.

These allotropes can differ for two main reasons, which are:

1. Its tomicity: the number of atoms that make up the molecule;

2. Structure: The arrangement of atoms in the crystal lattice is different for each allotropic form.

Sulfur (S) is an element that has the widest variety of allotropic forms, including molecules of S2, S4, S6and s8, which differ by the number of atoms in each molecule. All these sulfur allotropes can be found in places prone to volcanic eruptions, between the temperatures of 444.6 °C and 1000 °C. However, above 1000 °C, the S2 begins to dissociate into atomic sulfur (S0).

However, the most important allotropes of sulfur are two: o rhombic sulfur it's the monoclinic sulfur.

Both are formed by the same molecular formula, s8, the difference between the two being in the spatial arrangement of the atoms in the crystal lattice.

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Sulfur crystal structure (S8)

As you can see in the image below, the rhombic sulfur is in the form of yellow and transparent crystals, while the monoclinic one is needle-shaped and opaque:

Rhombic and monoclinic sulfur crystals and needles shape

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Between these two allotropic forms of sulfur, rhombic is the most common. It has a density equal to 2.07 g/cm3 at 20 °C, melting point equal to 112.8 °C and boiling point equal to 444.6 °C.

The monoclinic has a density of 1.96 g/cm3 at 20 ºC, melting point equal to 119 ºC and the same boiling point as rhombic sulfur. Under ambient conditions, both are a yellow powder, odorless, insoluble in water and very soluble in carbon sulphide (CS2).

A powdered sulfur storage area

Sulfur has several industrial uses, the main one being in the production of sulfuric acid, H2ONLY4, which, in turn, is widely used in the manufacture of various products. Due to its economic importance, the consumption of sulfuric acid can often indicate the degree of development of a country.

Another important application of sulfur is in rubber vulcanization, which is the addition of 2 to 30% sulfur to rubber, under heating and in the presence of catalysts, forming a three-dimensional polymer, with sulfur serving as a bridge between the chains carbonic. This makes natural rubber more resistant and flexible, ready for various uses, such as making tires.

Sulfur is also used in the production of black powder, insecticides, cosmetics and pharmaceuticals, such as sulfa-based antibiotics.

* Editorial credits for the image: emran /

By Jennifer Fogaça
Graduated in Chemistry

Would you like to reference this text in a school or academic work? Look:

FOGAÇA, Jennifer Rocha Vargas. "Sulfur Allotropy"; Brazil School. Available in: Accessed on June 28, 2021.


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