Allotropes of Sulfur - Different Forms of Sulfur

Sulfur which is indicated by the symbol "S", is one of the most common elements on the earth. It belongs to group 16 of the periodic table and its atomic number is 16. One of its unique qualities is its ability to exist in different forms, known as allotropes. These allotropes of sulfur have the same chemical composition but differ in structure and properties.

Allotropes of Sulfur

Sulfur can appear as crystals, powders, or even as a soft, sticky substance. Each allotrope of sulfur has its own characteristics and uses, making sulfur an important element in various fields, from medicine to agriculture.

Sulfur exists in crystalline or amorphous form. Crystalline sulfur has orderly arranged sulfur atoms while amorphous sulfur has randomly arranged sulfur atoms.

Sulfur forms two crystalline allotrophs and two amorphous allotrophs. Crystalline sulfur consists of S8 rings. These S8 rings can arrange into two different ways to form Rhombic sulfur and Monoclinic sulfur.

Rhombic sulfur

Rhombic sulfur is the most stable form of the sulfur allotrophs. It forms a crown-shaped S8 ring. These S8 rings form octahedral-shaped crystals. Rhombic sulfur crystals are transparent and yellow in color. Since it is a non-polar compound, rhombic sulfur cannot be dissolved in polar solvents like water.

• Density: 2.06 g/cm³
• Melting point: 113 ⁰C

Preparation of rhombic sulfur

Rhombic sulfur is prepared to dissolve powdered sulfur in Carbon sulfide solution (CS₂) and let the solvent to be evaporated.

1. First, powdered sulfur is dissolved in a CS₂ solution at room temperature
2. Then the solvent is filtered to remove any undissolved matter
3. The filtrate is collected and the solvent is evaporated slowly.
4. After the solvent is evaporated, it can observe octahedral-shaped rhombic sulfur crystals.

Monoclinic sulfur

Monoclinic sulfur is also a crystalline sulfur that consists of S8 rings. These are needle-like (Rod-shaped) crystals. Only stable above 96 ⁰C. When the temperature decreases below 96 ⁰C, it will turn into rhombic sulfur.  Conversely, monoclinic sulfur can be prepared by heating rhombic sulfur above 96 ⁰C.

• Density: 1.96 g/ cm³
• Melting point: 119 ⁰C

Preparation of monoclinic sulfur

1. Powdered sulfur is heated until it melts in an evaporating dish.
2. Then it is let to be cooled.
3. It will form a crust when it is cooling
4. Make small holes in the crust and remove the remaining liquid.
5. remove the crust and observe needle-like monoclinic sulfur at the lower side of the crust

Plastic sulfur

Plastic sulfur is an amorphous sulfur. It is a brown color rubber-like material. It has malleable properties. Unlike rhombic sulfur, plastic sulfur is insoluble in CS₂ solvent. Plastic sulfur can be formed by sudden cooling of molten sulfur powder. Plastic sulfur will also turn into rhombic sulfur after a while.

Preparation of plastic sulfur

1. Powdered sulfur is heated until it reaches its boiling point.
2. It is poured into a beaker that contains cold water for rapid cooling
3. After it is completely cooled, it can observe plastic sulfur.

Milk of sulfur

If sulfur and water are resulted as products in a chemical reaction, in such a situation milk of sulfur is formed. Milk of sulfur is a dispersion of sulfur in water. (Solid-liquid dispersion). Therefore, the solution appears as a milky white or yellow color solution.

Chemical reactions where the milk of Sulfur is formed