Difference Between Rhombic and Monoclinic Sulphur

The key difference between rhombic and monoclinic sulphur is that the rhombic sulphur is the most stable allotropic form of sulphur that exists as rhombic octahedral crystals whereas the monoclinic sulphur exists as long, needle-shape prisms but, it is stable only at temperatures between 96◦C and 119◦C.

Sulphur, also spelt it as “sulfur”, is a chemical element having the chemical symbol S and atomic number 16. It is a nonmetal and occurs in nature in different allotropic forms. Furthermore, at room temperature, it is readily available as bright yellow colored crystals. The major sources of Sulphur include natural gas, extraction from beneath the earth’s crust and as byproducts of other chemical processes. Rhombic and monoclinic Sulphur are two allotropic forms; allotropes are the different forms of the same chemical element that exists in the same physical state, i.e. structural modifications. Not only the structure but also the method of preparation of these allotropes are also different from each other.

CONTENTS

1. Overview and Key Difference
2. What is Rhombic Sulphur
3. What is Monoclinic Sulphur
4. Side by Side Comparison – Rhombic vs Monoclinic Sulphurin Tabular Form
5. Summary

What is Rhombic Sulphur?

Rhombic Sulphur, or alpha-Sulphur, is a crystalline allotropic form of Sulphur that has rhombic octahedral crystals. It is the most stable form of allotrope among other allotropes of Sulphur. Therefore, almost all the other allotropes eventually convert into the rhombic form.

When considering the method of preparation, first we should dissolve sulphur powder in carbon disulphide (at room temperature); it is insoluble in water. Then we can filter the mixture using a filter paper. After the filtration, we have to keep the filtrate in a beaker, covered with a filter paper. This allows the carbon disulphide to evaporate slowly, leaving the alpha sulphur crystals. The density of these crystals is around 2.06 g/mL, and the melting point is at 112.8◦C. If we heat rhombic sulphur to about 96◦C slowly, it converts into the monoclinic form.

What is Monoclinic Sulphur?

Monoclinic sulphur is a crystalline allotropic form of sulphur that has needle-like, long crystals. These crystals appear as prisms; hence we can call these crystals as prismatic sulphur. It is not stable as much as rhombic sulphur, therefore, it converts into the rhombic form when heated to about 94.5◦C slowly. The monoclinic form is stable above 96◦C.

The density of this allotropic form is about 1.98 g/mL, and the melting point is 119◦C. at temperatures below 96◦C, it converts into rhombic form. When considering the method of preparation of this form, first we should heat sulphur powder on an evaporating dish, until the sulphur powder melts. Then we should allow it to cool until a solid crust forms over the surface. After the formation of this crust, we should make two holes on the crust and pour out the molten sulphur out of it. On the lower side of the crust, we can see monoclinic sulphur crystals.

What is the Difference Between Rhombic and Monoclinic Sulphur?

Rhombic Sulphur is a crystalline allotropic form of Sulphur which has rhombic octahedral crystals. It is the most stable form of allotrope among other allotropic forms of sulphur. Therefore, other allotropes also tend to convert into the rhombic form. Monoclinic sulphur is a crystalline allotropic form of sulphur that has needle-like, long crystals. It is stable at temperatures between 96◦C and 119◦C. This is the main difference between rhombic and monoclinic sulphur. Further to the structural difference between rhombic and monoclinic sulphur, they too differ slightly in some properties as well as in the method of preparation.

Summary – Rhombic vs Monoclinic Sulphur

Sulphur is an inorganic substance that has many allotropic forms that exist in the same physical state. Rhombic form and monoclinic form are such two allotropes. The difference between rhombic and monoclinic sulphur is that rhombic sulphur exists as rhombic octahedral crystals whereas monoclinic sulphur exists as long, needle-shape prisms.

Reference:

1. “Rhombic Sulphur.” Modes of Nutrition. Available here   
2. “Monoclinic Sulfur.” Modes of Nutrition. Available here  

Image Courtesy:

1.’PSM V83 D031 Crystals of rhombic sulphur’By Unknowm – Popular Science Monthly Volume 83, (Public Domain) via Commons Wikimedia 
2.’PSM V83 D031 Crystals of needle like or monoclinic sulphur’By Unknowm (Public Domain) via Commons Wikimedia