There are a number of chemical elements that can be found in two or mor forms, these chemical elements are referred to as allotropes. Allotropes generally differ in arrangement of atoms. Tin, carbon, oxygen, sulphur are some of the elements that exhibit allotropes. Monoclinic (?- sulphur) and yellow rhombic sulphur (?- sulphur) are two of the most common allotropes of sulphur. This article will focus on discussing the allotropes of sulphur.

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Sulphur: Details

Sulphur is a non-metal with atomic number \(16\) and atomic mass \(32.07{\rm{u}}.\) Its electronic configuration is \({\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{,2}}{{\rm{s}}^{\rm{2}}}{\rm{,2}}{{\rm{p}}^{\rm{6}}}{\rm{,3}}{{\rm{s}}^{\rm{2}}}{\rm{,3}}{{\rm{p}}^{\rm{4}}}.\) It is placed in group \(16\) and period \(3.\) The elementary nature of Sulphur was first established by Lavoisier in \(1777.\) In general, Sulphur is pale yellow, brittle, and has a faint smell.

Allotropy: Definition

There are elements that exist in two or more forms that differ in their chemical properties however, the physical properties remain the same. These elements are referred to as allotropes or allotropic forms and the phenomenon itself is identified as allotropy.

Allotropes of Sulphur: Types

Sulphur exists in several allotropic forms. All these forms can be divided into two categories:

Crystalline Allotropes

1. Rhombic sulphur or alpha \(\left( {\rm{\alpha }} \right)\) sulphur
2. Monoclinic sulphur or beta \(\left( {\rm{\gamma }} \right)\) sulphur

Amorphous Allotropes

1. Plastic sulphur or gamma \(\left( \gamma \right)\) sulphur
2. Milk of Sulphur
3. Colloidal Sulphur

Allotrope of Sulphur-Rhombic Sulphur

Rhombic Sulphur is the most common and most stable form of Sulphur. It is also called alpha \(\left( \alpha \right)\) sulphur or octahedral Sulphur.

Preparation: Rhombic sulphur is prepared by dissolving powdered Sulphur in carbon disulfide at room temperature. After that, the mixture is filtered. Thereafter, the filtrate is stored in a tiny beaker lined with filter paper. The carbon disulfide will eventually evaporate, leaving massive octahedral rhombic Sulphur crystals behind.

Properties:

1. Rhombic Sulphur exists as a rhombic octahedral crystal.
2. It is pale yellow.
3. Its density is \(2.06\;\,{\rm{g}}/{\rm{mL}}\).
4.  It melts at \({\rm{112}}{\rm{.}}{{\rm{8}}^{\rm{o}}}\,{\rm{C}}{\rm{.}}\)
5. It is stable at room temperature. When rhombic Sulphur is slowly heated at \({\rm{9}}{{\rm{6}}^{\rm{o}}}\,{\rm{C,}}\) it changes into monoclinic Sulphur. But, on cooling below \({\rm{9}}{{\rm{6}}^{\rm{o}}}\,{\rm{C}}\) it returns to rhombic form.
6. It is insoluble in water and soluble in carbon disulfide.
7. Rhombic Sulphur exists as \({{{\rm{S}}_{\rm{8}}}}\) unit at room temperature. The eight sulfur atoms present in a molecule form a puckered ring.

Monoclinic Sulphur

Monoclinic Sulphur is also called \(\beta – \)Sulphur or prismatic Sulphur. It is stable only above \({\rm{396}}\,{\rm{K}}\) and changes into rhombic Sulphur below this temperature.

Preparation: Sulphur is heated slowly in an evaporating dish till it melts. The molten Sulphur is then allowed to cool slowly. During the cooling process, a solid crust will be formed over the surface. As this crust is being formed, two holes are made in the crust. The molten Sulphur is then poured out of these holes. The crust is then removed. On the lower side of the crust, long needle-shaped crystals of monoclinic Sulphur are formed.

Properties:

1. Monoclinic Sulphur exists as a long needle-like prism: hence it is called prismatic Sulphur.
2. It melts at \({119^{\rm{o}}}\,{\rm{C}}{\rm{.}}\)
3. Its density is \(1.98\;\,{\rm{g}}/{\rm{mL}}{\rm{.}}\)
4. It is insoluble in water but readily soluble in carbon disulfide.
5. Monoclinic Sulphur exists as Ss molecules which possess puckered ring structure similar to those of rhombic Sulphur, but it differs from the rhombic Sulphur in symmetry.

Plastic Sulphur

Plastic Sulphur is also called \(\gamma – \)sulphur. 

Preparation: Plastic Sulphur is prepared by suddenly cooling the molten Sulphur (nearly its boiling point) by pouring it into cold water. Yellow rubbery ribbons of Sulphur are formed.

Properties

1. It is an amorphous form of Sulphur. It is soft and elastic like rubber in the beginning but hardens on standing and cooling and gradually changes into rhombic Sulphur.
2. It has a rubber-like transparent yellow thread and is insoluble in carbon disulfide and water.
3. Its density is \(1.92\;\,{\rm{g}}/{\rm{mL}}{\rm{.}}\)
4. It does not have a sharp melting point.
5. Plastic Sulphur consists of a zig-zag chain and sometimes \({{\rm{S}}_8}\) and other rings.

Milk of Sulphur

When flowers of Sulphur are boiled with milk of lime, a mixture of calcium pentasulphide \(\left( {{\rm{Ca}}{{\rm{S}}_5}} \right)\) and calcium thiosulphate \(\left( {{\rm{Ca}}{{\rm{S}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}} \right)\) is obtained. The mixture treated with hydrochloric acid gives a white amorphous precipitator known as milk sulphur. 

\(3{\rm{Ca}}{({\rm{OH}})_2} + 12\;{\rm{S}} \to 2{\rm{Ca}}{{\rm{S}}_5} + {\rm{C}}{{\rm{a}}_2}\;{{\rm{S}}_2}{{\rm{O}}_3} + 3{{\rm{H}}_2}{\rm{O}}\)

\(2{\rm{Ca}}{{\rm{S}}_5} + {\rm{C}}{{\rm{a}}_2}\;{{\rm{S}}_2}{{\rm{O}}_3} + 8{\rm{HCl}} \to 12\;{\rm{S}} + 4{\rm{CaC}}{{\rm{l}}_2} + 4{{\rm{H}}_2}{\rm{O}}\)

Milk of Sulphur is soluble in carbon disulfide. It tends to revert to the rhombic variety on standing for a long time. It is largely used in medicines.

Colloidal Sulphur

Colloidal form of Sulphur may be obtained by passing hydrogen sulphide through nitric acid or by treating sodium thiosulphate solution with dilute hydrochloric acid.

\({{\rm{H}}_{\rm{2}}}{\rm{S}} + 2{\rm{HN}}{{\rm{O}}_3} \to 2{{\rm{H}}_2}{\rm{O}} + 2{\rm{N}}{{\rm{O}}_2} + {\rm{S}}\)

\({\rm{N}}{{\rm{a}}_2}\;{{\rm{S}}_3}{{\rm{O}}_3} + 2{\rm{HCl}} \to 2{\rm{NaCl}} + {\rm{S}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} + {\rm{S}}\)

It may also be obtained by treating hydrogen sulphide with sulphur dioxide.

\(2{{\rm{H}}_2}\;{\rm{S}} + {\rm{S}}{{\rm{O}}_2} \to 2{{\rm{H}}_2}{\rm{O}} + {\rm{3S}}\)

Engel’s Sulphur

Engel’s Sulphur is also called cyclo\({\rm{ – }}{{\rm{S}}_{\rm{6}}}\) or \(\varepsilon – \)sulphur. It is unstable and contains \({{\rm{S}}_{\rm{6}}}\) rings. In this, the ring adopts the chair form.

At higher temperatures, open chains consisting of sulphur atoms covalently bonded to each other are formed. Above \({\rm{1000}}\,{\rm{K,}}\) it exists as \({{\rm{S}}_2}\) molecule.

What are the Uses of Sulphur?

Some of the users of Sulphur are as follows:

1. Sulphur is rarely used in the manufacture of sulphur dioxide, which is used for fumigation, bleaching and the manufacture of sulphuric acid.
2. It is used in the manufacture of matches, fireworks, gunpowder, and dyes.
3. It is used in the preparation of several drugs.
4. It is used as a fungicide.
5. It is used in the vulcanization of rubber.

Summary

In this article, you have explored the types of crystalline and amorphous salts, their preparation, properties, etc. Because of its applications, sulphur has grown in importance. Its applications are not limited to industries; it also plays an important role in our ecosystem by influencing plant growth. As a result, many sulphur-containing fertilisers have been developed.

FAQs on Allotropes of Sulphur

Here are some of the frequently asked questions related to allotropes of Sulphur.

Q.1. How many allotropes of Sulphur are there? Ans: There are two main types of allotropes of Sulphur.  Q.2. What are the two main types of allotropes of Sulphur? Ans: Allotropes of Sulphur can be classified into two types which are, crystalline Sulphur and amorphous Sulphur.  Q.3. Give one example of an allotrope? Ans: Carbon is the most common example of an allotrope. Diamond and graphite are some of the examples of allotropes of Carbon. Q.4. What is the most common allotrope of Sulphur? Ans: Rhombic Sulphur or alpha(α) Sulphur is considered to be the most common allotrope of Sulphur. Q.5. What is the difference between the allotropes of Sulphur? Ans: The arrangement of atoms and other chemical properties contribute towards the difference in different allotropes of Sulphur. Q.6. Are allotropes of Sulphur soluble in water? Ans: Allotropes of Sulphur are not soluble in water.